# A taxonomic revision and cranial description of Terrestrisuchus gracilis (Archosauria, Crocodylomorpha) from the Upper Triassic of Pant-y-Ffynnon Quarry (southern Wales)

Stephan N. F. Spiekman, Vincent Fernandez, Richard J. Butler, Kathleen N.

Dollman, Susannah C. R. Maidment

► To cite this version:

Stephan N. F. Spiekman, Vincent Fernandez, Richard J. Butler, Kathleen N. Dollman, Susannah C. R. Maidment. A taxonomic revision and cranial description of Terrestrisuchus gracilis (Archosauria, Crocodylomorpha) from the Upper Triassic of Pant-y-Ffynnon Quarry (southern Wales). Papers in Palaeontology, 2023, 9 (6), pp.e1534. 10.1002/spp2.1534. hal-05095250

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# A taxonomic revision and cranial description of *Terrestrisuchus gracilis* (Archosauria, Crocodylomorpha) from the Upper Triassic of Pant-y-Ffynnon Quarry (southern Wales)

by STEPHAN N. F. SPIEKMAN<fnref n="1" />,<fnref n="2" />,<fnref n="4" />,* ID, VINCENT FERNANDEZ<fnref n="1" />,<fnref n="3" /> ID,
RICHARD J. BUTLER<fnref n="2" /> ID, KATHLEEN N. DOLLMAN<fnref n="3" /> ID and
SUSANNAH C. R. MAIDMENT<fnref n="1" />,<fnref n="2" /> ID

Typescript received 29 April 2023; accepted in revised form 3 August 2023

**Abstract:** Non-crocodyliform crocodylomorphs, formerly referred to the informal group 'Sphenosuchia', are the earliest known crocodylomorph precursors of extant crocodylians. They are therefore crucial for our understanding of early crocodylian evolution and the origin of typical crocodylian characteristics, such as the formation of a secondary palate, complex cranial pneumaticity, and a reinforced braincase. *Terrestrisuchus gracilis*, known from the Upper Triassic fissure fill deposits of Pant-y-Ffynnon in southern Wales, is represented by almost 200 specimens, comprising articulated, partially associated, and isolated remains. In this contribution we provide a taxonomic revision of *Terrestrisuchus gracilis* and redescribe its cranial anatomy in detail, based in part on novel micro-computed tomography data. The posterior skull region is extensively pneumatized as indicated, among other features, by large pre- and

postcarotid recesses on the parabasisphenoid, and a large pneumatic cavity in the articular of the mandible. In contrast, the quadrate forms only a small, unfused contact with the prootic, suggesting that complex pneumatization of the postorbital region pre-dated the co-ossification of the quadrate and braincase in Crocodylomorpha. *Terrestrisuchus gracilis* preserves an ossified basihyal, which represents the first occurrence of this bone in non-avemetatarsalian archosaurs. Finally, we show that *Terrestrisuchus gracilis* was probably cathemeral (i.e. active in a range of light levels), based on a phylogenetic flexible discriminant analysis of the relative dimensions of the sclerotic ring and orbit.

**Key words:** *Terrestrisuchus*, Crocodylomorpha, Triassic, CT scanning, cranial osteology, braincase pneumaticity.

### *Non-crocodyliform* crocodylomorphs had a wide distribution between the early Late Triassic (Carnian) and Late Jurassic (Irmis *et al.* 2013). Historically, this group has been referred to as 'Sphenosuchia', but current phylogenetic analyses suggest that they do not form a single (monophyletic) clade, but rather a grade of several lineages leading up to Crocodyliformes (Leardi *et al.* 2017; Ruebenstahl *et al.* 2022; Spiekman 2023). Although they were relatively rare components of Late Triassic ecosystems (Irmis *et al.* 2013), crocodylomorphs are the only pseudosuchians to survive the Triassic–Jurassic mass extinction event, possibly undergoing a rapid adaptive radiation in the Early Jurassic (Toljagić & Butler 2013). Despite their relative rarity, non-crocodyliform crocodylomorphs were a diverse group of terrestrial faunivores. Most taxa are small with a slender build but some were much larger, with an estimated skull length of over 50 cm (Nesbitt *et al.* 2005; Irmis *et al.* 2013; Drymala &

Zanno 2016). Due to their phylogenetic position and unique ecomorphology, non-crocodyliform crocodylomorphs are crucial to our understanding of early crocodylomorph evolution and the order of acquisition of crocodylian apomorphies, such as the origin of an extensive system of air-filled sinuses (pneumaticity) surrounding the braincase, the reduction of the parabasisphenoid, and the fusion of the quadrate with the braincase, among others (Pol *et al.* 2013; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). Prior to the current study, *Sphenosuchus acutus* represented the crocodylomorph taxon most distantly related to Crocodyliformes (i.e. the earliest diverging crocodylomorph) for which the anatomy had been studied in great detail (Walker 1990). In certain respects, this taxon already exhibits a relatively derived condition, including considerable braincase pneumaticity and a connection, albeit a limited one, between the quadrate and braincase (Walker 1990; Leardi *et al.* 2020).

<footnote n="1">¹Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK; stephanspiekman@gmail.com</footnote>

<footnote n="2">²School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK</footnote>

<footnote n="3">³European Synchrotron Radiation Facility, 71 Avenue des Martyrs, Grenoble 38000, France</footnote>

<footnote n="4">⁴Current address: Staatliches Museum für Naturkunde Stuttgart, Rosenberg 1, 70191 Stuttgart, Germany</footnote>

<footnote>*Corresponding author</footnote>
*Terrestrisuchus gracilis* is known from the Upper Triassic fissure fill of Pant-y-Ffynnon Quarry, one of several Upper Triassic to Lower Jurassic fissure fill localities from the Bristol Channel area that are well-known for preserving a diverse fauna of predominantly small-bodied tetrapods (Whiteside *et al.* 2016). *Terrestrisuchus gracilis* is represented by numerous isolated and associated remains, comprising almost 200 specimens in total. This material was described in detail by Peter J. Crush in his unpublished PhD dissertation, completed at University College London (Crush 1980), and later formally described and assigned to *Terrestrisuchus gracilis* in a comparatively brief publication (Crush 1984). The aim of the current contribution is to expand upon Crush's original description and to provide a detailed comparative description of the anatomy of *Terrestrisuchus gracilis*.

*Terrestrisuchus gracilis* has highly elongate and gracile limbs and a very slender bauplan. Even among non-crocodyliform crocodylomorphs *Terrestrisuchus gracilis* is particularly small and highly gracile, with a known body length estimated at *c.* 760 mm by Crush (1984), who probably considerably overestimated the length of the tail (Allen 2010; SNFS pers. obs.). Recent phylogenetic analyses suggest that *Terrestrisuchus gracilis* represents an early diverging crocodylomorph only distantly related to Crocodyliformes (Leardi *et al.* 2017; Ruebenstahl *et al.* 2022). *Terrestrisuchus gracilis* was recently reassigned to Saltoposuchidae, a clade of small-bodied, highly gracile crocodylomorphs that also includes *Saltoposuchus connectens* and *Litargosuchus leptorhynchus* (Spiekman 2023).

Due to its extensive and well-preserved remains and phylogenetic position near the origin of Crocodylomorpha, *Terrestrisuchus gracilis* is a key taxon for studying the earliest evolution of the crocodylian lineage, but it has been undervalued due to limited morphological descriptions and uncertainty surrounding its taxonomy and specimen composition. Here, we provide a detailed redemption of the cranial anatomy of *Terrestrisuchus gracilis*, aided by x-ray micro-computed tomography (µCT) of its most complete skull, preserved in NHMUK PV R 7591, including a 'reassembled' three-dimensional (3D) model of this specimen and a complete skull reconstruction. In addition, we present a revised diagnosis, provide the first complete hypodigm for the species, and investigate its diel activity patterns through a quantitative analysis of the orbit and sclerotic ring.

## MATERIAL AND METHOD

### X-ray micro-computed tomography

NHMUK PV R 7591 represents the best preserved skull among the material referred to *Terrestrisuchus gracilis*. It

comprises a mostly complete posterior half of the skull and mandible (i.e. most of the circumorbital region and all cranial elements posterior to this), which have been strongly lateromedially compressed, obscuring many morphological details. The two blocks containing the skull material were characterized with µCT using a Nikon Metrology HMX ST system at the Natural History Museum, London, UK (NHMUK). The UltraFocus x-ray source was fitted with a tungsten target and set to 160 kV and 187 µA (30 W), and filtered with 1 mm of copper. The acquisition consisted of 3142 projections recorded over 360°, with an exposure of 1 s per projection and a frame averaging of 2. CT reconstruction was performed with CT pro 3D (Nikon Metrology) with automated beam hardening mode and an isotropic voxel size of 38.0 µm.

The µCT data were segmented in Avizo v2020.1 (https://www.thermofisher.com). The resulting surface files (available in Spiekman *et al.* 2023) were subsequently exported into Blender v3.2 (https://blender.org). To attenuate artefacts resulting from manual segmentation, meshes were smoothed in Blender using a series of modifiers (i.e. remesh/smooth, smooth and remesh/voxel). Although this procedure removed the artefacts efficiently, it resulted in fading of some minute yet informative osteological details. Meshes were then imported into VGstudio max 3.5 (Volume Graphics), along with the µCT data, and converted to regions of interest (ROIs). To match the ROIs more closely to the bones, the refinement tool was used with a search distance of 1 voxel and no threshold. ROIs were subsequently converted back to meshes using the grid-based watertight approach. Imported in Blender v3.2, a combination of two modifiers (remesh/smooth with an octree value of 8 and scale of 0.99, and remesh/voxel with the original voxel size of 38 µm) removed the blocky aspect of the meshes created by the watertight approach in VGstudio max 3.5. Final colours and texture were applied to the bones, and the skull and mandible were 'reassembled' by digitally repositioning the individual elements into their inferred *in vivo* positions. Images of the reconstructed digital models were also rendered in Blender v3.2.

### Sclerotic ring analysis

In order to determine the diel activity patterns of *Terrestrisuchus gracilis*, we included it in the dataset of Choi niere *et al.* (2021), which is an expanded version of the dataset from Schmitz & Motani (2011). These analyses use phylogenetic flexible discriminant analysis (pFDA) on measurements of the sclerotic ring and orbit in a variety of extant birds and squamates to infer potential daily activity patterns in extinct taxa. We measured the orbit
height and the internal and external diameters of the scle-
rotic ring from the 3D reconstruction of the skull of
NHMUK PV R 7591 based on µCT data. The dataset
of Choiniere *et al.* (2021) uses orbit length, but the length
of the orbit was not fully preserved in NHMUK PV R
7591 therefore we assumed a circular orbit and used our
orbit height measurement to represent orbit length. We
added these data to those of Choiniere *et al.* (2021; see
Benson *et al.* 2021, eye_data.csv). In order to conduct
analyses, it was necessary to first add *Terrestrisuchus gracilis*
to the 50 timescaled phylogenetic trees of archosaurs
used by Choiniere *et al.* (2021; see Benson *et al.* 2021,
Sclerotic_ring_tree_04_August_2020.tre). To do this we
used an R script provided by RBJ Benson to add *Terre-
trisuchus* to the base of each of these 50 trees and calcu-
late missing branch lengths based upon the Rhaetian age
proposed for this taxon. The data and tree file used for
our analysis are provided in Appendix S1.

Choiniere *et al.* (2021) performed multiple pFDA ana-
lyses and found the best classification rates in their analysis
5, which used extant bird and lizard training data, and only
two diel activity bins, nocturnal versus non-nocturnal. We
therefore replicated this analysis (using R scripts available
in Benson *et al.* 2021) with our data added. Furthermore,
to explore the activity patterns of *Terrestrisuchus gracilis* we
ran an additional analysis (analysis 4 of Choiniere
*et al.* 2021), which also used both extant lizard and bird
training data but which had three diel activity bins corre-
sponding to nocturnal, diurnal and cathemeral patterns.

*Institutional abbreviations.* AMNH, American Museum of Natural History, New York, NY, USA; BP, Evolutionary Studies Institute, University of Witwatersrand, Johannesburg, South Africa; CM, Carnegie Museum of Natural History, Pittsburgh, PA, USA; NHMUK, Natural History Museum, London, UK; SMNS, Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany; VMNH, Virginia Museum of Natural History, Martinsville, VA, USA; YPM, Yale Peabody Museum, New Haven, CT, USA.

SYSTEMATIC PALAEOONTOLOGY

Archosauria Cope, 1869-1870 sensu Gauthier & Padian 2020

Pseudosuchia Zittel, 1887-1890

Crocodylomorpha Hay, 1930 sensu Nesbitt 2011

(= Crocodilomorpha emended Walker 1970)

Saltoposuchidae Crush, 1984 sensu Spiekman 2023

*Terrestrisuchus gracilis* Crush, 1984

Figures 1–9

Holotype. NHMUK PV R 7557a-h (Figs 1, 2).

*Referred material.* Almost 200 specimens are referred to *Terre-
trisuchus gracilis*, consisting of both associated and isolated

remains. A full overview of all referred specimens is provided in Table S1 (see also Appendix S2 for a detailed overview of all cranial remains and their distribution among the *Terrestrisuchus gracilis* specimens). The main specimens preserving associated remains besides the holotype are NHMUK PV R 37600, NHMUK PV R 7553, NHMUK PV R 7561, NHMUK PV R 7562, NHMUK PV R 7571, NHMUK PV R 7591 and NHMUK PV R 7593 (Figs 3–9).

Revised diagnosis. *Terrestrisuchus gracilis* is a small-sized non-crocodyliform crocodylomorph distinguished by the following combination of character states (autapomorphies among non-crocodyliform crocodylomorphs, in part based on the phylogenetic analysis in Spiekman (2023), are indicated with *; traits that are potentially ontogenetically variable are indicated with †*: squamosal without a longitudinal crest on the dorsal surface (also absent in *Litargosuchus leptorhynchus*)†; transverse width of anterior process of squamosal wider than supratemporal fenestra (also present in *Saltoposuchus connectens* and *Litargosuchus leptorhynchus*); absence of sagittal crest on parietal (also absent in *Litargosuchus leptorhynchus* and *Kayentasuchus walkeri*)†; presence of a large quadrate fenestra (also present in *Saltoposuchus connectens* and *Almadasuchus figarii*); perilymphatic loop of the otoccipital extending approximately as far ventrally as the subcapsular buttress in lateral view*; presence of an otic bulla on the otoccipital*; enlarged, heavily pneumatized parabasisphenoid body (also present in *Junggarsuchus sloani*, *Almadasuchus figarii* and *Macelognathus vagans*); elongate anterior process on the lateral head of the ectopterygoid (also present in *Saltoposuchus connectens* and *Junggarsuchus sloani*); retroarticular process elongated, triangular shaped and dorsally facing*; large pneumatic cavity with dorsal and ventral openings in articular*; distinct postzygodiapophyseal laminae on posterior cervical and anterior dorsal vertebrae (also present in *Hesperosuchus agilis*); narrow teardrop-shaped, poorly sculptured paramedian osteoderms*; proximal end of radius having a medial process (also present in *Hesperosuchus agilis*); proximal ends of metacarpals overlapping*; metacarpal I more slender than other metacarpals (also present in *Junggarsuchus sloani*); lateral condyle of distal end of femur projecting further distally than medial condyle (also present in *Macelognathus vagans*); tibia longer than femur (also present in *Hallopus victor* and *Macelognathus vagans*); pedal digit V having two phalanges (at least one phalanx also present in *Hesperosuchus agilis*).

Remarks. Some authors considered *Terrestrisuchus gracilis* to be synonymous with *Saltoposuchus connectens* from the Stubensandstein (Löwenstein Formation, Norian) of southwestern Germany (Huene 1921), with *Terrestrisuchus gracilis* representing an earlier ontogenetic stage than the type material of *Saltoposuchus connectens* (e.g. Clark 1986; Benton & Clark 1988; Allen 2003a, 2003b). However, others have outlined clear diagnostic differences between the two forms (e.g. Sereno & Wild 1992; Irmis *et al.* 2013), and a recent revision of *Saltoposuchus connectens* found strong support for maintaining *Terrestrisuchus gracilis* and *Saltoposuchus connectens* as separate but closely related species within *Saltoposuchidae* (Spiekman 2023).
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**FIG. 1.** *Terrestrisuchus gracilis* NHMUK PV R 7557a (holotype). A, complete slab. B, overview of central region of the slab. C, close up of the cranial and mandibular remains. D, close up of a series of neural arches of dorsal vertebrae and associated osteoderms. *Abbreviations*: cav, caudal vertebra; de, dentary; dv, dorsal vertebra; fe, femur; fi, fibula; fr, frontal; il, ilium; l., left; mx, maxilla; na, nasal; nea, neural arch; os, osteoderm; pu, pubis; r, radius; r., right; ra, radiale; sa, surangular; sq, squamosal; ti, tibia; u, ulna; ul, ulnare. Scale bars represent: 50 mm (A, B); 10 mm (C, D).

Crush (1980, 1984) described *Terrestrisuchus gracilis* exclusively on material collected from Pant-y-Ffynnon Quarry. However, crocodylomorphs are also known from other Upper Triassic fissures in the Bristol Channel area (Whiteside et al. 2016). Material from Ruthin Quarry is restricted to jaw fragments, teeth and osteoderms, and thus is undiagnostic to lower taxonomic levels (Skinner et al. 2020). Similarly, it is likely that a tentatively identified ulna and partial femur from Durdham Down Quarry represent crocodylomorph remains, but they cannot be assigned to a genus or species (Foffa et al. 2014). Two distal caudal vertebrae from Tytherington Quarry were assigned to *Terrestrisuchus* and considered likely to be conspecific with *Terrestrisuchus gracilis* (Mussini et al. 2020). However, these remains are extremely limited and largely undiagnostic. Furthermore, the dating of the fissure fills is controversial due to its unique depositional setting (e.g. Walkden et al. 2021; Whiteside & Benton 2021), and it is therefore possible that the various fissures are separated by considerable time intervals. Consequently, these remains cannot be confidently attributed to *Terrestrisuchus gracilis*, and are considered as Crocodylomorpha indet. here. Finally, substantial crocodylomorph material, including articulated remains, is known from several fissures in Cromhall Quarry. This material was treated in brief by Fraser et al. (2002), who found that two separate morPHS were represented at Cromhall that both differed from *Terrestrisuchus gracilis*. Nevertheless, both were preliminarily generically assigned to *Terrestrisuchus*. Allen (2010) provided a brief description of some Cromhall specimens and synonymized them with *Terrestrisuchus gracilis* from Pant-y-Ffynnon without further motivation. More detailed description of the Cromhall material has thus far not been conducted, in part due to uncertainty regarding the composition and morphology of the *Terrestrisuchus gracilis* material from Pant-y-Ffynnon. Comparisons of the Cromhall specimens, predominantly the most complete specimen, NHMUK PV R 10002, with the now revised Pant-y-Ffynnon material of *Terrestrisuchus gracilis* indicate distinct morphological differences, for instance in limb proportions and carpal morphology (SNFS pers. obs.). Therefore, *Terrestrisuchus gracilis* can currently be restricted to the fissures at Pant-y-Ffynnon Quarry. The description of the Cromhall crocodylomorph material will be the subject of a separate study.

*Ontogenetic assessment. Terrestrisuchus gracilis* currently represents one of the smallest crocodylomorphs (Irmis et al. 2013), with known specimens not exceeding 1 m in total body length (Crush 1984). For this reason, as outlined above, *Terrestrisuchus gracilis* was previously considered to be represented by

skeletally immature specimens, with the taxon assigned as a junior synonym of *Saltoposuchus connectens*. Conversely, it has also tentatively been suggested that the body size of the preserved fauna at Pant-y-Ffynnon, which is represented by small-bodied tetrapods, is a result of insular dwarfism (Skinner et al. 2020), thus explaining the small size of *Terrestrisuchus gracilis*. However, it is possible that larger bodied animals are absent at Pant-y-Ffynnon due to a preservational bias related to the depositional conditions of the fissures (Spiekman et al. 2021). Resolution of this issue is complicated by the generally isolated, fragmentary nature of specimens for most taxa, hampering observations on skeletal maturity. Life history has been poorly studied in early crocodylomorphs, and the level of maturity is therefore unclear for specimens of most taxa. A recent histological investigation of *Saltoposuchus connectens* found that one of the larger specimens known for this taxon is likely to represent an individual that was a sub-adult (i.e. not representing an early juvenile but also not skeletally mature; Spiekman 2023).

Several features are present in the *Terrestrisuchus gracilis* material that suggest that at least the majority of specimens were immature:

1. The parietals are unfused in all known specimens of *Terrestrisuchus gracilis* preserving these elements, including NHMUK PV R 7591, on which much of the cranial description is based (Figs 10–12). In contrast, no suture is discernible in dorsal view between the parietals of other known non-crocodyliform crocodylomorphs, except for a partially fused parietal suture in *Litargosuchus sloani*, which also represents a particularly small, probably immature crocodylomorph specimen (BP/1/5237, Walker 1990; Wu & Chatterjee 1993; Harris et al. 2000; Clark & Sues 2002; Leardi et al. 2020; Ruebenstahl et al. 2022). Parietal fusion is generally considered to take place during postnatal development in archosaurs (Nesbitt 2011; Bailleul et al. 2016).

2. The articulars of the mandible of NHMUK PV R 7591 possess large pneumatic openings, which correspond to the condition in juvenile individuals of *Alligator mississippiensis* (DufEAu & Witmer 2015), and thus also might reflect skeletal immaturity in this specimen. However, the relationship between articular pneumatization and ontogeny is poorly understood in early crocodylomorphs (see Discussion on Pneumatic cavities below).

3. The neurocentral sutures of most postaxial vertebrae, including all known dorsal vertebrae, are unfused in the *Terrestrisuchus gracilis* material, which is a relatively robust indicator of skeletal immaturity in crocodylomorphs and saurians more widely (Brochu 1996; Griffin et al. 2021).
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**FIG. 2.** *Terrestrisuchus gracilis* NHMUK PV R 7557b-i (holotype). A, NHMUK PV R 7557b, which forms the counterslab to NHMUK PV R 7557a. B, left scapula NHMUK PV R 7557e in lateral view. C, NHMUK PV R 7557f, left pubis in anteromedial view (left) and associated gastralia (right). D, distal end of left humerus NHMUK PV R 7557h in posterior view. E, left maxilla NHMUK PV R 7557c in lateral view. F, axis NHMUK PV R 7557g in right lateral view. *Abbreviations*: dc, distal carpal; fe, femur; im, impression; j, jugal; l, left; mn, manus; ot, otoccipital; pr, prootic; ps, pisiform; r, radius; r., right; ra, radiale; so, supraoccipital; sq, squamosal; sv, sacral vertebra; u, ulna; ul, ulnare. Scale bars represent: 20 mm (A); 10 mm (B–C, E); 5 mm (D, F).

NHMUK PV R 7591 also possesses a large orbit relative to its skull size (orbit height is 21 mm and skull width across squamosals is 33 mm). However, although a large orbit size is sometimes used to assess ontogeny, mature individuals of small-bodied taxa also possess large orbits relative to body size (Griffin *et al. 2021*), and relative orbit size alone thus does not represent a secure measure to assess ontogeny in this case. *Terrestrisuchus gracilis* shares extremely gracile limb proportions (e.g. narrow limb shafts relative to limb bone lengths) with *Litargosuchus leptorhynchus*, and in the latter taxon this was considered to represent an indicator of skeletal immaturity (Clark & Sues 2002). Although relative robusticity is indeed ontogenetically variable, without an adequate growth series,

including confidently identified, skeletally mature specimens, the use of relative limb ‘robustness’ cannot be considered an adequate measure of ontogeny (Griffin *et al. 2021*). Nevertheless, *Terrestrisuchus gracilis* is likely to have had more gracile limb proportions than *Saltoposuchus connectens* throughout its ontogeny, given that the scaling ratio in limb proportions between *Terrestrisuchus gracilis* and the larger *Saltoposuchus connectens* exceeds the allometric scaling seen intraspecifically in both extant crocodylians and cursorial quadrupeds (Irmis *et al. 2013*).

While many features therefore indicate skeletal immaturity for the known specimens of *Terrestrisuchus gracilis*, it is likely that the majority of known specimens do not represent perinatal

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**FIG. 3.** *Terrestrisuchus gracilis* NHMUK PV R 7591 in: A, right; B, left lateral view. *Abbreviations*: cav, caudal vertebra; co, coracoid; hu, humerus; ic, interclavicle; l., left; os, osteoderm; psv, presacral vertebra; r, radius; r. right; sc, scapula; u, ulna. Scale bar represents 50 mm.
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**FIG. 4.** *Terrestrisuchus gracilis* NHMUK PV R 7593. A, NHMUK PV R 7593a (middle), 7593b (bottom) and 7593c (top). B, NHMUK PV R 7593d proximal end of left humerus in anterior/ventral view. C, NHMUK PV R 7593e, paired osteoderms. *Abbreviations*: an, angular; ax, axis; cav, caudal vertebra; cev, cervical vertebra; co, coracoid; cr, cervical rib; dr, dorsal rib; fr, frontal; hu, humerus; ic, interclavicle; j, jugal; l, left; la, lacrimal; mn, manus; mt, metatarsal; mx, maxilla; na, nasal; os, osteoderm; pa, parietal; pl, palatine; po, postorbital; psv, presacral vertebra; pu, pubis; q, quadrate; r, radius; r. right; sc, scapula; sq, squamosal; u, ulna. Scale bars represent: 50 mm (A); 10 mm (B); 5 mm (C).

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**FIG. 5.** *Terrestrisuchus gracilis* associated specimens NHMUK PV R 7561 and NHMUK PV R 37788. A, NHMUK PV R 7561a (left) and 7561b (right). B, NHMUK PV R 7561c, right ilium in lateral view. C, NHMUK PV R 37788, right ilium in lateral view. *Abbreviations*: cav, caudal vertebra; de, dentary; dv, dorsal vertebra; fe, femur; hu, humerus; ic, interclavicle; il, ilium; is, ischium; l., left; mt, metatarsal; mx, maxilla; os, osteoderm; pa, parietal; pu, pubis; r, radius; r., right; sq, squamosal; ti, tibia; u, ulna. Scale bars represent: 50 mm (A); 10 mm (B, C).

individuals. Osteoderms are known to ossify a considerable time after hatching in at least some crocodylomorphs (Vickaryous & Hall 2008; de Buffrénil *et al.* 2015; Griffin *et al.* 2021), and all specimens that preserve articulated parts of the vertebral column, including the holotype NHMUK PV R 7557 and NHMUK PV R 7591, possess fully formed paramedian osteoderms.

*Locality & horizon.* Upper Triassic fissure fill deposits of PantyhFynnon Quarry in southern Wales. The dating of the fissure fill deposits of the Bristol Channel area is contentious. Some recent studies suggest that the Triassic fissure fill faunas post-date the

Rhaetian transgression (Whiteside & Marshall 2008; Whiteside *et al.* 2016; Mussini *et al.* 2020), whereas others suggest a considerably older Carnian or Norian age (Walkden *et al.* 2021).

## CRANIAL DESCRIPTION

Most cranial elements are present among the material assigned to *Terrestrisuchus gracilis*, except for the premaxilla, prefrontal and vomer. Only fragmentary remains are
<figure><img src="image_10.png" /><figcaption>**FIG. 6.** *Terrestrisuchus gracilis* NHMUK PV R 7562. *Abbreviations:* cav, caudal vertebra; cev, cervical vertebra; dc, distal carpal; dv, dorsal vertebra; fe, femur; ga, gastralia; il, ilium; im, impression; l., left; mc, metacarpal; ns, neural spine; os, osteoderm; ps, pisiform; pu, pubis; r, radius; r. right; ra, radiale; sv, sacral vertebra; u, ulna; ul, ulnare. Scale bar represents 50 mm.</figcaption></figure>

identified for the laterosphenoid and stapes, whereas all other cranial elements are represented by either complete or mostly complete remains. Many skull elements are present as isolated or disassociated remains in various specimens, although partially articulated skulls are present in NHMUK PV R 7557a, NHMUK PV R 7591 and NHMUK PV R 7593 (Figs 1–4). The description of the postorbital skull region, including the braincase, is predominantly based on the µCT data of the partial skull of NHMUK PV R 7591 (Figs 10–12). Based on our new

findings, a new reconstruction of the skull and mandible of *Terrestrisuchus gracilis* is presented in Figure 13.

### Snout

*Maxilla. Terrestrisuchus gracilis* has a virtually completely straight tooth-bearing margin of the maxilla (Fig. 14), as is also the condition in *Litargosuchus leptorhynchus* and *Kayentasuchus walkeri* (Clark & Sues 2002). In contrast,
<figure><img src="image_11.png" /><figcaption>FIG. 7. *Terrestrisuchus gracilis* NHMUK PV R 7553a (left) and 7553b (right). *Abbreviations*: br, braincase element; cav, caudal vertebra; co, coracoid; dv, dorsal vertebra; ic, interclavicle; l., left; psv, presacral vertebra; r., right; sq, squamosal. Scale bar represents 20 mm.</figcaption></figure>

the tooth-bearing margin of the maxilla is convex an-
teriorly in lateral view in other non-crocodyliform croco-
dylomorphs (Saltoposuchus connectens, Spiekman 2023;
Junggarsuchus sloani, Clark et al. 2004; Ruebenstahl
et al. 2022; Dromicosuchus grallator, Sues et al. 2003;
Sphenosuchus acutus, Walker 1990; Redondavenator
quayensis, Nesbitt et al. 2005; Dibothrosuchus elaphros,
Wu & Chatterjee 1993; Ruebenstahl et al. 2022; Pseudhe-
sperosuchus jachaleri, Clark et al. 2000; Hesperosuchus agi-
lis, Colbert 1952; CM 29894, Clark et al. 2000; and
Trialestes romeri, Lecuona et al. 2016). Note that there is
confusion surrounding the taxonomic assignment of CM
29894, which was originally referred to Hesperosuchus
agilis (Clark et al. 2000), an interpretation that has been
questioned by others (Nesbitt 2011; Irmis et al. 2013,
although see Leardi et al. 2017). Here, we refer to this
specimen, as well as YPM 41198, which both derive from

the *Coelophysis* Quarry in the upper part of the Chinle
Formation, separately from *Hesperosuchus agilis*, which in
our comparisons is restricted to the holotype AMNH
FARB 6758 from the Blue Mesa Member in the lower
part of the Chinle Formation (see also Spiekman 2023).

The complete posterior process of the maxilla gradually
tapers posteriorly and gradually curves laterally in ventral
view (Fig. 14B). The anterodorsal margin of the ascending
process of the maxilla is gently convex at its base and
gradually straightens distally (Fig. 14A, C). The angle
formed between the tooth-bearing margin and the
ascending process of the maxilla (excluding the antorbital
fossa) is strongly acute, being c. 15°. In contrast, in
the relatively undistorted specimens of *Saltoposuchus connectens*
(SMNS 12596 and SMNS 12352) this angle is
c. 25–30° (Spiekman 2023). The anterior margin of the
antorbital fenestra is gently rounded and the antorbital
<figure><img src="image_12.png" /><figcaption>**FIG. 8.** *Terrestrisuchus gracilis* NHMUK PV R 7571. Note that the *Clevosaurus* dentary is assigned a separate specimen number: NHMUK PV R 37015. *Abbreviations*: cav, caudal vertebra; co, coracoid; dr, dorsal rib; dv, dorsal vertebra; r, radius; r. right; sc, scapula; u, ulna. Scale bar represents 50 mm.</figcaption></figure>

fossa is formed by a thin sheet of bone (Fig. 14C). In
contrast to the reconstruction by Crush (1984), the antor-
bital fossa is also exposed ventral to the antorbital fenes-
tra in lateral view, although it does not reach far
posteriorly, similar to the condition in *Dibothrosuchus
elaphros* (Wu & Chatterjee 1993) and *Carnufex carolinен-
sis* (Drymala & Zanno 2016). In other non-crocodyliform
crocodylomorphs the antorbital fossa is exposed in lateral
view along the entire length of the ventral margin of the
antorbital fenestra on the maxilla. Anteriorly, the

antorbital fossa forms a conspicuously deep concavity or
pit that extends considerably under the base of the
ascending process of the maxilla (Fig. 14A). The maxillae
of *Terrestrisuchus gracilis* can easily be distinguished from
those of *Agnospitys cromhallensis* (VMNH 1751), the
only other described maxilla of a predatory archosaur
from the fissure fill fauna (Fraser *et al.* 2002), by the
presence of a promaxillary foramen on the anterior part
of the antorbital fossa in the latter. Small nutrient foram-
ina are present on the lateral surface of the maxilla dorsal

<caption>**FIG. 9.** *Terrestrisuchus gracilis* NHMUK PV R 37600. A, NHMUK PVR 37600a (far right), 37600b (centre left), 37600d (far left), and 37600e (centre right). B, NHMUK PV R 37600c, isolated left astragalus in anterior view. *Abbreviations*: cav, caudal vertebra; dt, distal tarsal; fe, femur; fi, fibula; il, ilium; is, ischium; l., left; mc, metacarpal; mt, metatarsal; ps, pisiform; pu, pubis; r, radius; r., right; ra, radiale; sv, sacral vertebra; ti, tibia; u, ulna; ul, ulnare. Scale bars represent: 50 mm (A); 5 mm (B).</caption>
<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>
**FIG. 10.** The digitally reconstructed skull and associated postcranial elements of NHMUK PV R 7591 in: A, right lateral; B, left lateral or ventral view. *Abbreviations*: an, angular; ar, articular; atlic, atlas intercentrum; atlna, atlantal neural arch; atplc, atlas pleurocentrum; atlr, atlantal rib; ax, axis; axr, axial rib; bo, basioccipital; bshy, basihyal; cav, caudal vertebra; cev, cervical vertebra; cr, cervical rib; crb, ceratobranchial I; ect, ectopterygoid; fr, frontal; j, jugal; l, left; ls, laterosphenoid; os, osteoderm; ot, otoccipital; pa, parietal; pbs, parabasisphenoid; pl, palatine; po, postorbital; pr, prootic; pra, prearticular; pt, pterygoid; q, quadrate; qj, quadratojugal; r., right; sa, surangular; scl, scleral ring; sq, squamosal. Scale bar represents 10 mm.

to the alveoli. The dorsal surface of the posterior process of the maxilla possesses an anteriorly placed foramen from which an elongate longitudinal groove extends posteriorly (Fig. 14E), which facilitated the maxillary branch (V2) of the trigeminal nerve (Witmer 1997).

The palatal process is well preserved (Fig. 14B, D) and resembles that of *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Walker 1990; Ruebenstahl *et al.* 2022). It is separated from the alveolar margin by a shallow longitudinal groove. The posterior margin of the process, which formed the anterior border of the choana, is anteriorly concave and forms a sharp and acute angle with the medial margin of the process. The palatal process contacted its antimere along its straight medial margin (Crush 1984; Fig. 13C). Based on the comparably far posteriorly reaching palatal process, this contact is homologous to the secondary palate present in other non-crocodyliform crocodylomorphs and protosuchids (Dollman *et al.* 2018; Dollman & Choiniere 2022). The palatal process also extends considerably anteriorly, leaving only a narrow slot between its anterolateral margin and the tooth-bearing portion of the maxilla (Fig. 14B). No clear facet for the premaxilla and its palatal contribution can be discerned.

**Nasal.** The nasal is almost 10-fold longer than wide (Table S2), and it is very dorsoventrally thin (Fig. 15). Its medial margin is virtually straight in ventral view, whereas the lateral margin is slightly concave on its anterior half and straight to very slightly convex on its posterior half. The medial and lateral margins are oriented parallel to each other, and the element roughly maintains its width along most of its anteroposterior length. In this regard the element can be clearly differentiated from early dinosaur and dinosauromorph nasals, which widen considerably posteriorly (e.g. Colbert 1989; Yates 2003; Nesbitt *et al.* 2020; Nesbitt & Sues 2021). The nasal is generally similar but comparatively much more lateromedially slender than those of most other non-crocodyliform crocodylomorphs (e.g. *Saltoposuchus connectens*, Spiekman 2023; *Junggarsuchus sloani*, Clark *et al.* 2004; Ruebenstahl *et al.* 2022; *Dromicosuchus grallator*, Sues *et al.* 2003; *Dibothrosuchus elaphros*, Wu & Chatterjee 1993; CM 29894, Clark *et al.* 2000), with the exception of *Litargosuchus leptorhynchus*, which has a similarly slender nasal (Clark & Sues 2002). The ventral surface of

the nasal is gently transversely concave on its anterior half in *Terrestrisuchus gracilis*, whereas the dorsal surface is transversely convex (Fig. 4A). Anteriorly, the nasal has two anteriorly directed processes: a large anteromedial process that gradually tapers distally, and a much smaller anterolateral process that is very narrow and which terminates in a sharp point (Fig. 15). The postnarial process of the premaxilla is likely to have articulated with the notch formed between these two anterior nasal processes (Crush 1980; Clark *et al.* 2000; Fig. 13A, B). Posteriorly, the nasal terminates in a blunt process that medially articulated with the frontals, which entered between the posterior processes of the paired nasals as in several other non-crocodyliform crocodylomorphs (e.g. CM 29894, *Litargosuchus leptorhynchus* and *Saltoposuchus connectens*; Clark *et al.* 2000; Clark & Sues 2002; Spiekman 2023) and laterally with the prefrontal (Fig. 13B). The lateral margin of the nasal articulated on its anterior half with the ascending process of the maxilla and on its posterior half with the lacrimal and prefrontal.

**Lacrimal.** The lacrimal is similar to that of other non-crocodyliform crocodylomorphs. It has an inverted L shape in medial view, which is formed by anterior and ventral processes with long axes that are approximately perpendicular to each other (Fig. 16). The ventral process forms the posterior margin of the antorbital fenestra anteriorly and the anterior margin of the orbit posteriorly (Fig. 13A). Its ventral end is expanded posteroventrally and slightly anteroventrally (Fig. 16A), where it articulates with the jugal and maxilla, respectively. Both the anterior and posterior margins of the ventral process are gently concave. A clear, mostly straight, longitudinal ridge is present on the medial surface of the ventral process, dividing the process in approximately equally anteroposteriorly wide anterior and posterior portions. A similar ridge is also present in *Trialestes romeri* (Lecuona *et al.* 2016), and a possibly homologous ridge might be present in *Carnufex carolinensis*, but in the latter the ridge curves strongly posterodorsally to anteroventrally along the medial surface of the ventral process (Drymala & Zanno 2016). The lateral surface of the ventral process anterior to this ridge contributed to the antorbital fossa, and is continuous with the wide, plate-like contribution of the anterior process of the lacrimal to the antorbital
<figure><img src="image_15.png" /><figcaption>**FIG. 11.** Digitally reassembled reconstruction of the skull and associated postcranial elements of NHMUK PV R 7591 in: A, right lateral; B, dorsal view. Note that left frontal, left postorbital, left quadratojugal, left quadrate, left jugal, left ectopterygoid, left palatine, left scleral ring, right atlantal neural arch and tentative right proatlas represent mirror copies of the originals from the opposite side. *Abbreviations*: an, angular; ar, articular; atlr, atlantal rib; ax, axis; axr, axial rib; cev, cervical vertebra; cr, cervical rib; ect, ectopterygoid; fr, frontal; j, jugal; l, left; ot, otoccipital; pa, parietal; pbs, parabasisphenoid; pl, palatine; po, postorbital; pra, prearticular; pt, pterygoid; q, quadrate; qj, quadratojugal; r, right; sa, surangular; scl, sclerotic ring; sq, squamosal. Scale bar represents 10 mm.</figcaption></figure>
fossa (see below), as in *Carnufex carolinensis* (Drymala & Zanno 2016), *Saltoposuchus connectens* (Spiekman 2023), *Litargosuchus leptorhynchus* (Clark & Sues 2002), *Dromicosuchus grallator* (Sues et al. 2003), *Sphenosuchus acutus* (Walker 1990), *Pseudhesperosuchus jachaleri* (Bonaparte 1972) and *Dibothrosuchus elaphros* (Ruebenstahl et al. 2022).

The anterior process of the lacrimal is *c*. 1.5-fold longer than the ventral process. Its ventral margin formed most of the dorsal margin of the antorbital fenestra (Fig. 13A). Its dorsal margin articulated with the nasal anteriorly and with the prefrontal posteriorly, whereas its anterior margin articulated with the ascending process of the maxilla. The dorsal margin of this process is virtually straight whereas the ventral margin, which is very thin lateromedially, is straight to very gently concave (Fig. 16). The anterior process has a triangular outline. Most of the anterior process (except for its dorsalmost margin), as well as the anterior half of the ventral process, are convex and raised in medial view, indicating that in lateral view these parts of the bone formed the lacrimal contribution to the antorbital fossa (Fig. 16). This wide, sheet-like contribution to the antorbital fossa with a triangular outline that tapers anteriorly, is similar to that seen in *Saltoposuchus connectens* (Spiekman 2023), *Dibothrosuchus elaphros* (Wu & Chatterjee 1993; Ruebenstahl et al. 2022), CM 29894 (Clark et al. 2000), *Litargosuchus leptorhynchus* (Clark & Sues 2002), *Dromicosuchus grallator* (Sues et al. 2003) and *Carnufex carolinensis* (Drymala & Zanno 2016), but differs from the dorsoventrally narrower contribution to the fossa of *Sphenosuchus acutus* and *Junggarsuchus sloani* (Walker 1990; Ruebenstahl et al. 2022).

In medial view, the posterodorsal end of the lacrimal forms a fossa with a concave medial surface (Fig. 16). This region has a triangular outline with a sharp, acute posterodorsal corner. On the anterodorsal portion of the fossa a small anteroposteriorly directed groove can be discerned (Fig. 16A). This triangular depression, including the groove, formed the articulation facet for the prefrontal (Crush 1980, 1984), similar to the configuration seen in *Sphenosuchus acutus* and *Dibothrosuchus elaphros* (Walker 1990; Wu & Chatterjee 1993). Ventral to this depression and just dorsal to the base of the ventral process, a small concavity is seen on the posterior surface of the lacrimal (Fig. 16A). Although it is not preserved as a clear foramen, it is most likely that this represents the opening for the lacrimal duct (Crush 1980, 1984), given that it corresponds in its relative location to the position of the foramen for the lacrimal duct in *Saltoposuchus connectens* (Spiekman 2023), *Sphenosuchus acutus* (Walker 1990) and *Junggarsuchus sloani* (Ruebenstahl et al. 2022).

Skull roof, orbital & postorbital region

*Frontal*. The frontal is an elongate, plate-like element (Fig. 17A). It is very thin dorsoventrally (Fig. 17B), similar to the condition in *Saltoposuchus connectens* (Spiekman 2023) and in contrast to the much thicker frontals of *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Walker 1990; Ruebenstahl et al. 2022). The frontals are unfused in all available specimens, as in other non-crocodyliform crocodylomorphs. The medial margin of the frontal is straight and articulated with its antimere. The lateral margin is more complex. Anteriorly it is oriented slightly anteromedially to posterolaterally, and the anterior portion of the frontal thus tapers gradually anteriorly in dorsal view (Fig. 17A). The anterior end is incomplete in all known examples of the frontal, but it probably terminated in a gently rounded point. Based on the shape of the frontals and nasals, the anterior ends of the frontals wedged in between, and were partially overlapped by, the posterior ends of the nasals (Crush 1984), similar to the condition in all other non-crocodyliform crocodylomorphs for which this feature is known. The central portion of the lateral margin is distinctly concave in dorsal view and curves posterolaterally, and as a consequence the frontal is considerably transversely expanded posteriorly (Fig. 17A, C, D). This curved margin forms the dorsal margin of the orbit. The posterior margin forms an uneven V shape in dorsal view, with a posteriorly directed central process (Fig. 17B, C) that wedged between the two processes present on the anterior margin of each parietal (see below). Consequently, the articulation between both frontals and parietals was W shaped in dorsal view (Figs 11B, 13B), as in *Saltoposuchus connectens*, *Dromicosuchus grallator*, *Dibothrosuchus elaphros* and *Almadasuchus figarii* (Sues et al. 2003; Leardi et al. 2020; Ruebenstahl et al. 2022; Spiekman 2023). The posteromedial margin of the central process of the frontal is longer and more posteriorly oriented than its posterolateral margin, and the process is positioned slightly laterally relative to the lateromedial midpoint of the frontal in dorsal view (Fig. 17B, C). Based on the morphologies of both elements, the articulation between the frontal and parietal was probably formed by an abutting contact, with the parietal partially overlapping the frontal. The dorsal surface of the posterior end of the frontal does not bear a well-demarcated concavity that is part of the supratemporal fossa, in contrast to *Hesperosuchus agilis*, *Pseudhesperosuchus jachaleri*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Dromicosuchus grallator*, *Saltoposuchus connectens* and CM 29894 (Colbert 1952; Bonaparte 1972; Walker 1990; Wu & Chatterjee 1993; Clark et al. 2000; Sues et al. 2003; Ruebenstahl et al. 2022; Spiekman 2023). A contribution of the posterior part of the frontal to the
<figure><img src="image_16.png" /></figure>
**FIG. 12.** Digitally reassembled reconstruction of the skull and associated postcranial elements of NHMUK PV R 7591 in: A, ventral; B, posterior view. Note that left frontal, left postorbital, left quadratojugal, left quadrate, left jugal, left ectopterygoid, left palatine, left sclerotic ring, right atlantal neural arch and tentative right proatlas represent mirror copies of the originals from the opposite side. *Abbreviations:* atlic, atlas intercentrum; atplc, atlas pleurocentrum; atlr, atlantal rib; ax, axis; axr, axial rib; bo, basioccipital; cev, cervical vertebra; cr, cervical rib; ect, ectopterygoid; fom, foramen magnum; j, jugal; l, left; ot, otoccipital; pbs, parabasisphenoid; pl, palatine; po, postorbital; potf, post-temporal fenestra; pr, prootic; pt, pterygoid; q, quadrate; qj, quadratojugal; r., right; scl, sclerotic ring; sq, squamosal. Scale bars represent 10 mm.

supratemporal fossa is also absent in *Litargosuchus leptorhynchus* (Clark & Sues 2002).

The dorsal surface of the frontal is mostly flat and smooth, but it possesses a slightly raised longitudinal ridge, as is also present in *Litargosuchus leptorhynchus*, *Dromicosuchus grallator* and *Saltoposuchus connectens* (Clark & Sues 2002; Sues et al. 2003; Spiekman 2023). This ridge is present as a more distinctly developed narrow crest in *Hesperosuchus agilis*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani* and CM 29894 (Fig. 17A; Colbert 1952; Walker 1990; Wu & Chatterjee 1993; Clark et al. 2000; Ruebenstahl et al. 2022). A poorly developed ridge, as in *Terrestrisuchus gracilis*, is also present in *Litargosuchus leptorhynchus*, *Dromicosuchus grallator* and *Saltoposuchus connectens* (Clark & Sues 2002; Sues et al. 2003; Spiekman 2023). In *Dibothrosuchus elaphros*, two additional parasagittal ridges are present on the dorsal surface of each frontal (Wu & Chatterjee 1993; Ruebenstahl et al. 2022). In *Terrestrisuchus gracilis* this bulge is positioned near, but does not reach, the medial margin of the frontal (Fig. 17A). It originates anteriorly just anterior to the level of the anteriormost extent of the orbital margin on the lateral side of the frontal, and it reaches posteriorly close to the posterior end of the bone. The bulge is most prominent and widest near its anterior end. In more derived crocodylomorphs and crocodyliforms the dorsal surface of the frontal is rugose and flattened (e.g. *Protosuchus haughtoni*, Gow 2000; *Almadasuchus figarii*, Pol et al. 2013; Leardi et al. 2020).

The ventral surface of the frontal is characterized by a sharp and distinct longitudinal crest near its lateral margin (Fig. 17B-D), identified as the crista cranii by Walker (1990). This crest is oriented parallel to the frontal contribution of the orbital margin and thus possesses a similar curvature. Although it is distinct, the crista cranii does not form a ventrally directed process as in for instance *Junggarsuchus sloani* and *Almadasuchus figarii* (Leardi et al. 2020; Ruebenstahl et al. 2022), and instead its condition is very similar to that present in *Hesperosuchus agilis* and *Saltoposuchus connectens* (Colbert 1952; Spiekman 2023). The straight medial margin of the frontal is also slightly raised in ventral view, and consequently the ventral surface of the frontal between this medial margin and the crista cranii is transversely concave, forming a distinct groove that transmitted the

olfactory tract. Just anterior to its maximum transverse constriction this ventral groove possesses a rounded concavity, which represents the mould for the olfactory bulb (Walker 1990; Ezcurra 2016). The anterior margin of the mould is not clearly preserved in any known specimen, but it certainly reached close to the anterior end of the frontal, as in *Saltoposuchus connectens* and *Macelognathus vagans* (Leardi et al. 2017; Spiekman 2023). In contrast, in *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* the mould for the olfactory bulb is considerably shorter relative to the length of the frontal, and it does not reach anteriorly until near the end of the bone (Walker 1990; Ruebenstahl et al. 2022). Near the anterolateral edge of the bone, a laterally facing groove is preserved lateral to the crista cranii (Fig. 17C). This narrow groove probably forms an articulation facet for the prefrontal. Directly medial to this facet, a small but distinct ventrally and slightly anteriorly directed process is preserved directly anterolateral to the anteriormost extent of the crista cranii. A process is also present in the same region on the frontal in *Saltoposuchus connectens* and *Sphenosuchus acutus* (Walker 1990; Spiekman 2023). This process also articulated with the prefrontal, as was suggested by Crush (1980), and as can be observed in *Sphenosuchus acutus* (Walker 1990). A posterolateral groove is present near the posterior end of the lateral margin of the frontal (Fig. 17B). It is present on the ventral surface of the frontal directly medial to the lateral margin of the element, which roofs this narrow groove, and as such it is open both laterally and ventrally. Such a distinct groove is absent in *Saltoposuchus connectens* (Spiekman 2023). It forms an articulation facet with the medial margin of the dorsomedial process of the postorbital. This articulation is partially preserved in NHMUK PV R 7591, although the posterior portion of this process has become disarticulated and is positioned slightly dorsally to the groove as a result of compression (Fig. 10A). The articulating portion of the medial margin of this process of the postorbital would have been covered by the lateral margin of the frontal in dorsal view. As in *Saltoposuchus connectens* and *Almadasuchus figarii* (Leardi et al. 2020; Spiekman 2023), the ventral surface of the frontal lacks the foramina for a complex system of canals that is present in the frontal of *Sphenosuchus acutus* (Walker 1990).
<figure><img src="image_17.png" /><figcaption>**FIG. 13.** Cranial reconstruction of *Terrestrisuchus gracilis* in: A, left lateral; B, dorsal; C, ventral; D, posterior view. The anatomy of the premaxilla, prefrontal and vomer, which is unknown for *Terrestrisuchus gracilis*, was reconstructed based on the articular surfaces on surrounding bones and the morphology in closely related taxa such as *Saltoposuchus connectens*, *Litargosuchus leptorhynchus* and *Sphenosuchus acutus*.</figcaption></figure>

*Parietal*. The parietals are unfused in all available specimens of *Terrestrisuchus gracilis* (Fig. 18), as in *Saltoposuchus connectens*, *Dromicosuchus grallator* and CM 29894 (Clark *et al.* 2000; Sues *et al.* 2003; Spiekman 2023). In

contrast, the parietals of *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani*, *Almadasuchus figarii* and *Phyllodontosuchus lufengensis* (Walker 1990; Wu & Chatterjee 1993; Harris *et al.* 2000; Leardi *et al.* 2020;
<figure><img src="image_18.png" /><figcaption>**FIG. 14.** Selected maxillae of *Terrestrisuchus gracilis*. A–B, left maxilla of NHMUK PV R 7557c in: A, lateral; B, ventral view. C–D, left maxilla of NHMUK PV R 7572b in: C, lateral; D, ventral view. E, left maxilla of NHMUK PV R 38290a in medial view. *Abbreviations*: al, alveolus; aofo, antorbital fenestra; aofo, antorbital fossa; ap, ascending process; ch, choana; gr, groove; mng, maxillary nerve groove; nf, nutrient foramen; palp, palatal process; pit, pit; pop, posterior process. Scale bars represent 10 mm.</figcaption></figure>

Ruebenstahl *et al.* 2022) are fused without a discernible suture. In BP/1/5237, the holotype and only known specimen of *Litargosuchus lektorhynchus*, the parietals are fused near their posterior ends, but a clear suture is discernible along the rest of their contact (Clark & Sues 2002). The medial margin is not complete in any of the parietals known for *Terrestrisuchus gracilis*, but the articular margin between both parietals was probably straight and simple, as in *Dromicosuchus grallator* (Sues *et al.* 2003). The medial portion of the parietal is dorsoventrally tall relative to rest of the parietal (Fig. 18D). The lateral margin of the parietal is continuously curved and concave in dorsal view (Fig. 18A), and it forms the medial margin of the supratemporal fenestra. The dorsal surface of the

parietal is mostly smooth, but it bears a single, curved longitudinal lamina that is oriented roughly parallel to the lateral margin of the bone. This poorly pronounced lamina forms the lateral demarcation of a weakly developed supratemporal fossa of the parietal. The anterior extent of this lamina cannot be discerned, but it continues posteriorly until the distal end of the posterolateral process of the parietal. The presence of this lamina precludes the presence of a distinct sagittal crest positioned along the medial margins of the parietal, given that this crest represents the medial demarcation of the supratemporal fossa in taxa that possess this feature. Among non-crocodyliform crocodylomorphs, a sagittal crest is present in *Pseudhesperosuchus jachaleri*, *Sphenosuchus acutus*,
Dibothrosuchus elaphros, Junggarsuchus sloani, Phyllodontosuchus lufengensis and Almadasuchus figarii (Bonaparte 1972; Walker 1990; Wu & Chatterjee 1993; Harris et al. 2000; Leardi et al. 2020; Ruebenstahl et al. 2022), whereas this crest is very poorly developed or absent in Saltoposuchus connectens, CM 29894, Litargosuchus leptorhynchus, Kayentasuchus walkeri and Dromicosuchus grallator (Clark et al. 2000; Clark & Sues 2002; Sues et al. 2003; Spiekman 2023). In the taxa in which the crest is poorly developed or absent, the supratemporal

fossa is delimited by a lamina similar to that seen in NHMUK PV R 7591. However, given that it occurs along the midline of fused parietals, the formation of the sagittal crest is ontogenetically variable, similar to parietal fusion.

The dorsal surface of the parietal is slightly convex in anterior or posterior view, such that the medial side of the surface faces dorsally and the lateral side, including the supratemporal fossa, faces slightly dorsolaterally (Fig. 18A, C). The anterior margin of the parietal is characterized by an anteromedial process and a shorter anterolateral process, which are separated by a concavity in dorsal view (Fig. 18A). The anteromedial process is directed anteriorly whereas the anterolateral process is directed anterolaterally.

The posterolateral process of the parietal is distinctly posterolaterally oriented, as in Saltoposuchus connectens, Dromicosuchus grallator and CM 29894 (Clark et al. 2000; Sues et al. 2003; Spiekman 2023), but in contrast to the fully laterally directed posterolateral processes of Dibothrosuchus elaphros, Phyllodontosuchus lufengensis, Sphenosuchus acutus, Junggarsuchus sloani, Almadasuchus figarii and crocodyliforms (Colbert & Mook 1952; Walker 1990; Wu & Chatterjee 1993; Harris et al. 2000; Leardi et al. 2020; Ruebenstahl et al. 2022). The posterolateral process of the parietal is strongly reduced in Litargosuchus leptorhynchus but is directed somewhat posterolaterally (Clark & Sues 2002). The posterior surface of the posterolateral process of Terrestrisuchus is distinctly sloped

<figure><img src="image_19.png" /><figcaption>FIG. 15. The left nasal of NHMUK PV R 37787 in ventral view. Abbreviations: alp, anterolateral process; amp, anteromedial process; im, impression of nasal; pop, posterior process. Scale bar represents 10 mm.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>FIG. 16. Selected lacrimals of Terrestrisuchus gracilis. A, left lacrimal of NHMUK PV R 7593a in medial view. B, right lacrimal of NHMUK PV R 37728 in medial view. Abbreviations: aofe, antorbital fenestra; aofo, antorbital fossa; ap, anterior process; dr, dorsal rib; gr, groove; is, ischium; l., left; ld, lacrimal duct; lr, longitudinal ridge; or, orbit; prff, prefrontal facet; vp, ventral process. Scale bars represent 10 mm.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>FIG. 17. Selected frontals of *Terrestrisuchus gracilis*. A, left frontal of NHMUK PV R 37731 in dorsal view. B, digital reconstruction of the posterior end of the right frontal of NHMUK PV R 7591 in dorsal (top), lateral (middle) and ventral (bottom) view. C, left frontal of NHMUK PV R 38290b in ventral view. D, right frontal of NHMUK PV R 37711 in ventral view. *Abbreviations*: bu, bulge; cc, crista cranii; cp, central process; olb, olfactory bulb mould; or, orbit; pof, postorbital facet; prff, prefrontal facet; vdp, ventrally directed process. Scale bars represent: 10 mm (A, C, D); 5 mm (B).</figcaption></figure>

posteroventrally and slightly medially, and it is concave. The posterior surface is dorsoventrally short and separated from the dorsal surface of the parietal by a distinct posterior rim, similar to the condition seen in *Saltoposuchus connectens* and *Dromicosuchus grallator* (Sues *et al.* 2003; Spiekman 2023). In contrast, the postero-lateral process of the parietal is dorsoventrally tall and separated from the dorsal surface of the parietal by a sharp crest in *Pseudhesperosuchus jachaleri*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani* and *Almadasuchus figarii* (Bonaparte 1972; Walker 1990; Wu & Chatterjee 1993; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). The distal tip of the posterolateral process is

incomplete in all specimens and its shape can therefore not be reconstructed in detail. Based on its articulation with surrounding elements and comparisons with other taxa (e.g. *Junggarsuchus sloani* and *Almadasuchus figarii*; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022), the lateral margin of the posterolateral process formed the medial margin of the post-temporal fenestra in posterior view (Figs 12B, 13; see also the description of the squamosal below). A deep and elongate groove is present on the lateral surface of the posterolateral process of the parietal. This represents the articulation facet for the anteriorly curved tip of the medial process of the squamosal (Fig. 18B, C). The groove is most prominent at the
<figure><img src="image_22.png" /><figcaption>FIG. 18. The digitally reconstructed right parietal of NHMUK PV R 7591 in: A, dorsal; B, ventral; C, lateral; D, medial; E, posterior view. *Abbreviations*: alp, anterolateral process; amp, anteromedial process; ll, longitudinal lamina; plp, posterolateral process; por, posterior rim; stfe, supratemporal fenestra; stfo, supratemporal fossa; sqf, squamosal facet. Scale bar represents 5 mm.</figcaption></figure>

proximal base of the posterolateral process, where it is demarcated dorsally by the lateral margin of the posterolateral process and ventrally by a prominent longitudinal crest present on the ventral surface of the posterolateral process. The latter crest is directed laterally and fades posteriorly. The remainder of the ventral surface of the parietal is smooth and flat to slightly concave. There are no discernible articulation facets for the supraoccipital, prootic and laterosphenoid, which would have articulated on the ventral surface and posterior edge of the parietal in life.

*Postorbital*. The postorbital is a triradiate bone with posterior, ventral and dorsomedial processes (Fig. 19). The ventral process is the longest (Table S2), most slender and most complex of the three. The ventral process can be subdivided in a distinct proximal and distal section, which are subequal in length. The proximal section is widest at its origin and tapers distally as a result of the posterodorsal to anteroventral orientation of the posterior margin in lateral view (Fig. 19A, B), whereas the anterior margin is mostly vertically oriented. The proximal section of the ventral process is directed ventrolaterally (Fig. 19A–D). Its lateral surface is mostly flat and confluent with the main portion of the postorbital, being directed laterally, anteriorly and somewhat dorsally (Fig. 19D). The base of the ventral process is not rugose,

in contrast to *Sphenosuchus acutus* (Walker 1990). The medial surface of the ventral process is characterized by a prominent longitudinal crest (Fig. 19B). The surface anterior to this crest forms the posterior margin of orbit. The surface posterior to the crest is wider and concave on the proximal section of the ventral process. This surface forms part of the articulation facet with the squamosal and continues dorsally onto the ventromedial surface of the posterior process (Fig. 11), similar to the condition seen in CM 29894, *Saltoposuchus connectens*, *Litargosuchus leporhynchus* and *Dromicosuchus grallator* (Clark et al. 2000; Clark & Sues 2002; Sues et al. 2003; Spiekman 2023). This overlapping squamosal–postorbital contact differs from that of *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani* and *Almadasuchus figarii*, in which the posterior process of the postorbital and anterior process of the squamosal form a stronger and more complex suture (Walker 1990; Pol et al. 2013; Ruebenstahl et al. 2022). The posterior margin of the proximal section of the ventral process is relatively thin lateromedially (Fig. 19C), whereas its anterior margin is wide and transversely rounded (Fig. 19D). The distal section of the ventral process is deflected ventrally relative to its proximal section, as can be seen in anterior and posterior views (Fig. 19C, D). The distal section of the ventral process mostly maintains its anteroposterior width in lateral view, but it slightly expands and flattens
near its terminus (Fig. 19A). At the terminus, the posteromedial surface is slightly concave to form the articulation facet for the distal end of the ascending process of the jugal (Fig. 19C). Thus, the articulation between the postorbital and jugal is formed only by a small overlapping surface on both bones, and the ascending process of the jugal articulated on the posterior side of the ventral process of the postorbital (Figs 11A, 13A), *contra* Crush (1980), who considered the jugal to articulate on the anterior side of the postorbital. The tip of the ventral process of the postorbital is gently rounded. This configuration is similar to that seen in *Sphenosuchus acutus* and *Litargosuchus leporhynchus* (Walker 1990; Clark & Sues 2002). In *Saltoposuchus connectens* the articular surface between the postorbital and jugal is much more considerable (Spiekman 2023), which might be a preservational artefact due to the strong dorsoventral compression in the only specimen preserving this configuration (SMNS 12591a). However, a similar configuration is also present in some non-crocodyliform crocodylomorphs that are more closely related to crocodyliforms (*Junggarsuchus sloani* and *Dibothrosuchus elaphros*; Wu & Chatterjee 1993; Ruebenstahl *et al.* 2022).

The posterior process of the postorbital is directed posteriorly. It is dorsoventrally flattened and triangular in outline in dorsal view (Fig. 19E). It is the shortest of the three postorbital processes. Its medial margin forms the anterolateral margin of the supratemporal fenestra (Figs 11B, 13B). Anteriorly, this margin merges with the posterior margin of the dorsomedial process to form the roundly curved anterolateral corner of the supratemporal fenestra. The lateral margin is mostly straight and oriented anterolaterally and slightly ventrally. The lateral margin of the posterior process is directed anteroventrally in lateral view (Fig. 19A) and anterolaterally in dorsal view (Fig. 19E), until it transitions into the posterior margin of the ventral process. The external surface of the posterior process faces dorsally but its anterior portion is angled slightly anterolaterally (Fig. 19A, D). This surface is mostly flat, but slightly concave anteriorly. Anteromedially, near the anterolateral corner of the supratemporal fenestra, the margin is curved ventromedially. The ventral surface of the posterior process is also mostly flat but slightly concave anteriorly (Fig. 19B, C, F). This surface faces ventrally and slightly posteromedially. The entire surface of the posterior process dorsally overlapped, and thus articulated with, the anterior end of the squamosal (Figs 11, 13A, B).

The dorsomedial process of the postorbital is directed dorsally, medially and anteriorly. Its dorsal surface is mostly flat and directed predominantly dorsally and slightly anteriorly and laterally (Fig. 19A, D, E). It lacks a concave supratemporal fossa, in contrast to *Sphenosuchus acutus* and CM 29894 (Walker 1990; Clark *et al.* 2000).

The ventral surface is slightly concave (Fig. 19F). The lateral margin of the dorsomedial process is curved and forms the posterodorsal margin of the orbit (Fig. 19E). Ventrally, this margin seamlessly transitions into the anterior margin of the ventral process. The medial margin possesses a knob-like protrusion on its posterior portion, which interlocked on the dorsal side of the lateral margin of the frontal. The medial margin of the dorsomedial process anterior to this protrusion is convex in dorsal view (Fig. 19E). This margin is dorsoventrally thicker and more rounded than the thin lateral margin, and it articulated with a groove on the frontal that is positioned ventromedially to its lateral rim (Fig. 11B; see also the description of the frontal above). The dorsomedial process similarly articulates with a groove on the posterolateral margin of the frontal in CM 29894, *Litargosuchus leporhynchus* and *Saltoposuchus connectens* (Clark *et al.* 2000; Clark & Sues 2002; Spiekman 2023), whereas in *Dibothrosuchus elaphros* and *Junggarsuchus sloani* the dorsomedial process articulates with both the frontal and parietal (Ruebenstahl *et al.* 2022).

**Jugal.** The jugal is a triradiate bone consisting of anterior, posterior and dorsal or ascending processes (Fig. 20). The anterior process is the longest. It is more than twice the length of the posterior process, whereas the ascending process is considerably shorter than the posterior process (Table S2). The posterior and anterior processes were horizontally oriented in lateral view in life (Figs 11A, 13A). This corresponds with the morphology seen in most non-crocodyliform crocodylomorphs, except *Junggarsuchus sloani* in which the anterior process is markedly oriented anteroventrally to posterodorsally (Clark *et al.* 2004; Ruebenstahl *et al.* 2022). Based on the digitally reconstructed skull of NHMUK PV R 7591, the posterior process of the jugal extended posterior to the posterior margin of the infratemporal fenestra (Fig. 11A), as in other non-crocodyliform crocodylomorphs (e.g. Walker 1990; Sues *et al.* 2003).

The anterior process is lateromedially thin (Fig. 20A, B) and virtually straight along its entire length in dorsal view. The dorsal margin of the anterior process, which forms the ventral margin of the orbit, is concave in lateral view on its posterior portion, with the margin curving gradually posterodorsally to form a confluent transition with the anterior margin of the ascending process (Fig. 20A, C). This curvature is the strongest at the posterior end of this margin. The central portion of both the dorsal and ventral margins of the anterior process is straight. Towards the anterior end of the anterior process, the dorsal margin curves anteroventrally and the ventral margin anterodorsally, resulting in a tapered anterior tip of the anterior process (Fig. 20C), which is also present in *Saltoposuchus connectens* and *Sphenosuchus acutus*
<figure><img src="image_23.png" /><figcaption>**FIG. 19.** The digitally reconstructed right postorbital of NHMUK PV R 7591 in: A, lateral; B, medial; C, posterior; D, anterior; E, dorsal; F, ventral view. *Abbreviations:* dmp, dorsomedial process; itf, infratemporal fenestra; jf, jugal facet; lc, longitudinal crest; or, orbit; pop, posterior process; sf, squamosal facet; vp, ventral process. Scale bar represents 5 mm.</figcaption></figure>

(Walker 1990; Spiekman 2023). In this regard, the jugal morphology differs from that seen in other non-crocodyliform crocodylomorphs (e.g. CM 29894, *Dibothrosuchus elaphros* and *Junggaruchus sloani*), which have a complex, partially interdigitating articulation with the maxilla (Wu & Chatterjee 1993; Clark et al. 2000; Ruebenstahl et al. 2022). Based on its simple and unexpanded anterior end, the anterior process very probably did not contribute to the margin of the antorbital fenestra, in contrast to *Dibothrosuchus elaphros* (Ruebenstahl et al. 2022).

The lateral surface of the anterior process is flat and laterally facing on its dorsal half, whereas the ventral half of the lateral surface is convex and ventrolaterally facing. The lateral surface is generally smooth, but at least 10 small nutrient foramina are present in NHMUK PV R 7593a (Fig. 20C). It lacks a low longitudinal ridge, a feature that is present in *Dromicosuchus grallator*, CM 29894 and *Junggaruchus sloani* (Clark et al. 2000; Sues

et al. 2003; Ruebenstahl et al. 2022). Medially, the surface of the central portion of the anterior process is flat and faces medially and slightly ventrally. Further posteriorly, an anteroposteriorly oriented, medially directed ridge rises from the medial surface (Fig. 20B). Anteriorly, this ridge is low and narrow, but it becomes gradually taller in height posteriorly. At the base of the anterior process the distal edge of this ridge widens dorsoventrally at its distal extent to form a flat, medially facing surface that is oval in medial view, being longer anteroposteriorly than wide dorsoventrally. This forms the articular surface for the lateral flange of the ectopterygoid (Figs 12A, 20B). Postero-dorsally, the articulation facet thins distinctly to form a narrow ridge that is oriented dorsoventrally at a slight anterodorsal to posteroventral tilt. This ridge continues posterodorsally until it gradually tapers out on the medial surface of the base of the ascending process. A similar ectopterygoid facet is also present in *Sphenosuchus acutus* (Walker 1990). In this latter taxon the facet is more
<figure><img src="image_24.png" /><figcaption>**FIG. 20.** Selected jugals of *Terrestrisuchus gracilis*. Digitally reconstructed right jugal of NHMUK PV R 7591 in: A, lateral; B, medial view. C, right jugal of NHMUK PV R 7593a in lateral view. *Abbreviations*: ap, anterior process; apr, anteroposterior ridge; asp, ascending process; dvr, dorsoventral ridge; ectf, ectopterygoid facet; itf, infratemporal fenestra; nf, nutrient foramen; or, orbit; pit, pit; pp, posterior process; qjf, quadratojugal facet. Scale bars represent 5 mm.</figcaption></figure>

circular in medial view, and there are no anteriorly or posterodorsally directed ridges that project from it. A distinct pit-like concavity is formed directly dorsal to the articulation facet where the orientation of the ridge transitions from anteroposterior to dorsoventral (Fig. 20B). The ventral base is also concave, but to a lesser extent. Ventral to this articulation facet, the medial surface of the anterior process is mediodorsally facing and flat. The medial surface dorsal to the articulation facet is also flat and faces medially.

The posterior process of the jugal gradually tapers posteriorly throughout its length, terminating in a sharp point (Fig. 20). The posterior end of the posterior process is slightly curved posteromedially in dorsal view. The ventral margin of the posterior process is transversely thin and very slightly convex in lateral view (Fig. 20C). The lateral surface of the posterior process is smooth. It is slightly transversely convex and faces ventrolaterally. A single nutrient foramen is preserved on the anterodorsal edge of the lateral surface in NHMUK PV R 7593a (Fig. 20C). Anteriorly, the dorsal margin of the posterior process is lateromedially wide, forming a dorsally facing platform. Posteriorly, this platform narrows and gradually

faces dorsomedially and finally medially on the distal half of the process. The dorsal margin of the posterior process forms the ventral margin of the infratemporal fenestra (Figs 11A, 13A). This margin is slightly concave in its posterior half in lateral view, but it becomes more distinctly concave anteriorly, where this margin curves anterodorsally to transition into the posterior margin of the ascending process of the jugal (Fig. 20A, C). The medial surface of the posterior process is slightly concave anteriorly, and becomes more conspicuously concave posteriorly ventral to the medially oriented platform present on the dorsal portion of the process (Fig. 20B). The posterior process of the jugal articulated with the quadratojugal along virtually its entire length, with only its posteriormost end connecting to the quadrate (Fig. 11A). This articulation with the quadrate was formed by a simple overlapping suture, as is also the case in *Sphenosuchus acutus* and *Junggarsuchus sloani* (Walker 1990; Ruebensstahl *et al.* 2022).

The ascending process of the jugal is directed mostly dorsally and slightly posterodorsally in lateral view (Fig. 20), similar to the condition in *Carnufex carolinensis* (Drymala & Zanno 2016) but contrasting with the
completely dorsally directed ascending processes in
the jugals of *Saltoposuchus connectens*, *Litargosuchus leptorhynchus* and *Sphenosuchus acutus* (Walker 1990; Clark & Sues 2002; Spiekman 2023). The ascending process is considerably more slender than both the anterior and posterior processes, as in *Litargosuchus leptorhynchus* and *Dromicosuchus grallator* (Clark & Sues 2002; Sues et al. 2003). The process becomes narrower very gradually in lateral view and its cross-section at mid-height is oval, being slightly longer anteroposteriorly than wide lateromedially. The lateral surface of the ascending process is smooth and its posterior edge is not roughened (Fig. 20A, C), in contrast to the condition in *Sphenosuchus acutus* (Walker 1990). In lateral view, the posterior and anterior margins are completely straight for most of their lengths, whereas in several other non-crocodyliform crocodylomorph taxa the posterior margin of the ascending process is distinctly convex (e.g. *Sphenosuchus acutus* and *Junggarsuchus sloani*; Walker 1990; Ruebenstahl et al. 2022). The anterior margin of the ascending process forms the posterior margin of the orbit, whereas the posterior margin forms the anterior margin of the infratemporal fenestra (Figs 11A, 13A). At the base of the process these margins are both concave as the ascending process widens ventrally and the posterior margin curves posterоventrally to transition into the dorsal margin of the posterior process, and the anterior margin curves anteroventrally to transition into the dorsal margin of the anterior process. At its dorsal end the ascending process possesses a kink (Fig. 20A, B) and as a result, the distal tip of the process is directed completely dorsally. This projection forms the short articulation facet of the jugal with the ventral process of the postorbital (Fig. 11A; see also the description of the *postorbital* above). The projection terminates in a rounded tip.

*Squamosal*. The squamosal is a wide, plate-like bone (Fig. 21). Its outer surface is convex and faced predominantly dorsolaterally. The medial margin of the squamosal, which forms the lateral edge of the supratemporal fenestra (Figs 11B, 13B), is a thickened rim that faces medioventrally due to a distinct medioventral curvature of the squamosal surface directly lateral to this rim at an angle of more than 90° relative to the main dorsal surface (Fig. 21F). It is directed slightly anterolaterally to posteromedially in dorsal view (Fig. 11B). The medial margin is slightly concave in dorsal view (Fig. 21A). The plate-like dorsolateral surface of the squamosal is widest near the posteriormost extent of the supratemporal fenestra. From here, the lateral margin of the squamosal curves dorsomedially in both directions, with this curvature being most prominent posteriorly. Consequently, the lateral margin is clearly convex in both lateral and dorsal views (Fig. 21A, E). There is no longitudinally oriented dorsolateral crest

on the outer surface of the squamosal, in contrast to the
condition in all other known non-crocodyliform crocody-
lomorphs except *Litargosuchus leptorhynchus* (Clark &
Sues 2002; Spiekman 2023), nor is there an ear flap
groove as has been described for *Kayentasuchus walkeri*
and protosuchids (Busbey & Gow 1984; Gow 2000; Clark
& Sues 2002). A transversely very wide squamosal of
which the external surface faces predominantly dorsally is
also present in *Saltoposuchus connectens* and *Litargosuchus
leptorhynchus* (Clark & Sues 2002; Spiekman 2023), as
well as protosuchids (Gow 2000). In contrast, the squa-
mosal is narrower in *Sphenosuchus acutus*, CM 29894,
*Pseudhesperosuchus jachaleri*, *Kayentasuchus walkeri* and
*Dibothrosuchus elaphros* (Bonaparte 1972; Walker 1990;
Wu & Chatterjee 1993; Clark *et al.* 2000; Clark &
Sues 2002). In *Junggarsuchus sloani* and *Almadasuchus fig-
arii* the squamosal is also quite wide but its outer surface
faces distinctly dorsolaterally rather than predominantly
dorsally (Leardi *et al.* 2020; Ruebenstahl *et al.* 2022).
The squamosal narrows anteriorly in dorsal view and its an-
terior end is widely rounded. The anterior end is over-
lapped by the posterior process and main body of the
postorbital, as well as the proximal portion of its ventral
process (Fig. 11B; see also the description of the *postor-
bital* above). The anterior margin of the squamosal is
curved strongly ventromedially (Fig. 21B, E) and its an-
terior face formed a tight articulation with the posterior
surface of the longitudinal crest present on the medial
surface of the ventral process of the postorbital.

The posteromedial end of the squamosal is characterized by a distinct anteromedially curved, hook-shaped process (Fig. 21A), which is widely present among crocodylomorphs (e.g. *Saltoposuchus connectens* and *Junggarsuchus sloani*; Ruebenstahl et al. 2022; Spiekman 2023). This process articulated with the distinct groove present on the ventrolateral surface of the posterolateral process of the parietal and, as such, its medial margin was covered dorsally by the parietal, as in the articulated skull BP/1/5237 of *Litargosuchus leptorhynchus* (Clark & Sues 2002). Nevertheless, the majority of this process is exposed on the dorsal skull table (Fig. 11B). In more derived crocodylomorphs with a deeper posterolateral process of the parietal (e.g. *Dibothrosuchus elaphros*, *Almadasuchus figarii*), this portion of the squamosal was placed deeply within the supratemporal fenestra (Wu & Chatterjee 1993; Leardi et al. 2020). The posterior surface of the squamosal is complex and separated from the dorsal surface by a weakly developed rim. A similarly poorly developed rim is present in *Litargosuchus leptorhynchus* (Clark & Sues 2002), whereas this rim forms a distinct ridge in many other non-crocodyliform crocodylomorphs (e.g. *Saltoposuchus connectens*, *Sphenosuchus acutus*; Walker 1990; Spiekman 2023). This rim originates at the anterior end of the base of the hook-shaped
<figure><img src="image_25.png" /><figcaption>**FIG. 21.** Digitally reconstructed squamosals of NHMUK PV R 7591. A–C, right squamosal in: A, dorsal; B, ventral; C, posterior view. D, partial left squamosal in posterior view. E–F, right squamosal in: E, lateral; F, medial view. *Abbreviations:* cx, convexity; otf, otoccipital flange; pit, pit; pmp, posteromedial process; pof, postorbital facet; popf, paroccipital process facet; por, posterior rim; ptf, post-temporal fenestra; ptfl, post-temporal flange; qf, quadrate facet; stfe, supratemporal fenestra. Scale bar represents 5 mm.</figcaption></figure>

posteromedial process in *Terrestrisuchus gracilis* and is
directed posterolaterally from there. It is curved, with its
convex side facing posteriorly in lateral view (Fig. 21E).
The rim is less pronounced laterally, but it extends to the
lateral end of the squamosal. The posterior surface of the
squamosal is generally convex, particularly in its dorsal
portion (Fig. 21A). However, a clear and pronounced
concavity is present on the ventrolateral portion of the
posterior surface. This represents the articular surface for
the paroccipital process of the otoccipital, which articu-
lated with this facet on the anterior surface of its distal
end (Fig. 21D). This facet extends ventrally onto a flat-
tened platform, which is present on a ventromedially
directed process named the otoccipital flange (*sensu*
Spiekman 2023). This flange has a straight medial margin

and a convex lateral margin, and it terminates in a point
ventrally in posterior view (Fig. 21D). The flattened plat-
form that forms the articulation facet for the ventral por-
tion of the anterior surface of the paroccipital process
faces predominantly posteriorly, but also slightly medially
and dorsally. At its distal end the flange is very thin
anteroposteriorly but it gradually thickens proximally,
and it is confluent with the ventrolateral surface of the
squamosal through a curved lamina. In more derived cro-
codylomorphs (e.g. *Almadasuchus figarii*) the otoccipital
flange reaches further ventrally, where it connects with
the quadrate to form a fully enclosed otic aperture that
opens on the lateral side of the skull and which is bor-
dered anteriorly by the quadrate and posteriorly by the
squamosal (Leardi *et al.* 2020). In *Terrestrisuchus gracilis*
the otic aperture is clearly not fully enclosed but open posteriorly (Figs 11A, 12B), as is also the case in *Sphenosuchus acutus* (Walker 1990). Another process, the post-temporal flange (*sensu* Spiekman 2023), extends from the posterior surface of the squamosal, medial to the otocipital flange (Fig. 21A–C, F). It extends from the ventral surface of the squamosal and is directed mainly ventromedially at its base and slightly posteriorly. The process curves strongly medially and tapers along its entire length into a pointed terminal tip that is directed medially and slightly dorsally in posterior view and, as such, the process has a similar hook-shaped outline as the posteromedial process (Fig. 21C, D). The ventral margin of this process is distinctly convex in posterior view whereas the medial margin is strongly concave. The ventral margin of the post-temporal flange posteriorly overlapped the dorsal portion of the base of the paroccipital process. The lateral and dorsal margins formed the lateral and ventral border of the post-temporal fenestra (Figs 12B, 13D). This configuration most strikingly resembles that of *Litargosuchus leptorhynchus* and *Saltoposuchus connectens* (Spiekman 2023; SNFS pers. obs., BP/I/5237), but a similar condition is also present in other crocodylomorphs. For instance, a similar distinct, hook-shaped post-temporal flange of the squamosal is present in *Kayentasuchus walkeri* and *Junggarsuchus sloani* (Clark & Sues 2002; Ruebenstahl et al. 2022). A distinct pit of unknown function is preserved directly dorsal to the base of the flange (Fig. 21F). A shallow groove is directed mediodorsally from this pit on the ventromedial margin of the squamosal, which fades near the base of the hook-shaped posteromedial process. The medial margin of the post-temporal flange curves dorsomedially to form the dorsomedial extension of the occipital contribution of the squamosal. Its medial portion would have articulated with the distal end of the posterolateral process of the parietal (Fig. 12B).

The ventral surface of the squamosal is generally concave and smooth (Fig. 21B). It bears the articulation facet for the dorsal head of the quadrate, which is formed by a distinct excavation on the posteromedial part of the ventral surface. The posterior end of this excavation is placed directly anterior to the posteroventral process, where it is most deeply excavated, and it terminates anteriorly directly lateral to the base of the posteromedial process. The dorsal head of the quadrate was thus placed far medially in the skull, directly adjacent to the braincase (Fig. 21B, F). It is oval in outline in ventral view, with its

long axis being oriented anteroposteriorly and slightly lateromedially, as in *Saltoposuchus connectens* (Spiekman 2023). Finally, a slight elevated mount-like convexity is present on the posterior end of the ventral surface, in between the posteroventral process and the otoccipital flange (Fig. 21F).

*Quadrate*. The quadrate is a pillar-like bone (Fig. 22A–F). The posterior margin of its shaft is continuously and strongly concave in lateral view (Fig. 22A). This margin is relatively thin lateromedially and slightly directed posteromedially, as can be observed in posterior view (Fig. 22E). The main surfaces of the shaft face posterolaterally and anteromedially, respectively. The posterolateral surface is smooth and slightly concave. It is characterized by a very prominent longitudinal excavation on its lateral portion, which represents the internal quadrate fenestra (Fig. 22A, C, E, G), which is pneumatic in nature (Leardi *et al.* 2020). Among non-crocodyliform crocodylomorphs, a similar-shaped internal quadrate fenestra is present in *Saltoposuchus connectens* and *Dromicosuchus grallator* (Sues *et al.* 2003; Spiekman 2023), whereas internal quadrate fenestrae of differing morphologies occur in *Sphenosuchus acutus*, *Macelognathus vagans*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani* and *Almadasuchus figarii* (Walker 1990; Leardi *et al.* 2017; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). In *Terrestrisuchus gracilis* this fenestra nearly reaches the ventral condyle ventrally and the level of the proximodorsal margin of the pterygoid ramus in lateral view dorsally (Fig. 22A). The fenestra is deepest dorsolaterally, where it reaches anteriorly to form the excavated base of the pterygoid ramus. It is demarcated laterally by a distinct rim on the lateral edge of the shaft. The quadratojugal did not form part of the lateral margin of the quadrate fenestra (*contra* Leardi *et al.* 2017; Leardi *et al.* 2020). In derived non-crocodyliform crocodylomorphs (hallopodids *sensu* Leardi *et al.* 2017, *Dibothrosuchus elaphros* and *Junggarsuchus sloani*), the pneumatic quadrate fenestra is directly connected to internal diverticula of the quadrate, also known as the infundibular diverticulum (Dufeu & Witmer 2015; Leardi *et al.* 2017; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). Such an internal connection is absent in *Terrestrisuchus gracilis*, as in *Saltoposuchus connectens*, *Sphenosuchus acutus* and *Hesperosuchus agilis* (Leardi *et al.* 2020; Ruebenstahl *et al.* 2022; Spiekman 2023). Based on the µCT data, internal diverticula are absent in the quadrate body of

**FIG. 22.** Selected quadrates of *Terrestrisuchus gracilis*. A–F, digitally reconstructed and reassembled right quadrate of NHMUK PV R 7591 in: A, lateral; B, medial; C, dorsal; D, ventral; E, posterior; F, anterior view. G, right quadrate, exposed in lateral view, and associated elements of NHMUK PV R 7593b. H–I, right quadrate of NHMUK PV R 37727 in: H, medial; I, posterior view. *Abbreviations*: adf, anterodorsal flange; cr, crest; dh, dorsal head; iqf, internal quadrate fenestra; mac, mandibular condyle; prof, prootic facet; ptp, pterygoid process (*sensu* Walker 1990); ptra, pterygoid ramus; tc, tympanic crest. Scale bars represent 5 mm.
<figure><img src="image_26.png" /></figure>
Terrestrisuchus gracilis. The anterior margin of the lateral rim of the quadrate articulated with the posterior margin of the quadratojugal for most of its length, whereas its posterior margin probably formed the anterior insertion site for the tympanic membrane, as was also indicated by Crush (1980). The lateral rim is mostly straight in lateral view, but it curves slightly anterodorsally dorsal to the internal quadrate fenestra (Fig. 22A). This anterodorsal projection forms the anterior margin of a very thin anterodorsal flange of the quadrate (= anterodorsal process of Walker 1990; Fig. 22A–C, G, H). The anterior margin of this flange is slightly concave in lateral view (Fig. 22A). Its dorsal margin is virtually straight and is posteriorly confluent with the dorsal head of the quadrate. The dorsal margin of the anterodorsal flange aligns directly with the portion of the medial margin of the squamosal that contributes to the border of the supratemporal fenestra, and it articulated with this margin in life (Fig. 11B; note that due to the displacement of the anterodorsal flange in NHMUK PV R 7591 it is directed more medially than it would have been in life, and it thus extends into the supratemporal fenestra). This dorsal margin is slightly thickened in the right quadrate of NHMUK PV R 7593b (Fig. 22G), but such a thickening is absent in the restored right quadrate of NHMUK PV R 7591 (Fig. 22C). This thickening was erroneously identified as a tympanic crest and the anterodorsal margin of the tympanic membrane by Crush (1980). The anterodorsal flange is oriented anteriorly and slightly laterally and ventrally, and its anterior margin articulated with the quadratojugal, as in *Sphenosuchus acutus* (Walker 1990).

Medial to the lateral rim, the pterygoid ramus of the quadrate projects anteriorly, medially and slightly ventrally (Fig. 22B, D, F). The ramus curves gently ventrally along its length, and its ventral margin is distinctly concave, whereas its dorsal margin is slightly convex. The pterygoid ramus formed an overlapping lateral contact with the quadrate flange of the pterygoid. It is unclear whether the pterygoid ramus is complete in the right quadrate of NHMUK PV R 7591, but its preserved morphology is similar to that of *Sphenosuchus acutus* (Walker 1990). As in *Sphenosuchus acutus*, it preserves a small anteroventrally projecting process on the ventral portion of the anterior end of the ramus, which is separated from the remainder of the ramus by a small notch (the ‘pterygoid process’ sensu Walker 1990; Fig. 22A, B). A split distal end of the pterygoid ramus is also present in *Saltoposuchus connectens*, *Macelognathus vagans* and *Almadasuchus figarii* (Leardi et al. 2017; Leardi et al. 2020; Spiekman 2023). The remainder of the pterygoid ramus terminates in a blunt tip. Three crests connect to the pterygoid ramus: one dorsally and two ventrally (Fig. 22B, F). The dorsal crest is thin and straight in medial view (Fig. 22B). Like the anterodorsal process of

the quadrate anterolateral to it, this crest is displaced medially in the digitally reconstructed right quadrate of NHMUK PV R 7591, and it would have faced anteromedially in roughly the same plane as the pterygoid ramus in life. The dorsal crest is oriented anteroventral to posterodorsal. It maintains its height in its ventral half but slowly reduces in height in its dorsal half. It terminates close to the dorsal margin of the quadrate. The two ventral crests are best observed in anterior view and diverge from the pterygoid ramus ventromedially and ventrolaterally, respectively (Fig. 22F). A clear concavity is formed between them on the anterior surface of the quadrate, with a distinct pit being present on the ventral base of the pterygoid ramus (Fig. 22D, F). The ventromedial crest is most pronounced and forms a continuation of the ventral margin of the pterygoid ramus. It is thin and straight in anterior view, and the crest faces anteromedially in approximately the same plane as the pterygoid ramus (Fig. 22F). The crest is concave in medial view and terminates on the anteromedial side of the ventral condyle of the quadrate (Fig. 22B). The ventrolateral crest is not as well developed, and it fades away into the anterior surface of the quadrate ventrally. It is straight in anterior view and faces anteromedially (Fig. 22F).

A connection between the quadrate and the braincase is present in all crocodyliforms. Among non-crocodyliform crocodylomorphs this connection has been observed in *Sphenosuchus acutus*, *Kayentasuchus walkeri*, *Dibothrosuchus elaphros*, *Macelognathus vagans*, *Almadasuchus figarii* and *Junggarsuchus sloani* (Walker 1990; Clark & Sues 2002; Leardi et al. 2017; Leardi et al. 2020; Ruebenstahl et al. 2022). The absence of this connection has not been established confidently in any crocodylomorph to date, indicating that this feature might be synapomorphic to Crocodylomorpha, as was also previously suggested (Gower 2002). In *Terrestrisuchus gracilis* the dorsal head of the quadrate is positioned on the posterodorsal end of the quadrate (Fig. 22). It articulated with a facet on the posteromedial part of the ventral surface of the squamosal (see description of the squamosal above), and additionally articulated on its medioventral side to the portion of the prootic directly dorsal to its crista prootic (see description of the prootic below), thus establishing a connection to the braincase (contra Crush 1980, 1984). Despite the disarticulation of the braincase and associated elements of NHMUK PV R 7591, the connection between the quadrate and prootic can be unambiguously established based on the dorsomedially directed dorsal head of the quadrate, the alignment of the dorsal margin of the quadrate and the crista prootica of the prootic, the positioning of the quadrate relative to the other elements framing the temporal canal (Fig. 23), the additional articulation between the quadrate and pterygoid anteriorly and the quadrate and articular ventrally, and based on close
comparison with *Litargosuchus leptorhynchus* and *Sphenosuchus acutus* (Walker 1990; SNFS pers. obs., BP/I/5237). The articulation facet with the squamosal is predominantly oriented dorsally, but also medially and posteriorly (Fig. 22C). This facet is convex and it is trapezoidal in outline in dorsal view, with a lateromedially short anterior margin and a considerably wider posterior margin. Both these margins are oriented anteromedially to posterolaterally. The medial part of the facet is formed by a relatively thin medial expansion from the main body of the quadrate (Fig. 22B, C). It is unclear whether the thin, posteriorly directed, posterolateral expansion of the facet in the right quadrate of NHMUK PV R 7591 represents the original morphology, or whether it is an artefact of the digital reconstruction or diagenetic distortion. The dorsal head formed the lateral margin of the temporal canal (Fig. 23).

The main axis of the ventral end of the quadrate is oriented posteromedially to anterolaterally (Fig. 22D). In posterior view the medial half of the ventral margin of the quadrate is straight (Fig. 22E), whereas the lateral half curves anteriorly and dorsally. The latter is thus strongly convex in posterior and ventral views (Fig. 22D, E). The posterior surface directly dorsal to the ventral margin of the quadrate is smooth and gently concave (Fig. 22A). In ventral view, the medial end of the

mandibular condyle is expanded anteriorly into a point, which is formed by the ventral termination of the ventromedial crest that diverges from the pterygoid ramus (Fig. 22D). This forms a triangular surface that corresponds in shape to the glenoid fossa on the articular, thus forming the facet of the quadrate with the mandible (Fig. 11A). The ventral surface of the quadrate is slightly convex.

**Quadratojugal.** The quadratojugal is very thin and plate-like in lateral view (Fig. 24), rather than L shaped as reconstructed by Crush (1980, 1984). Its lateral surface is distinctly concave and its medial surface correspondingly convex. Although diagenetic deformation cannot be fully excluded, it most probably reflects the original morphology of the bone. This configuration is inverted along the ventral margin of the bone, which has a convex lateral surface and concave medial surface. This ventral margin articulated along the medial surface of the posterior process of the jugal and is obscured by it in lateral view (Figs 11A, 13A). This articulation forms along the entire anteroposterior length of the quadratojugal, and reached from near the posterior tip of the posterior process of the jugal until the onset of the posterior margin of its ascending process. The ventral margin of the quadratojugal is straight to very slightly concave in lateral and ventral views; it is slightly concave on its lateral surface and convex on its medial surface.

The posterior margin of the quadratojugal is oriented mostly dorsoventrally and slightly posteroinventrally to anterodorsally. In lateral view the posterior margin is concave, and in posterior view the margin is somewhat concave laterally and convex medially, although it is unclear whether this is due to diagenetic distortion. The posterior margin articulated on the anterior side of the lateral end of the quadrate, including the anterodorsal flange dorsally, forming an abutting contact (Figs 11A, 13A). The anterior margin is largely incomplete and therefore not much can be said about its morphology. At least in its ventral half the anterior margin was thin and mostly straight in lateral view, being oriented predominantly dorsoventrally. There is a concavity near the anterior margin at about mid-height of the bone that is probably the result of diagenetic distortion. The dorsal portion of the quadratojugal is the most poorly preserved region of the bone. It presumably formed a simple abutting contact with the squamosal. It is unclear whether the quadratojugal also reached the postorbital anteriorly.

**Sclerotic ring.** The remains of a largely complete sclerotic ring are preserved inside the right orbit of NHMUK PV R 7591 (Fig. 10A). A sclerotic ring is distributed widely among reptiles but it is absent in crown crocodylians

<figure><img src="image_27.png" /><figcaption>**FIG. 23.** Close-up of the temporal canal in the digitally reassembled skull model of NHMUK PV R 7591 in an internal, oblique anterior view. Note the contact between the dorsal head of the quadrate and the crista prootica of the prootic. The anterodorsal flange of the quadrate is directed anteromedially inside the supratemporal fenestra due to the deformation of this part of the quadrate. It would have articulated along the medial edge of the squamosal in life, as explained in the text. *Abbreviations:* adf, anterodorsal flange; crpr, crista prootica; dcr, dorsal crest; dh, dorsal head; pa, parietal; pr, prootic; pr-q c, prootic-quadrate contact; q, quadrate; sq, squamosal; tpc, temporal canal.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**FIG. 24.** Digitally reconstructed right quadratojugal of NHMUK PV R 7591 in: A, lateral; B, medial view. *Abbreviations:* jf, jugal facet; qf, quadrate facet; sqf, squamosal facet. Scale bar represents 5 mm.</figcaption></figure>

(Atkins & Franz-Odendaal 2016). Among early crocodylomorphs, limited remains of scleral ossicles are also present in *Junggarsuchus sloani*, but they are otherwise not known from any non-marine crocodylomorph (Nesbitt et al. 2013) despite the presence of several excellent and virtually completely preserved skulls of this group. Among aquatic crocodylomorphs, sclerotic rings are known only for deep-diving pelagic metriorhynchoid thalatosuchians (Young et al. 2010). The sclerotic ring of NHMUK PV R 7591 is poorly preserved and it is not possible to identify the individual scleral plates, either through external observation or in the µCT data (Figs 3A, 10A). Nevertheless, because this ring has a similar texture and a similar density to the cranial bones in the µCT data, it is clear that it represents the poorly preserved remains of the scleral ossicles rather than the fossilized remains of the scleral cartilage (Atkins & Franz-Odendaal 2016). The sclerotic ring of NHMUK PV R 7591 was not identified by Crush (1980, 1984), but it was separately observed and discussed in brief by Allen (2010) and Nesbitt et al. (2013).

## Palate

**Palatine.** The palatine is a dorsoventrally flat bone with anteriorly and posteriorly directed rami (Fig. 25). The posterior ramus is almost twice as long as the anterior ramus (Table S2). In dorsal view the medial margin of the anterior ramus is convex and its lateral margin very slightly concave, whereas for the posterior ramus the medial margin is slightly concave and the lateral margin is sinusoidal, being concave at the base and convex towards the terminus of the process. The entire medial margin of the palatine is dorsoventrally very thin. The anterior ramus articulated with the vomer, whereas the pterygoid articulated with the palatine along its medial margin and with its posterior ramus (Crush 1984; Fig. 13C). The lateral margin of the palatine is dorsoventrally thickened, mostly straight, and it articulated with the maxilla. It extended anteriorly and posteriorly to form the curved posterolateral margin of the choana, and the anterolateral margin of the suborbital fenestra, respectively (Figs 13C, 25B).
<figure><img src="image_29.png" /><figcaption>**FIG. 25.** Selected palatines of *Terrestrisuchus gracilis*. A, left palatine of NHMUK PV R 7593a in dorsal view. B, right palatine of NHMUK PV R 38291 in ventral view. *Abbreviations*: ara, anterior ramus; ch, choana; chr, choanal ridge; f, fossa; lm, lateral margin; pora, posterior ramus; r., right; ri, ridge; sof, suborbital fenestra; sq, squamosal. Both scale bars represent 10 mm.</figcaption></figure>

Two small fossae are positioned close to each other on the ventral surface of the palatine. One is subcircular in outline and located directly posterior to the posteriormost margin of the choana (Fig. 25B, f1). The other is located anteromedial to the first fossa, and it extends anteriorly along the posteromedial margin of the choana (Fig. 25B, f2). A very low, step-like ridge is directed anterolaterally to posteromedially across the ventral surface of the palatine and, as a result, the portion of the palatine medial to this ridge is displaced ventrally very slightly relative to the lateral portion (Fig. 25B; Crush 1980). Another, poorly developed ridge forms the anterior margin of the first fossa (f1). This represents the choanal ridge (*sensu* Dollman & Choiniere 2022) or ‘spout-like’ ridge (*sensu* Walker 1990). The choanal ridge is well developed in *Sphenosuchus acutus*, *Kayentasuchus walkeri*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Walker 1990; Wu & Chatterjee 1993; Nesbitt 2011; Dollman & Choiniere 2022; Ruebenstahl *et al.* 2022). *Terrestrisuchus gracilis* lacks the clear striations present on the medial side of the ventral surface of the palatine in *Saltoposuchus connectens* (Spiekman 2023).

The dorsal surface of the palatine is mostly flat. A small but deep fossa is present on the anterolateral corner of the palatine (Fig. 25A). It is separated from the lateral margin of the palatine and the anterolateral margin of the anterior ramus by a small ridge. This fossa forms the attachment site for part of the *M. pterygoideus* (Witmer 1997) and its morphology deviates strongly from that of *Sphenosuchus acutus*, in which this fossa is much more distinct and positioned further posteriorly, framing the anterior border of the suborbital fenestra (Nesbitt 2011, ch. 90; Walker 1990, fig. 3).

*Ectopterygoid*. The ectopterygoid is a curved element with two bifurcating ends (Fig. 26). The lateral end articulated with the jugal, whereas the medial end articulated with the transverse flange of the pterygoid (Figs 12A, 13C, 26H). The lateral end possesses two processes that are set at an angle of 180° relative to each other, with one process projecting anteriorly and somewhat ventrally, and the other posteriorly and somewhat dorsally (Fig. 26E). Both processes are thin and terminate in a sharp tip. The anteroventral process is longer and more slender than the posterodorsal process. The anteroventral process curves
<figure><img src="image_30.png" /></figure>
**FIG. 26.** Digitally reconstructed right ectopterygoid and surrounding elements of NHMUK PV R 7591. A–F, right ectopterygoid in: A, ventral; B, dorsal; C, anterior; D, posterior; E, lateral; F, medial view. G, hypothetical articulation between right ectopterygoid and jugal in oblique ventromedial view. H, hypothetical articulation between right ectopterygoid and pterygoid in oblique anteroventral view. *Abbreviations:* adp, anterodorsal process; avp, anteroventral process; cr, crest; ect, ectopterygoid; j, jugal; le, lateral end; mee, medial end; pdp, posterodorsal process; pt, pterygoid; pvp, posteroventral process; sh, shaft; tp, triangular process. Scale bar represents 5 mm.

anterodorsally and thus possesses a convex ventral margin and concave dorsal margin in lateral view. In contrast, the posterodorsal process is mostly straight but it is curved slightly posteroventrally at its distal tip. The entire lateral surface of the anterolateral end, including both processes, is convex and articulated with the longitudinal crest on the medial surface of the jugal (see description of jugal above). The exact articulation with the jugal is difficult to ascertain due to the disarticulation of the ectopterygoid from the jugal (Fig. 10B) but it probably formed along the dorsal margin of the medial ridge of the jugal, as was also described for *Sphenosuchus acutus* (Fig. 26G; Walker 1990). The dorsal side of the posterodorsal process possesses a small, triangular process in lateral view (Fig. 26E). Medially, this process extends along the shaft of the ectopterygoid as a crest on its dorsal surface, and a shallow groove is present along and directly posterior to the crest (Fig. 26B). The crest and groove fade away at the base of the bifurcating posteroventral end of the ectopterygoid.

The shaft of the ectopterygoid projects anteromedially and slightly dorsally from the lateral process before curving posteroventrally at about mid-length (Fig. 26A–F). The posterior margin of the shaft projects medially and posteroinventrally and posteriorly widens transversely. The shaft has an oval cross-section with the long axis angled anterodorsally to posteroinventrally. The bifurcating medial end of the ectopterygoid is wider than the lateral end and it is oriented anterodorsally to posteroinventrally (Fig. 26F), thus aligning with the lateral end of the transverse flange of the pterygoid. Its posterodorsal surface is convex in medial view whereas its anteroventral surface is distinctly concave. Both processes of the medial end are medially directed and the posteroventral process is longer, lateromedially wider and dorsoventrally thicker than the anterodorsal process (Fig. 26A, D). The anterodorsal process represents a flange that is very thin dorsoventrally (Fig. 26F). Its distal end is incomplete. It is directed medioventrally and slightly posteriorly. The posteroventral and anterodorsal processes of the medial end are separated from each other by a distinct, rounded, concave gap in ventral view (Fig. 26A). The posteroventral process curves medially in ventral view towards its distal end and its anterior margin is slightly concave, whereas its posterior margin is distinctly convex. In its proximal portion

the posteroventral process has the same dorsoventral height as the shaft directly proximal to it, but towards its distal end it somewhat increases in height (Fig. 26E, F). The posteroventral process tapers along its length in ventral view and terminates in a thickened, blunt end (Fig. 26A). The large and thickened posteroventral process of the medial end of the ectopterygoid very probably articulated with the ventral side of the pterygoid near its posteroventral end (Fig. 26H), similar to the condition seen in *Dibothrosuchus elaphros* (Ruebenstahl et al. 2022). The anterodorsal process of the medial end of the ectopterygoid probably articulated on the lateral side of the transverse flange of the pterygoid, near its anterodorsal end (Fig. 26H).

*Pterygoid.* The pterygoid is composed of three main processes: the anteriorly directed palatal ramus, the lateral directed transverse flange, and the mostly dorsally directed quadrate flange (Fig. 27). Of these three, the palatal ramus is the largest. It is narrow and tapers anteriorly (Fig. 27A, B). The medial margin of the palatal ramus is straight and oriented anteroposteriorly, whereas the lateral margin is straight to very slightly convex in ventral view and oriented mainly anteroposteriorly and slightly posterolaterally to anteromedially along its entire length. The palatal ramus terminates in a sharp point. A small, pointed process projects anterolaterally from the lateral margin of the palatal ramus, close to its base (Fig. 27A). A small notch is formed anterior to this process, into which the posterior end of the palatine fitted. Thus, the posterior end of the palatine articulated with the anteromedial margin of the small process and the lateral margin of the palatal ramus directly anterior to this process. This interpretation was previously suggested by Crush (1980, 1984), who referred to this structure as the palatine process. The ventral surface of the palatal ramus is mostly smooth and gently concave. However, a very faint, straight lamina projects posteromedially from the anterior margin of the palatine process along the ventral surface of the pterygoid (Fig. 28). The surface posterolateral to this lamina is slightly deflected dorsolaterally relative to the rest of the ventral surface. The ventral surface of the palatal ramus possesses a clear, thickened, longitudinal rim along its medial margin (Fig. 27A, C, F, H, I), which is also present in CM 29894, *Saltoposuchus*
<figure><img src="image_31.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>E</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>H</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>G</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>F</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>I</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>J</figcaption></figure>

<figure><img src="image_40.png" /></figure>
**FIG. 27.** Selected pterygoids of *Terrestrisuchus gracilis*. A, left pterygoid of NHMUK PV R 37791 in ventral view. B, left pterygoid of NHMUK PV R 37697, the counter slab to NHMUK PV R 37791. C–D, left pterygoid of NHMUK PV R 37791 in: C, medial; D, posterior view. E–J, digitally reconstructed and reassembled right pterygoid of NHMUK PV R 7591 in: E, dorsal; F, ventral; G, lateral; H, medial; I, anterior; J, posterior view. *Abbreviations*: bptpf, basipterygoid process facet; dep, depression; lg, longitudinal groove; lrim, longitudinal rim; mptf, medial pterygoid flange; par, palatal ramus; pit, pit; plpr, palatine process; pthk, pterygoid hook; qfl, quadrate flange; tfl, transverse flange. Scale bars represent: 10 mm (A–D); 5 mm (E–J).

<figure><img src="image_41.png" /></figure>

**FIG. 28.** Right pterygoid, exposed in ventral view, and the surrounding disarticulated elements of NHMUK PV R 37665. *Abbreviations*: lam, lamina; par, palatal ramus; plpr, palatine process; tfl, transverse flange. Scale bar represents 5 mm.

*connectens*, *Sphenosuchus acutus* and *Dibothrosuchus elaphros* (Walker 1990; Wu & Chatterjee 1993; Clark et al. 2000; Spiekman 2023). This rim is transversely convex and its crest is directed ventrally. This crest gradually fades anteriorly near the anterior tip of the palatal ramus. It continues posteriorly, curving posterolaterally at the base of the palatal ramus. From here, the crest is lateromedially thinner, forming a sharp, blade-like rim that is posterolaterally directed (the medial pterygoid flange *sensu* Walker 1990). This rim forms the ventral margin of the quadrate flange of the pterygoid. A deep, pit-like concavity is formed on the ventral surface of the pterygoid directly lateral to the medial pterygoid flange (Fig. 27A, F).

Only the posterior portion of the dorsal surface of the palatal ramus can be described. It is smooth like the ventral surface, and it is convex in anterior view (Fig. 27E, I). The longitudinal rim on the ventral side of the medial margin corresponds to a deeply excavated longitudinal groove on the dorsal surface. Medially, this groove is

demarcated along its entire length by a tall, thin, dorsally facing lamina. The medial face of this lamina articulated with its antimere along most of its length except for their posterior portion, and consequently a distinct interpterygoid vacuity was absent in *Terrestrisuchus gracilis* (Figs 12A, 13C; *contra* Crush 1980, 1984). This corresponds to the condition in *Saltoposuchus connectens*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Walker 1990; Ruebenstahl et al. 2022; Spiekman 2023). Posteriorly, the longitudinal groove opens into a larger depression on the dorsal surface of the pterygoid at the base of the palatal ramus (Fig. 27E). Posteriorly, the dorsally facing lamina widens dorsoventrally near the base of the palatal ramus, where the ventral rim of the pterygoid curves posterolaterally. Here, the dorsally facing lamina has a distinctly concave medial surface, which is directed medioventrally. The lamina terminates in a short, posteriorly directed process at the anteromedial base of the quadrate flange of the pterygoid
(= pterygoid hook sensu Crush 1980; Fig. 27A, D, F, H, J), and which is also present in *Sphenosuchus acutus* (Walker 1990).

The transverse flange is directed laterally from the pterygoid at the base of the palatal ramus. The flange is tilted anterodorsally to posteroventrally to a degree that is comparable to that seen in CM 29894, *Sphenosuchus acutus* and *Dibothrosuchus elaphros*, but the flange has a considerably larger posteroventral expansion in these other taxa (Walker 1990; Clark *et al.* 2000; Ruebenstahl *et al.* 2022). Although its inclination in this specimen cannot be determined due to diagenetic deformation, the transverse flange of *Saltoposuchus connectens* (SMNS 12591a; Spiekman 2023) is similar in relative anteroposterior length and morphology to that seen in *Terrestrisuchus gracilis*. The transverse flange increases in anteroposterior width and dorsoventral height laterally (Fig. 27A, E–G, I). The anterior and posterior margins are both concave in ventral view but this concavity is deeper in the former (Fig. 27A). The dorsal and ventral surfaces are both slightly transversely convex. The lateral margin of the transverse flange is widely rounded and dorsoventrally thickened. Posteroventrally, the lateral end of the transverse flange terminates in a small, sharp point (Fig. 27G).

The quadrate flange of the pterygoid was oriented predominantly dorsally and only slightly posteriorly and laterally (Fig. 27C, D), similar to the condition seen in *Sphenosuchus acutus* and *Dibothrosuchus elaphros* (Walker 1990; Wu & Chatterjee 1993). The process is thin and elongate, and it tapers along its length before it terminates in a sharp point at its distal end. The quadrate flange is very thin lateromedially and its medial surface faces posteromedially, whereas its lateral surface faces anterolaterally. The anterior and posterior margins of the quadrate flange are incompletely preserved but they were probably mostly straight. As in all other known non-crocodyliform crocodylomorphs with the possible exception of *Junggarsuchus sloani*, the quadrate flange is not fused to the pterygoid ramus of the quadrate (e.g. Walker 1990; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). Instead, the distal half of the quadrate flange was laterally overlapped by the pterygoid ramus of the quadrate, and the connection between both flanges was probably cartilaginous (Walker 1990). The quadrate flange of the pterygoid reaches far dorsally in the skull, as in *Dibothrosuchus*

*elaphros* (Wu & Chatterjee 1993; Ruebenstahl *et al.* 2022). The posteromedial surface of the quadrate flange bears a rounded concavity at its base, which is framed anterodorsally by the short pterygoid hook. This concavity received the anterior margin of the basipterygoid process of the parabasisphenoid. There is no indication of pneumatic cavities in the pterygoid, in contrast to *Junggarsuchus sloani* (Ruebenstahl *et al.* 2022).

## Braincase

*Basioccipital*. The basioccipital forms the posteroventral portion of the braincase (Figs 29B–D, 30). The occipital condyle of the basioccipital is spherical in shape but it possesses a small concavity on its posterior end, which represents a notochordal pit (Fig. 31A, C). This pit is also present in *Litargosuchus leptorhynchus* and *Sphenosuchus acutus* (BP/I/5237; Walker 1990), but absent in *Hesperosuchus agilis* and more derived crocodylomorphs such as *Almadasuchus figarii* (Colbert 1952; Leardi *et al.* 2020). The neck of the occipital condyle is strongly constricted ventrally, as can best be observed in lateral view (Fig. 31E, F, K, L). The neck of the condyle is not constricted laterally. As was also noted by Crush (1980), the occipital neck of the basioccipital of NHMUK PV R 7591 possesses a small but distinct longitudinal groove along its midline (Fig. 31I), which is missing in NHMUK PV R 37725. On the dorsal surface of the occipital condyle, two flattened to slightly concave surfaces are present on each lateral side. These represent the articular surfaces for the otoccipital contributions to the occipital condyle. These facets are directed dorsolaterally and somewhat posteriorly (Fig. 31A, D–F). The facets do not touch along the midline and, as such, the basioccipital contributed to the floor of the foramen magnum, similar to the condition in *Sphenosuchus acutus* (Walker 1990). In *Junggarsuchus sloani* the occipital condyle is probably completely formed by the basioccipital, whereas the exact suture between the basioccipital and otoccipital on the occipital condyle is currently unclear in *Dibothrosuchus elaphros* (Ruebenstahl *et al.* 2022; *contra* Wu & Chatterjee 1993).

The basal tubera are visible ventrolateral to the occipital condyle in posterior view (Fig. 31A, G). They are pyramidal shaped, tapering ventrally. The basal tubera are

**FIG. 29.** Associated braincase elements of *Terrestrisuchus gracilis*. A, right otoccipital and prootic of NHMUK PV R 7557b in lateral view. B–D, digitally reassembled reconstruction of the braincase elements of NHMUK PV R 7591 in: B, posterior; C, right lateral; D, left lateral view. *Abbreviations*: bo, basioccipital; bptp, basipterygoid process; bt, basal tuber; cif, crista interfenestralis; clp, clinoid process; cn, cranial nerve; cor, cochlear recess; crpr, crista prootica; fom, foramen magnum;fov, fenestra ovalis; oc, occipital condyle; ocf, occipital foot; ol, otoccipital ledge; ot, otoccipital; pbs, parabasisphenoid; pbspl, parabasisphenoid plate; plf, perilymphatic foramen; ll, perilymphatic loop; pop, paroccipital process; pr, prootic; ptr, posterior tympanic recess; sbr, sub-basisphenoid recess; scbt, subcapsular buttress. Scale bars represent: 5 mm (A); 10 mm (B–D).
<figure><img src="image_42.png" /></figure>
<figure><img src="image_43.png" /><figcaption>**FIG. 30.** Digitally reassembled reconstruction of the braincase elements of NHMUK PV R 7591 in: A, dorsal; B, ventral view. *Abbreviations:* bo, basioccipital; bor, basioccipital recess; bptp, basipterygoid process; bt, basal tuber; clp, clinoid process; cn, cranial nerve; oc, occipital condyle; ot, otoccipital; pbs, parabasisphenoid; pbspl, parabasisphenoid plate; pop, paroccipital process; pr, prootic; sbr, sub-basisphenoid recess. Scale bar represents 10 mm.</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**FIG. 31.** Selected basioccipitals of *Terrestrisuchus gracilis*. A–F, isolated basioccipital NHMUK PV R 37725 in: A, posterior; B, anterior; C, ventral; D, dorsal; E, left lateral; F, right lateral view. G–L, digitally reconstructed basioccipital of NHMUK PV R 7591 in: G, posterior; H, anterior; I, ventral; J, dorsal; K, left lateral; L, right lateral view. *Abbreviations*: antm, anterior margin; bor, basioccipital recess; bt, basal tuber; btf, fossa of basal tuber; coc, cochlear contribution; gr, groove; np, notochordal pit; oc, occipital condyle; otfc, otoccipital facet. Scale bars represent 5 mm.</figcaption></figure>
connected via a low ridge that has the shape of an inverted V in posterior or anterior view. The lateral surfaces of the basal tubera are distinctly concave at their base, forming a marked fossa that faces posteriorly and somewhat laterally (Fig. 31A, E, F, G, L). A lamina projects anteriorly from the basal tubera, forming a thin ridge on each side of the basioccipital. This ridge is oriented slightly posteroventrally to anterodorsally in lateral view (Fig. 31E, F, K, L). The ventral surface medial to this ridge and the basal tubera is deeply excavated, forming the pneumatized basioccipital recess s.s. (Leardi et al. 2020; basioccipital fossa sensu Crush 1984). The floor of this recess possesses a gradual but distinct convexity along its midline but it does not bear the distinct lamina that separates the recess into two sides in other non-crocodyliform crocodylomorphs (CM 29894, *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Macelognathus vagans*, *Almadasuchus figarii* and possibly *Junggarsuchus sloani*; Walker 1990; Wu & Chatterjee 1993; Leardi et al. 2020; Ruebenstahl et al. 2022). The recess extends further laterally than can be seen in ventral view, and consequently the lateral surface of the basioccipital is very thin. In contrast to other non-crocodyliform crocodylomorphs, there is no evidence for the presence of a separate sub-basisphenoid recess on the basioccipital anterior to basioccipital recess (Leardi et al. 2020).

The dorsal surface of the basioccipital anterior to the occipital condyle is smooth and slightly concave, with slightly elevated lateral ends and a depressed median surface (Fig. 31D, J). The sides lateral to the dorsal surface articulated with both the otoccipital and the prootic (Figs 29B, C, 30). A surface is present on either antero-lateral side of the basioccipital that is thick, slightly concave, somewhat rugose, and directed mostly laterally but also somewhat laterally and anteriorly. This surface formed the posteromedial surface of the floor of the cochlear or lagenar recess (Fig. 29C). This surface is roughly positioned along the lateral side of the basioccipital between its anterior end and the lateral surface directly anterior to the basal tubera (Fig. 31B, D-F, H, J-L). It was previously erroneously interpreted as an articulation facet for the parabasisphenoid by Crush (1984). Instead, the parabasisphenoid connected to the basioccipital only on the anterior surface of the latter and ventrolaterally on the rim of the basioccipital recess (Figs 29C, 30; see also the description of the *parabasisphenoid* below). The basioccipital is transversely widest slightly anterior to the basal tubera. From here, the basioccipital gradually narrows anteriorly in dorsal view (Fig. 31J). The anterior margin of the basioccipital forms a thin, sheet-like, dorsoventrally oriented surface that serves as the anterior border of the basioccipital recess and the primary articular surface with the parabasisphenoid (Figs 30, 31I). The anterior margin is roughly straight in dorsal view (Fig. 31J).

**Supraoccipital.** The supraoccipital is a plate-like bone with anterior expansions on each lateral side of the element (Fig. 32). The supraoccipital is fully fused in the midline in both known examples for *Terrestrisuchus gracilis*, as is the case in other crocodylomorphs except for possibly *Kayentasuchus walkeri* and *Junggarsuchus sloani*, in which the supraoccipitals of the holotype specimens might be paired (Clark & Sues 2002; Ruebenstahl et al. 2022). The central, plate-like portion of the supraoccipital is thin anteroventrally to posterodorsally and octagonal in shape in posterodorsal view (Fig. 32D-G). The posterodorsal surface is mostly flat, although a slight elevation is present along the midline (Fig. 32A). The anteroventral surface of the plate is clearly concave in anterodorsal and posteroventral view (Fig. 32F, G). The supraoccipital plate is wider lateromedially than tall anterodorsally to posteroventrally, as in *Litargosuchus leptorhynchus* and *Almadasuchus figarii* (Clark & Sues 2002; Leardi et al. 2020). In contrast, the supraoccipital of *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani* and *Kayentasuchus walkeri* is considerably taller than wide (Walker 1990; Wu & Chatterjee 1993; Clark & Sues 2002; Ruebenstahl et al. 2022).

The dorsal margin of the supraoccipital possesses a clear facet for the parietal on each side of the dorsolateral part of the plate (Fig. 32A). Here, the supraoccipital is considerably thickened compared with the rest of the plate. The facets are directed anterodorsally and laterally and they are oval in shape, being longer dorsoventrally than wide anteroposteriorly. Directly ventral to the dorsal margin, the posterodorsal surface of the supraoccipital plate is slightly sculptured (Fig. 32A). The margins of the bone positioned directly ventrolaterally to the parietal facets are distinctly concave and face dorsolaterally (Fig. 32A, B).

The outline of the ventral margin of the supraoccipital is hard to discern in the known specimens, but it appears that the central portion of the ventral margin is directed fully ventrally and that it is concave in posterodorsal view (Fig. 32A). This portion represents the contribution of the supraoccipital to the border of the foramen magnum. The supraoccipital contributes to the foramen magnum in most non-crocodyliform crocodylomorphs (Walker 1990; Clark & Sues 2002; Leardi et al. 2020; Ruebenstahl et al. 2022), except for *Junggarsuchus sloani* (Ruebenstahl et al. 2022). Lateral to this central portion, the margin curves abruptly so that it faces ventrolaterally on each side. These faces formed the articular surfaces with the otoccipitals (Crush 1980, 1984), but their exact outline cannot be determined because they are either covered in matrix on each side or incomplete (Fig. 32A, B).

A large projection extends from both lateral sides of the anteroventral surface of the supraoccipital, which corresponds to the epiotic portion of the supraoccipital
<figure><img src="image_45.png" /><figcaption>**FIG. 32.** Known supraoccipitals of *Terrestrisuchus gracilis*. A, supraoccipital of NHMUK PV R 7557b in posterodorsal view. B–G, supraoccipital NHMUK PV R 37673 in: B, posterodorsal; C, anteroventral; D, left lateral; E, right lateral; F, anterodorsal; G, posteroventral view. *Abbreviations*: asc, anterior semicircular canal; feov, foramen for external occipital vein; fm, foramen magnum; otfc, otoccipital facet; ot/prof, otoccipital and prootic facet; paf, parietal facet. Scale bars represent 5 mm.</figcaption></figure>

(Walker 1990; Kuzmin *et al.* 2021). These projections extend far anterodorsally to approximately the anterodorsal-most part of the occipital plate. The anterodorsal extensions are roughly triangular in outline, both in anteroventral and posterodorsal view, and in lateral view (Fig. 13B–E). The posteroventral extent of these extensions cannot be determined because they are either incomplete or obscured by matrix. Along their anterolateral and anterodorsal margin, the projections articulated with the otoccipital and prootic. These articulation facets are distinctly roughened (Fig. 13C–E). Near the anterodorsal apex of the projections small anterodorsally directed openings for the anterior semicircular canals are visible (Fig. 13C, F). The surface medial to the triangular anterodorsal processes of the supraoccipital projections is distinctly concave and

excavated. On the medial side of this excavation a foramen is present on either side of the supraoccipital, exposed in anterodorsal view (Fig. 13F). Based on their relative positions compared with extant crocodylians, these foramina are likely to have facilitated the passage of the external occipital vein (Kuzmin *et al.* 2021). As in other non-crocodyliform crocodylomorphs, there is no indication of pneumatic cavities in the supraoccipital (Leardi *et al.* 2020); such features are present in crocodyliforms, including extant crocodylians, in the form of the intertympanic pneumatic recess related to the mastoid antrum (Herrera *et al.* 2018; Kuzmin *et al.* 2021).

*Otoccipital*. As in other crocodylomorphs (e.g. Leardi *et al.* 2020), the exoccipital and opisthotic are co-ossified
without a discernible suture to form an otoccipital. In posterior view the medial margin of the otoccipital forms the lateral border of the foramen magnum (Figs 13D, 29B). This margin is continuously concave in posterior view (Fig. 33C, G). Ventrally, the posteroventral portion of the otoccipital slightly expands transversely to form the occipital foot, which articulates on the dorsolateral side of the basioccipital and which contributes to the occipital condyle (Figs 29B, 33). Its articular surface is slightly concave. The articulation between the otoccipital and basioccipital continued anteriorly to approximately the level of the basal tubera of the basioccipital (Figs 29C, 30A). The dorsal extent of the contribution of the otoccipital to the margin of the foramen magnum can be discerned from the left otoccipital of NHMUK PV R 7591 (Fig. 33G). Here, this margin is somewhat raised (i.e. it extends further posteriorly), and it is curved distinctly dorsomedially. A similar raised margin is also present in *Sphenosuchus acutus*, and it was hypothesized to form an articular surface for the proatlas by Walker (1990). The otoccipitals did not meet in the midline to exclude the supraoccipital from the foramen magnum (Fig. 29B), in contrast to extant crocodylians and many crocodyliforms (e.g. Barrios *et al.* 2018; Kuzmin *et al.* 2021). The supraoccipital also contributes to the margin of the foramen magnum in other non-crocodyliform crocodylomorphs (e.g. *Sphenosuchus acutus*, Walker 1990; *Dibothrosuchus elaphros*, Ruebenstahl *et al.* 2022; *Almadasuchus figarii*, Leardi *et al.* 2020), except for *Junggarsuchus sloani* (Clark *et al.* 2004; Ruebenstahl *et al.* 2022). The dorsal surface of the otoccipital, which forms the articular surface for the supraoccipital posteriorly and further anteriorly for the prootic, is concave. This articular surface is curved in dorsal view, with the anterior end being directed anteroposteriorly, whereas the posterior end is oriented anterolaterally to posteromedially.

In posterior view both the dorsal and ventral margins of the paroccipital process are concave, whereas the lateral margin is convex (Fig. 33G, I). The paroccipital process is slightly dorsoventrally expanded towards the distal end. However, Crush (1984) reconstructed the paroccipital process as expanding much more strongly dorsoventrally, and as being shorter proximodistally. This was based on observations of specimen P65/67 (field number) (Crush 1980, fig. 147), which is not present in the NHMUK collections. This discrepancy might be attributed to ontogenetic variation or, less likely, because the isolated otoccipital of P65/67 might not have belonged to *Terrestrisuchus gracilis*. Distally, the paroccipital process gradually flattens in the anteroposterior plane (Fig. 33E). The posterior surface of the paroccipital process is smooth and convex for most of its length, being curved along its longitudinal axis; the distalmost part of the posterior surface is flat (Fig. 33G). The posterior surface is

directed mostly posteriorly and slightly dorsally. The anterior surface is conversely concave and smooth, and it is directed anteriorly and slightly ventrally (Fig. 33E). This concave anterior surface is probably homologous with the deeply recessed surface seen in other crocodylomorphs (e.g. *Dibothrosuchus elaphros*, *Junggarsuchus sloani*, *Almadasuchus figarii*; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022), which was named the posterior tympanic recess (PTR) by Wu & Chatterjee (1993), and probably corresponds to the otoccipital diverticulum of extant crocodylians (Dufeu & Witmer 2015; Leardi *et al.* 2020). However, because this recess is only poorly expressed in *Terrestrisuchus gracilis*, similar to the condition in *Litargosuchus leptorhynchus* and *Sphenosuchus acutus* (Walker 1990; KND pers. obs.) and differing from the clearly expressed recess of *Dibothrosuchus elaphros*, *Maceognathus vagans* and *Junggarsuchus sloani* (Leardi *et al.* 2017; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022), it is unclear whether it housed a diverticulum in this taxon. The paroccipital process articulated with the squamosal and this articulation is described in detail in the section dealing with the latter.

The lateral surface of the otoccipital directly anterior to the occipital foot is distinctly concave and excavated (Figs 29A, 33A). Near the dorsal end of the depression a small, single, circular hypoglossal foramen for cranial nerve XII is preserved in both otoccipitals of NHMUK PV R 7591 (Fig. 33A, E). This singular hypoglossal foramen is also present in medial view in NHMUK PV R 37796 (Fig. 33H) and also occurs in *Litargosuchus leptorhynchus* (KND pers. obs.). Other known non-crocodyliform crocodylomorphs, such as *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Almadasuchus figarii*, possess two separate hypoglossal foramina (Walker 1990; Wu & Chatterjee 1993; Leardi *et al.* 2020). Thalattosuchians also preserve one or two openings for cranial nerve XII, whereas extant crocodylians possess two or three (Herrera *et al.* 2018; Kuzmin *et al.* 2021). The position of the foramen in *Terrestrisuchus gracilis* at the most deeply excavated part of the concavity, near its dorsal end, corresponds to the position of the larger of the two openings in these other crocodylomorph taxa. A separate opening for the internal carotid artery, as present in *Junggarsuchus sloani*, *Almadasuchus figarii*, and crocodyliforms (Clark *et al.* 2004; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022), is absent in all specimens of *Terrestrisuchus gracilis*. Crush (1980) suggested that the internal carotid artery passed lateral to a small knob that he had identified near the posteroventral end of the subcapsular buttress in P65/67. A small ventral projection can be seen in the right otoccipital of NHMUK PV R 7591, which might represent the same structure identified by Crush (1980; Fig. 33A, kn?).

The concavity that houses the hypoglossal foramen is anteriorly bordered by a large flange that is confluent
<figure><img src="image_46.png" /><figcaption>FIG. 33. Selected otoccipitals of *Terrestrisuchus gracilis*. A–D, digitally reconstructed right otoccipital of NHMUK PV R 7591 in: A, lateral; B, medial; C, posterior; D, anterior view. E–G, digitally reconstructed left otoccipital of NHMUK PV R 7591 in: E, lateral; F, medial; G, posterior view. H–I, right otoccipital NHMUK PV R 37796 in: H, medial; I, posterior view. Abbreviations: cif, crista interfenestralis; cn, cranial nerve;fov, fenestra ovalis; fpr, fenestra pseudorotunda; kn, knob; lcs, loop-closure suture; lsc, lateral semicircular canal; ocf, occipital foot; ol, otoccipital ledge; otbul, otic bulla; plf, perilymphatic foramen; pop, paroccipital process; psc, posterior semicircular canal; ptr, posterior tympanic recess; rst, recessus scala tympani; scbt, subcapsular buttress; so/prof, supraoccipital and prootic facet; vrc, vestibular recess. Scale bars represent 5 mm.</figcaption></figure>
with the ventral margin of the paroccipital process. This
flange, which is part of the exoccipital contribution of the
otoccipital, forms a very thin, posterodorsally to antero-
ventrally oriented lamina in lateral view (Figs 29A; 33A,
E). This flange was identified as the subcapsular process
by Crush (1980; 1984) and later the subcapsular buttress
by Walker (1990). It is also known as the extracapsular
buttress sensu Kuzmin et al. (2021). Its posterior surface
is convex and its anterior surface consequently concave.
The subcapsular buttress reaches slightly further ventrally
than the occipital foot of the otoccipital, and it connected
to the laterodorsal margin of the basioccipital (Fig. 29C),
as is also the case in *Sphenosuchus acutus* (Walker 1990).
The lateral margin of the buttress curved laterally and
slightly anteriorly. The ventral end of the buttress curves
ventrally so that it is virtually vertically oriented in lateral
view (Fig. 33F). The subcapsular buttress forms the pos-
terior margin of the metotic foramen or vagus foramen,
which facilitates the passage of cranial nerves IX–XI. This
opening is slit-like and is oriented parallel to the subcap-
sular buttress, and thus also directed posterodorsally to
anteroventrally. This is similar to the condition seen in
*Sphenosuchus acutus* (Walker 1990). As in *Dibothrosuchus
elaphros*, *Junggarsuchus sloani* and *Almadasuchus figarii*
(Leardi et al. 2020; Ruebenstahl et al. 2022) there is no
separation within the metotic foramen (Figs 29A, 33A),
whereas in *Sphenosuchus acutus* the exits for cranial nerve
IX and X–XI are separated by a thickening of the opisto-
tic (Walker 1990).

The region of the otoccipital anterior to the metotic
foramen is formed by a large ring of bone that is oval in
lateral view. This represents the perilymphatic loop
(Fig. 29C), which is a derivative of the ventral ramus of
the opisthotic and a classic crocodylian feature that also
occurs in certain aetosaurian pseudosuchians (Walker
1985; Walker 1990; Gower & Walker 2002; Kuzmin
et al. 2021). The perilymphatic loop encloses a large
space, the recessus scala tympani (Fig. 33A). The antero-
dorsal portion of the perilymphatic loop is formed by the
crista interfenestralis (Walker 1990; Kuzmin et al. 2021;
referred to as the interfenestral bar by Crush 1980). The
recessus scala tympani encompassed the perilymphatic
sac, a large lateral extension of the inner ear, from the
cochlear recess (Walker 1990; Kuzmin et al. 2021).
The anteromedial opening of the recessus scala tympani is
called the perilymphatic foramen (Fig. 33D), which facili-
tated the passage of the perilymphatic sac out of the otic
capsule (Kuzmin et al. 2021). An adjacent, postero-
laterally positioned opening represents the fenestra pseu-
dorotunda (Fig. 33D), which was covered by the
secondary tympanic membrane (Kuzmin et al. 2021). The
crista interfenestralis is directed posterodorsally to antero-
ventrally, roughly parallel to the subcapsular buttress
(Figs 29A; 33A, B, D). Proximally, it arises from the base

of the paroccipital process just anterodorsal to the con-
cavity that could represent the posterior tympanic recess.
Here, the crista interfenestralis is very thin lateromedially.
The crista interfenestralis gradually widens both laterome-
dially and dorsoventrally, before reaching its widest extent
at the strong curve at the anteroventral end of the peri-
lymphatic loop (Fig. 33A). This curve is dorsally concave
and ventrally convex, forming a cup at the bottom of the
loop. The perilymphatic loop extends far anteroventrally,
reaching approximately as far ventrally as the subcapsular
buttress in lateral view (Figs 29A, 33A). In the other early
crocodylomorphs *Sphenosuchus acutus*, *Dibothrosuchus
elaphros*, *Junggarsuchus sloani* and *Almadasuchus figarii*,
the perilymphatic loop is much more dorsally restricted
(Walker 1990; Leardi et al. 2020; Ruebenstahl et al. 2022).
The posterior portion of the perilymphatic loop is called
the opisthotic ledge (sensu Walker 1990) or otoccipital
ledge (sensu Kuzmin et al. 2021). It is dorsoventrally thin
but lateromedially wide (Figs 29A, 33B), and thus differs
from the otoccipital ledge of *Sphenosuchus acutus*, which
is prominently expanded in lateral view (Walker 1990).
The otoccipital ledge is oriented posterodorsally to
anteroventrally, thus being roughly parallel in orientation
to the crista interfenestralis and the subcapsular buttress
(Fig. 29A). It forms the separation between the metotic
foramen posteriorly and the perilymphatic foramen an-
teriorly. The otoccipital ledge connects to the subcapsular
buttress at the ventral end of both structures. A small
separation can be seen between the otoccipital ledge and
the main body of the otoccipital in the right element of
NHMUK PV R 7591; this probably represents the loop-
closure suture (Fig. 33B), where the curved perilymphatic
loop reconnects to the opisthotic contribution of the
otoccipital (Walker 1990; Gower 2002).

The fenestra ovalis, through which the stapes connected
to the vestibule of the inner ear, is enclosed by the otocci-
pital posteroventrally and the prootic anterodorsally
(Fig. 29A). It forms the largest opening of the braincase
after the foramen magnum and it is oval to subrectangu-
lar in lateral view, with its longitudinal axis being ori-
ented anteroposteriorly. It is dorsoventrally tallest near its
anterior end. The otoccipital margin of the fenestra ovalis
is formed by the crista interfenestralis.

In medial view, anterior to the occipital foot and the
medial exit of the hypoglossal foramen, the thin and elon-
gate, posterodorsally to anteroventrally oriented metotic
foramen is visible (Fig. 33B, H). Anterodorsally, the
metotic foramen is bordered by an anteroposteriorly thin
and lateromedially wide flange. This was identified as the
tympanic bulla by Crush (1980) and it is probably
homologous to the bulla seen in extant crocodylians
(= otic bulla of Kuzmin et al. 2021). Interestingly, an otic
bulla has not been described in other non-crocodyliform
crocodylomorphs, and it was certainly absent in
*Sphenosuchus acutus, Dibothrosuchus elaphros* and *Junggar-
*suchus sloani* (Walker 1990; Ruebenstahl *et al.* 2022), and
consequently its presence might be an autapomorphy for
*Terrestrisuchus gracilis* among non-crocodyliform crocody-
lomorphs. The otic bulla is anteromedially directed and
also curved distinctly anteromedially (Fig. 33B–D, H). Its
posteromedial surface is convex and its anterolateral sur-
face, concave. The latter surface formed the posterior
margin of the vestibular recess (Fig. 33B, D, F). The ves-
tibular recess is a large space on the medial side of the
otoccipital, which housed the vestibular portion of the
inner ear (Kuzmin *et al.* 2021). From this recess a wide,
oval canal that is taller dorsoventrally than wide antero-
posteriorly, is directed laterally into the otoccipital.

Two openings of the semicircular canals can be dis-
cerned on the otoccipital. Both are small and circular. An
opening of the posterior semicircular canal is present on
the dorsal surface of the otoccipital on its medial margin,
directly dorsal to the vestibular recess (Fig. 33B). This
canal continued dorsally into the supraoccipital. The sec-
ond opening is part of the lateral semicircular canal. It
opens anterolaterally from the otoccipital directly dorsal
to the base of the crista interfenestralis of the perilympha-
tic loop (Fig. 33D), and it continued into the prootic.

*Prootic*. The prootic of *Terrestrisuchus gracilis* articulated with the supraoccipital posterodorsally, the otoccipital posteriorly, the parietal dorsally, the laterosphenoid anteriorly and the basioccipital and parabasisphenoid anteroventrally. Additionally, a small connection was present between the posterior end of the prootic and the dorsal head of the quadrate (*contra* Crush 1980, 1984). Extensive contact and fusion between the braincase and quadrate is considered a defining feature of Crocodyliiformes, although this condition is also known in non-crocodyliform crocodylomorphs closely related to crocodyliforms such as *Kayentasuchus walkeri*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani*, *Macelognathus vagans* and *Almadasuchus figarii* (Clark & Sues 2002; Leardi *et al.* 2017; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). In *Sphenosuchus acutus* this connection is restricted to the dorsal end of the quadrate and a small articulation on the prootic (Walker 1990), similar to the condition in *Terrestrisuchus gracilis* based on the morphology of the quadrate, squamosal and prootic, and the reconstructed configuration of the post-temporal fenestra and temporal canal (Figs 12, 23).

The transversely thin, anteroposteriorly directed dorsal
margin of the prootic formed the articulation with the
ventral surface of the parietal. This margin is very slightly
convex in lateral view (Fig. 34A). From the dorsal margin
the prootic slopes down almost vertically on both its lat-
eral and medial sides, forming a distinct dorsal prootic
crest. In posterior view the lateral side of this crest is

very slightly convex and the medial side is slightly con-
cave (Fig. 34D). Posteriorly, the dorsal prootic crest ends
abruptly in an edge that is slightly concave in lateral view
(Fig. 34A). Here, the prootic articulated with the
supraoccipital. Directly lateral to the articulation facet for
the supraoccipital, a large opening was present, framed
ventrally and medially by the prootic, dorsally by the
parietal and squamosal, and laterally by the quadrate.
This represents the anterior opening of the temporal
canal, which housed the stapedial arteries and which was
named the anterior temporal foramen by Walker (1990; =
temporoorbital foramen of Leardi et al. 2020; Figs 12B, 23,
34A, D). This canal is similarly framed in *Junggarsuchus
sloani* (Ruebenstahl et al. 2022), whereas in *Sphenosuchus
acutus* the anterior temporal foramen is framed only by the
prootic and squamosal (Walker 1990). In at least some cro-
codyliforms (e.g. *Eopneumatosuchus colberti*) and in extant
crocodylians, the prootic is excluded from the anterior
temporal foramen by the parietal and it is additionally
framed by the squamosal, and in some cases also the quad-
rate (Walker 1990; Kuzmin et al. 2021; Melstrom
et al. 2021). The position of the anterior temporal foramen
on the prootic was previously also identified by
Crush (1980). Its floor is distinctly concave in lateral view
(Fig. 34A). Due to the disarticulation of the elements that
formed the temporal canal, it is not possible to accurately
determine the presence or position of the postquadrate
foramen, although this opening was probably present in
*Terrestrisuchus gracilis* to transmit the stapedial arteries, as
in other crocodylomorphs (e.g. Walker 1990; Kuzmin
et al. 2021; Ruebenstahl et al. 2022). The anterior end of
the dorsal margin slopes down more gradually than the
posterior end, and it is distinctly convex in lateral view
(Fig. 34A). At its base a notch is formed, which represents
the opening for the trigeminal nerve (cranial nerve V), sim-
ilar to the condition seen in *Sphenosuchus acutus*
(Walker 1990).

The lateral side of the prootic is characterized by a very
prominent crest (Fig. 34A, C). This structure is homolo-
gous to the crista prootica of other archosauromorphs
(Gower 1997) and has also been termed the prootic-
basisphenoid flange (Walker 1990), lateral lamina of the
prootic (Kuzmin et al. 2021), or the otosphenoidal crest
(Leardi et al. 2020). The crista prootica continues far pos-
teriorly, extending considerably posteriorly beyond the
rest of the prootic. Here, the crista prootica is confluent
with the dorsal margin of the paroccipital process of the
otoccipital, to which it connects (Fig. 29C). The quadrate
connects to the prootic on the surface dorsal to this pos-
terior extension of the crista prootica (Fig. 34A, D). In
*Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggar-
*suchus sloani* the crista prootica terminates posteriorly in
a bulge near the posterior end of the prootic, named the
prootic buttress by Walker (1990), which is also present
<figure><img src="image_47.png" /><figcaption>FIG. 34. Digitally reconstructed right prootic of NHMUK PV R 7591 in: A, lateral; B, medial; C, ventral; D, posterior view. Abbreviations: asc, anterior semicircular canal; atf, anterior temporal foramen; aunf, auditory nerve foramina; bof, basioccipital facet; cn, crani al nerve; cor, cochlear recess; crpr, crista prootica; dcr, dorsal crest; flo, flocculus;fov, fenestra ovalis;itr, intertympanic recess; lsc, lateral semicircular canal; otfc, otoccipital facet; paf, parietal facet; pfr, prootic facial recess; prcc, precochlear cavities; qf, quadrate facet; sofc, supraoccipital articulation facet; vrc, vestibular recess. Scale bar represents 5 mm.</figcaption></figure>

in crocodyliforms, including extant crocodylians (Kuzmin *et al.* 2021; Melstrom *et al.* 2021; Ruebenstahl *et al.* 2022). The crista prootica is distinctly curved, so that its ventral margin is concave and its dorsal margin convex in lateral view for most of its length, with only the anterior and posterior ends being straight (Fig. 34A). The dorsal sur- face of the crista prootica is convex in anterior or pos- terior view, with the ventral surface correspondingly being concave (Fig. 34A, C). The anterior half of the crista prootic is oriented posterodorsally to antero- ventrally, reaching towards the wide dorsum sellae of the parabasisphenoid (Figs 29C, 30A, 35A). However, due to the disarticulation of NHMUK PV R 7591 it is unclear whether the prootic attached to the parabasisphenoid and

thus, whether the crista prootica continued onto the lat- ter, as has been described for *Sphenosuchus acutus* (Walker 1990).

There are two large concavities and a single smaller one on the lateral surface of the prootic that are roofed by the crista prootica, and which represent pneumatic recesses (Fig. 34A, C). Pneumatic recesses in the prootic have previously been established in *Kayentasuchus walkeri*, *Hesperosuchus agilis*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani*, *Macelognathus vagans* and *Almadasuchus figarii* among non-crocodyliform crocody- lomorphs (Walker 1990; Wu & Chatterjee 1993; Clark & Sues 2002; Leardi *et al.* 2017; Leardi *et al.* 2020; Rueben- stahl *et al.* 2022), and the identification of these cavities
in *Terrestrisuchus gracilis* represents the most basal occur-
rence of this feature in crocodylomorphs (i.e. the
occurrence of this feature most distant to the crown
within Crocodylomorpha). Here we follow the terminol-
ogy of Kuzmin *et al.* (2021) for the pneumatic recesses of
the prootic. The most anterior of the pneumatic cavities
on the lateral surface of the prootic is the prootic facial
recess (= facial antrum of Walker 1990) and it represents
the deepest of the three pneumatic excavations. The two
posterior cavities together form the intertympanic recess
(= mastoid antrum of Walker 1990). The anterior of the
two is the anteroposteriorly wider excavation, whereas
the posterior excavation of the intertympanic recess rep-
resents the least excavated and smallest of the pneumatic
fossae. The intertympanic recess is also subdivided into
two fossae in *Dibothrosuchus elaphros* and *Almadasuchus
figarii* (Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). In
extant crocodylians and some crocodyliforms (e.g. *Eop-
neumatosuchus colberti*), the intertympanic recess forms a
very large opening in the side of the braincase (Kuzmin
*et al.* 2021; Melstrom *et al.* 2021), whereas in thalattosu-
chians this recess is absent (Herrera *et al.* 2018).

The region ventral to the prootic facial recess is com-
plex and possesses three connected openings, two being
visible in lateral view (Fig. 34A) and one in medial view
(Fig. 34B). They correspond in their relative position to
the opening for the facial nerve (cranial nerve VII) and
the precochlear cavities described by Walker (1990) for
*Sphenosuchus acutus*, with the precochlear cavities being
pneumatic in origin. These structures have otherwise not
previously been identified among Crocodylomorpha. Of
the two openings visible in lateral view, one is positioned
posterodorsal to the other on the lateral surface of the
prootic. It is placed posteriorly and slightly ventrally to
the prootic facial recess. Its opening is directed postero-
dorsally and somewhat laterally, and the opening is
framed by a short posterior groove, which is formed ven-
trally by a distinct laterally facing lip (Fig. 34A). The
other foramen is positioned directly ventral to the prootic
facial recess and it faces anteroventrally and laterally. The
medial opening is positioned directly medial to the
anterolateral opening and it faces medially and slightly
posteriorly (Fig. 34B). All three openings are subcircular
in outline. Two further openings are present on the an-
terior portion of the vestibular recess on the prootic, and
they are visible in medial and posterior view (Fig. 34B,
D). They were identified as the openings for the anterior
branch of the auditory nerve in *Sphenosuchus acutus*
(Walker 1990). The anterior of the two is c. 1.5-fold the
size of the posterior, and both are oval, being anteropos-
teriorly longer than dorsoventrally tall. The anterior
opening is directed ventrally and slightly anteriorly
whereas the posterior opening is directed anteroventrally
and somewhat medially.

In *Sphenosuchus acutus* the prootic continues ventral to
the opening for the trigeminal nerve and anteroventral
to the openings for the facial nerve as a depressed lateral
surface that connects to the lateral side of the parabasi-
sphenoid plate (Walker 1990). This part of the prootic is
not present in NHMUK PV R 7591, leaving a large open-
ing in the braincase between the cochlear portion of the
prootic and the parabasisphenoid plate (Fig. 29C).
The only part of the prootic preserved ventral to the
facial nerve is a large, ventrally and slightly anteriorly
directed flange that covered the cochlea of the inner ear
anterolaterally, which was identified as the cochlear
prominence by Crush (1980). It is positioned directly an-
terior to the crista interfenestralis of the otoccipital and
on its anteroventral side this flange connected to the pos-
terior portion of the basioccipital (Fig. 29C). The antero-
lateral surface of this flange is convex, as can be observed
in lateral view (Fig. 34A). Its distal margin is incompletely
preserved but it is nevertheless unmistakably convex in
posterior view (Fig. 34D). The posteromedial surface of
this flange is distinctly concave, and it is particularly
deeply excavated at its base (Fig. 34C, D). This surface
represents the cochlear recess of the prootic (Walker 1990;
Kuzmin et al. 2021).

In posterior view a large opening is present dorso-
medially to the cochlear recess (Fig. 34D). This represents
the vestibular recess of the prootic, which housed a large
part of the vestibule of the inner ear. The outline of this
recess in posterior view is somewhat triangular, with a
lateromedially oriented dorsal margin and a dorsome-
dially to ventrolaterally oriented medial margin. The ves-
tibular recess reaches anteriorly until the anterior end of
the prootic, excluding the anterior extension of the crista
prootica. At its anterior end the anterior base of the an-
terior semicircular canal is positioned. From its base, the
anterior semicircular canal is directed dorsally, before
curving posteriorly through the dorsal crest of the pro-
otic. The medial side of the dorsal crest of the prootic is
partially broken, and thus the anterior semicircular canal
can be seen in medial view (Fig. 34B). It exits the prootic
at the posterior margin of the dorsal crest of the prootic
at the height of the final third of this crest in a postero-
dorsal direction, as is visible in posterior view (Fig. 34D).
Here, the anterior semicircular canal entered the supraoc-
cipital. The opening for the lateral semicircular canal can
also be observed in posterior view on the ventral part of
the concave articular surface for the otoccipital
(Fig. 34D). Both openings are roughly subcircular in out-
line. The medial side of the dorsal crest of the prootic
possesses a distinct depression near its posteroventral end,
formed by a clear concavity in posterior view. This repre-
sents the indentation of the flocculus, the cerebral lobe
involved in regulating head and eye stabilization (Voogd
& Wylie 2004; Ferreira-Cardoso et al. 2017).
<figure><img src="image_48.png" /><figcaption>**FIG. 35.** Digitally reconstructed parabasisphenoid of NHMUK PV R 7591 in: A, right lateral; B, left lateral; C, anterior; D, posterior; E, dorsal; F, ventral; G, oblique anterodorsolateral view. *Abbreviations:* bptp, basipterygoid process; bof, basioccipital facet; carp, carotid pillar; clp, cultriform process; cn, cranial nerve; cofl, cochlear flange; dors, dorsum sellae; hypfo, hypophyseal fossa; mdcl, median cleft; pbspl, parabasisphenoid plate; pocr, postcarotid recess; prcr, precarotid recess; ptfc, pterygoid facet; sbr, sub-basisphenoid recess; tr, trough. Scale bar represents 5 mm.</figcaption></figure>

*Parabasisphenoid.* The parabasisphenoid of NHMUK PV R 7591 is badly broken but virtually complete (Fig. 35). It is complex and highly pneumatized. The

parabasisphenoid has a thickened, transversely oriented posterior margin, which articulated with the anterior surface of the basioccipital (Fig. 35D). It is partially broken
in NHMUK PV R 7591 and its preserved portion is con-
sequently oriented obliquely (Fig. 35E, F). The articular
surfaces between the basioccipital and parabasisphenoid
are aligned so that the dorsal surface of the basioccipital
is confluent with the dorsal surface of the posterior por-
tion of the parabasisphenoid, which was identified as the
(para)basisphenoid plate by Walker (1990). Together
these surfaces formed the floor of the endocranial cavity.
The dorsal surface of the parabasisphenoid plate is both
longitudinally and transversely gently concave, and it
curves anterodorsally (Fig. 35B, D, E). The plate is bro-
ken, with its posteroventral portion being broken away
from the rest of the plate on its right side, but its mor-
phology can still be discerned. A shallow but distinct cleft
runs longitudinally along the midline of the plate and it
possesses two small foramina near its anterodorsal end,
one being positioned close to the dorsal margin on the
left side and another being placed slightly further ven-
trally and medially on the right side of the plate
(Fig. 35D, E). These probably transmitted the abducens
nerve, cranial nerve VI (Walker 1990; Kuzmin
et al. 2021).

On each posterolateral side of the parabasisphenoid
plate, a dorsally directed flange was present. This flange is
preserved but badly broken on the left side in NHMUK
PV R 7591 (Fig. 35B, D–G). It represents the cochlear
flange, which covered the lateral side of the lagenar
(= ventral) part of the cochlear recess, and which proba-
bly connected to the ventral margin of the cochlear recess
of the prootic (Figs 29C, 35A, B). This flange is also pre-
sent in the parabasisphenoid of *Sphenosuchus acutus* and,
to a lesser extent, in *Macelognathus vagans* (Walker 1990;
Leardi *et al.* 2017). The cochlear flange curves dorso-
laterally, so that its lateral surface is convex and its medial
surface concave in posterior view (Fig. 35D). In lateral
view this flange is triangular, gradually tapering to a point
dorsally (Fig. 35B).

Anterior to the parabasisphenoid plate the parabasi-
sphenoid is characterized by a very large pneumatic open-
ing in lateral view, which is divided into two
compartments by a vertically oriented strut (Fig. 35A, B).
Such a large and dorsoventrally tall pneumatic opening in
the parabasisphenoid is absent in extant crocodylians
(DufEAU & Witmer 2015; Kuzmin et al. 2021), but repre-
sents a feature typical of non-crocodyliform crocodylo-
morphs (e.g. *Sphenosuchus acutus*, *Dibothrosuchus
elaphros*, *Junggarsuchus sloani*, *Macelognathus vagans*
and *Almadasuchus figarii*; Walker 1990; Leardi et al. 2017;
Leardi et al. 2020; Ruebenstahl et al. 2022). The postero-
dorsal compartment corresponds to the postcarotid
recess, and the anteroventral compartment to the precaro-
tid recess, and this subdivision is also present in other
crocodylomorphs and extant crocodylians. The strut sepa-
rating the two compartments was named the carotid

pillar by Walker (1990) given that it transmits the carotid
arteries through the hypophyseal fossa. The postcarotid
recess is less than half the size of the precarotid recess
and it is triangular in lateral view, whereas the precarotid
recess is roughly rectangular in lateral view (Fig. 35B).
The ventral floor of the postcarotid and precarotid
recesses is distinctly convex in anterior or posterior view,
with a dorsally curved central portion and depressed lat-
eral sides. The anterior surface of the parabasisphenoid
plate, which forms the posterior wall of the postcarotid
recess, is convex in ventral or dorsal view. Both the an-
terior and posterior surfaces of the carotid pillar are dis-
tinctly concave. On the posterior surface a wide groove
forms the concavity along most of the length of the pillar.
The groove is deepest and widest ventrally and gradually
fades and narrows dorsally. The anterior surface is charac-
terized by two pneumatic cavities in its ventrolateral cor-
ners. A prominent but transversely thin lamina that runs
longitudinally along the transverse midline of the pillar
separates these cavities. It gradually tapers dorsally in lat-
eral view. The lateral sides of the carotid pillar are
roughly flat. The exact morphology of the dorsal end of
the carotid pillar is hard to discern because this part
of the parabasisphenoid is badly damaged, but the pre-
served portion of the pillar bifurcates at its dorsal end in
lateral view (Fig. 35B). The posterior prong of the bifur-
cation extends further dorsally than the anterior one. The
split between the two prongs is an excavated surface, both
in lateral and (on its lateral portion) in ventral view. The
precarotid recess continues into the basipterygoid pro-
cesses (see below), which are almost completely hollowed
out as a result. Anteriorly, the precarotid recess also con-
tinues for at least a short distance into the cultriform
process (see below), where it forms the rostral recess of
the parabasisphenoid. The rostral recess is also present in
*Sphenosuchus acutus*, *Macelognathus vagans*, *Almadasuchus
figarii* and some extant crocodylians (Walker 1990; Leardi
et al. 2017; Leardi et al. 2020; Kuzmin et al. 2021). The
anterior extent of the rostral recess cannot be determined,
therefore it is unclear whether it was directly connected
to the V-shaped opening of the dorsal surface of the cul-
triform process.

The portion of the parabasisphenoid ventral to the
postcarotid and precarotid recesses forms the ventral
body of the parabasisphenoid. It is oriented antero-
ventrally to posterodorsally (Fig. 35A, B) but it is less ver-
tically oriented than that seen in *Sphenosuchus acutus*
(Walker 1990). The ventral body of the parabasisphenoid
is hourglass shaped in ventral view, being constricted in
its anteroposterior centre (Fig. 35F). Its ventral surface is
distinctly concave in anterior and posterior view, forming
a clear excavated surface, the sub-basisphenoid recess, also
known as the median pharyngeal recess (Fig. 35D, F; e.g.
Walker 1990; Leardi *et al.* 2020; Kuzmin *et al.* 2021;
Ruebenstahl et al. 2022). This structure is similar in shape to the recess that occurs on the ventral surface of the parabasisphenoid in many non-crocodylomorph archo-saurs (= parabasisphenoid recess of Nesbitt 2011) and that seen in *Saltoposuchus connectens* and *Sphenosuchus acutus* (Walker 1990; Spiekman 2023), as well as thalatto-suchians (Herrera et al. 2018). In contrast, more derived non-crocodyliform crocodylomorphs (e.g. *Almadasuchus elaphros* and *Junggarsuchus sloani*) and extant crocodylians lack such an elongate trough (Leardi et al. 2020; Kuzmin et al. 2021; Ruebenstahl et al. 2022). In these taxa the sub-basisphenoid recess or median pharyngeal recess forms a deep pit on the posterior end of the ventral body of the parabasisphenoid, which forms a continuous pneu-matic sinus with the basioccipital recess (s.s., following Leardi et al. 2020) of the basioccipital. This confluent opening between the basioccipital and parabasisphenoid is therefore also known as the basioccipital recess *s.l.* (Leardi et al. 2020). Although the posterior end of the parabasisphenoid is incomplete in NHMUK PV R 7591, the parabasisphenoid almost certainly lacks such a deep pit. A more detailed discussion of the ventral recesses of the basioccipital and parabasisphenoid is provided in the Discussion below.

Near the anterior end of the ventral body, on its ventrolateral sides, the well-developed basipterygoid processes of the parabasisphenoid are preserved (Fig. 35). Prominent basipterygoid processes are also present in *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and probably *Saltoposuchus connectens* among early crocodylomorphs (Walker 1990; Wu & Chatterjee 1993; Kuzmin et al. 2021; Ruebenstahl et al. 2022; Spiekman 2023). In contrast, in *Almadasuchus figarii* and crocodyliforms, including *Protosuchus richardsoni*, *Orthosuchus stormbergi* and extant crocodylians, the basipterygoid processes are lost and instead the parabasisphenoid and pterygoids are tightly sutured (Leardi et al. 2020; Kuzmin et al. 2021). Tentative basipterygoid processes were identified for *Junggarsuchus sloani* but they are distinctly displaced posteriorly relative to the basipterygoid of other crocodylomorphs, and the homologies of these structures are unclear (Ruebenstahl et al. 2022). The basipterygoid process in *Terrestrisuchus gracilis* is directed ventrolaterally and anteriorly from its base (Fig. 35D, F). The roughened surface of the basipterygoid process is similar to that of *Sphenosuchus acutus* and *Dibothrosuchus elaphros* (Walker 1990; Ruebenstahl et al. 2022), being roughly trapezoidal in ventral view, with a transversely narrower anterior portion and wider posterior portion (Fig. 35F). The roughened surface is mostly on the ventral surface of the process, which is directed ventrolaterally and anteriorly, but it also extends considerably anteriorly onto its anterior surface through a gradual, rounded transition from the ventral to anterior surface (Fig. 35B, C, G). It is this anterior

portion of the basipterygoid process that forms the pre-dominant articulation with the pterygoid (Fig. 12A), as is also the case in *Sphenosuchus acutus* and *Dibothrosuchus elaphros* (Walker 1990; Ruebenstahl et al. 2022). The posterior portion of the roughened ventral surface of the basipterygoid process extends posteriorly beyond the base of the process, thus forming a dorsoventrally thin flange (Fig. 35D). The anterior portion of the ventral surface of the basipterygoid process is convex in lateral and anterior views, whereas the posterior portion is roughly flat (Fig. 35A, F).

The dorsal roof of the precarotid and postcarotid recesses is formed by the dorsum sellae (Kuzmin et al. 2021). The dorsum sellae is a dorsoventrally thin, transversely wide plate that is positioned anterior to the parabasisphenoid plate, and which is oriented posterodorsally to anteroventrally (Fig. 35A–C, E, G). The dorsum sellae is damaged on its right side, with its left side being relatively well preserved. It is characterized by a very large anteroposteriorly elongate opening along its midline, the hypophyseal or pituitary fossa, which housed the hypophysis of the brain. The hypophyseal fossa is in a similar relative position to that seen in other early crocodylomorphs (Walker 1990; Leardi et al. 2017; Ruebenstahl et al. 2022), whereas in extant crocodylians it is positioned further anteroventrally on the parabasisphenoid, where it is confluent with the dorsal margin of the cultriform process (Kuzmin et al. 2021). The edges of the hypophyseal fossa are directed directly vertically relative to the dorsal surface of the dorsum sellae on all sides of the fossa. However, these inner margins of the fossa are all broken ventrally, where they presumably formed a contact with the carotid pillar on the lateral sides. The hypophyseal fossa occupies virtually the complete anteroposterior length of the dorsum sellae, and it is considerably longer longitudinally (i.e. anteroventrally to posterodorsally) than wide transversely. An elongate hypophyseal fossa has also been observed in *Almadasuchus figarii* and thalattosuchian crocodylomorphs such as *Steneosaurus bollensis* (Herrera et al. 2018; Leardi et al. 2020). The lateral edge of the dorsum sellae is homologous to the edge that formed the anterior continuation of the crista prootica on the parabasisphenoid in *Sphenosuchus acutus* according to Walker (1990) (= the prootic-basisphenoid flange therein). Due to the disarticulation of these elements it is unclear whether this edge connected directly to the crista prootica of the prootic in *Terrestrisuchus gracilis*, but they are approximately aligned with each other in the reconstructed braincase (Figs 29C, D, 30). The lateral margin flares ventrally along most of its anteroposterior length, providing a very low lip for the dorsal end of the postcarotid recess and the posterior part of the precarotid recess. This lateral lip is most pronounced at its posterior end, where it curves
<figure><img src="image_49.png" /><figcaption>**FIG. 36.** Digitally reconstructed partial stapes of NHMUK PV R 7591. A, *in situ* placement of the stapes after digital removal of several surrounding elements for clarity; elements are observed in oblique anterolateral view. B–E, partial stapes in: B, dorsal; C, ventral; D, anterior; E, posterior view; arrows point laterally. *Abbreviations:* bo, basioccipital; ot, otoccipital; pa, parietal; pbs, parabasisphenoid; pro, prootic; st, stapes. Scale bars represent: 5 mm (A); 2 mm (B–E).</figcaption></figure>

posteroinventrally to frame the anterodorsal portion of the parabasisphenoid plate (Fig. 35B). The dorsal surface of the dorsum sellae is roughly flat to slightly convex in its posterior portion, whereas the surface is distinctly concave on its anterior half on either side of the hypophyseal fossa (Fig. 35G). The dorsum sellae is roughly rectangular in anterodorsal view. It maintains its width along its anteroposterior length, and its lateral margins are straight to slightly convex in dorsal view (Fig. 35E). The posterolateral corners of the dorsum sellae form roughly right angles, whereas the anterolateral corners are more gradual and obtuse and, consequently, the anterior margins of the dorsum sellae are angled somewhat anteromedially to posterolaterally. Anteroventrally, the dorsum sellae is connected to the cultriform process by a short and narrow bridge of bone, which forms the anterodorsal margin of the precarotid recess (Fig. 35A, B).

The cultriform process (= parabasisphenoid rostrum of Kuzmin *et al.* 2021) of the parabasisphenoid is strongly elongated. It is dorsoventrally widest at its base, and it gradually tapers anteriorly, terminating in a sharp point. The ventral margin of the cultriform process is transversely narrow and convex in lateral view, with the dorsal margin being slightly concave (Fig. 35A, B). On its dorsal surface the cultriform process possesses a narrow but deep trough (Fig. 35F) and, as such, the process has a V shape in cross-section. An elongate cultriform process with a V-shaped cross-section is a typical feature plesiomorphic to archosaurs (e.g. Nesbitt 2011; Sobral

*et al.* 2016) and it is also present in *Sphenosuchus acutus* (Walker 1990), whereas this process is either absent or incipient in *Macelognathus vagans* and *Junggarsuchus sloani* (Leardi *et al.* 2017; Leardi *et al.* 2020; Ruebenstahl *et al.* 2022), or anteroposteriorly short and dorsoventrally tall in *Almadasuchus figarii* and crocodyliforms (Leardi *et al.* 2020; Kuzmin *et al.* 2021).

*Laterosphenoid.* Several plate-like, fragmented bones are preserved underneath the right parietal and frontal of NHMUK PV R 7591. They are strongly distorted and deformed and probably represent remains of the laterosphenoid (Fig. 10) but bear no clear morphological features.

*Stapes.* A small rod-like bone was recovered from the µCT data of NHMUK PV R 7591. It is positioned slightly anterior to the right otoccipital and near the posterior end of the right prootic, thus corresponding to the approximate position of the fenestra ovalis (Fig. 36A). In lateral view the element is covered by both the right quadrate and squamosal, and it is therefore not visible externally. The element is clearly incomplete, given that it is not long enough to reach from the fenestra ovalis to the level of the lateral margin of the quadrate in life. The preserved element consists of the proximal part of the stapes, which is likely to constitute less than half of the full-sized element. The preserved stapes is a simple rod-like structure that gradually dorsoventrally expands distally (Fig. 36D, E). As such, the proximal end of the stapes is
subcircular in cross-section, whereas the distal end of the preserved section is distinctly oval in cross-section, being considerably taller dorsoventrally than wide anteroposteriorly. The preserved section of the stapes is gradually curved, so that in anterior and posterior view the dorsal margin is convex and the ventral margin concave (Fig. 36D, E).

## Mandible

The mandible of *Terrestrisuchus gracilis* is a slender structure. The dentary is the longest element in the mandible, forming more than half of the length of the lower jaw (Fig. 37F). A distinct mandibular fenestra and a remarkably well developed and strongly dorsally upturned retroarticular process of the articular characterize the postdentary region of the mandible. Of the postdentary elements, the coronoid can only be tentatively identified for *Terrestrisuchus gracilis* based on a single, isolated element. Largely complete, disarticulated remains are preserved for the angular and splenial, whereas the surangular, prearticular and articular are represented only in the well-preserved skull of NHMUK PV R 7591. Although the surangular and prearticular are incomplete, the articular is excellently preserved.

**Dentary.** The dentary slightly increases in dorsoventral height posteriorly and is gently curved along its anteroposterior length, with the dental margin being slightly concave and the ventral margin slightly convex in lateral view (Fig. 38C). The anterior end of the dentary is rounded in lateral view (Fig. 38A). It is not dorsoventrally expanded, in contrast to *Saltoposuchus connectens*, *Hesperosuchus agilis*, CM 29894 and *Trialestes romeri* (Colbert 1952; Clark et al. 2000; Lecuona et al. 2016; Spiekman 2023). In lateral view the dorsal, tooth-bearing margin is medially offset relative to the rest of the lateral surface of the dentary, and it is ventrally demarcated by a ridge that is oriented parallel to the dorsal margin (Fig. 38A). A similar ridge is present in *Saltoposuchus connectens* (Spiekman 2023). In *Terrestrisuchus gracilis* this ridge gradually fades both anteriorly and posteriorly. Anteriorly this occurs at the anteroposterior level of the fifth preserved tooth (most probably the eighth tooth position overall) counted from anterior; posteriorly this occurs close to the posterior end of the preserved dentary. The lateral surface of the tooth-bearing margin is approximately flat, whereas the rest of the lateral surface of the dentary is convex. Several nutrient foramina and small pits are present on the anterodorsal and anteroventral portions of the dentary.

The Meckelian groove reaches far anteriorly on the medial surface of the dentary, as in *Hesperosuchus agilis*

and *Sphenosuchus acutus* (Colbert 1952; Walker 1990), being positioned at the anteroposterior level of the second tooth counted from anterior (Fig. 38A). The symphysis between the two mandibular rami was positioned directly anterior to this, but as in *Hesperosuchus agilis* and *Spheinosuchus acutus* (Colbert 1952; Walker 1990) no clear articulation facet or rugosity is present on the medial surface of the dentary. Anteriorly, the Meckelian groove is positioned slightly ventral to the dorsoventral mid-height of the dentary and it occupies approximately one-third of its total height. Here, a small posteromedially opening foramen is present on the ventral half of the Meckelian groove (Fig. 38C). Posteriorly the groove gradually widens dorsoventrally until its ventral margin becomes confluent with the ventral margin of the dentary. Slightly anterior to this confluence, on the dorsomedial surface of the Meckelian groove, a posteriorly facing foramen and associated groove are positioned, which were identified as the alveolar canal and considered the posterior terminus of the Meckelian canal by Crush (1980). The Meckelian canal was covered medially by the splenial in life. The dentary articulated medially with the splenial, posteroventrally with the angular, and posterodorsally with the surangular.

**Coronoid.** Crush (1980, 1984) tentatively identified a small, fragmentary bone that is preserved adjacent to the well-preserved splenial in NHMUK PV R 37734 as a right coronoid (Fig. 39). The element is probably incomplete but has similar proportions to the coronoid of the early crocodyliform *Protosuchus haughtoni* (Dollman et al. 2019), although it is considerably more curved and robust than the coronoid of the latter taxon. A separate coronoid could not be identified in the articulated right mandible of NHMUK PV R 7591. Due to its poor preservation and the lack of articulation with other mandibular elements, it remains unclear whether the element of NHMUK PV R 37734 represents a coronoid.

**Splenial.** The splenial is an elongate element that covered the mandibular ramus from just posterior to the symphysis (with a short gap between the posterior end of the symphysis and the anterior end of the splenial) until the mandibular fossa for the attachment of the jaw adductor musculature (Crush 1984). Both its lateral and medial surfaces are roughly flat. Its dorsal margin is gently, continuously concave in lateral view for most of its length except for the posterior end, which is slightly convex (Fig. 39). Its ventral margin is convex at its anterior and posterior portions but concave at the anteroposterior mid-portion in lateral view, which is noticeably different from the slightly but continuously convex ventral margin of the dentary. The ventral margin is folded laterally, which would have covered the dentary in ventral view. This folding extends along
<figure><img src="image_50.png" /><figcaption>**FIG. 37.** Selected regions of the mandible of *Terrestrisuchus gracilis*. A, skull and mandible of NHMUK PV R 7591 in right lateral view. B, right angular, exposed in medial view, and associated elements of NHMUK PV R 7593a. C, left angular of NHMUK PV R 37778 in lateral view. D–E, digitally reconstructed and partially reassembled posterior portion of the right mandibular ramus of NHMUK PV R 7591 in: D, lateral; E, medial view. F, reconstruction of the mandible in lateral view, modified from Crush (1984). The region including the surangular, angular and external mandibular fenestra was reconstructed by Crush (1984) based on NHMUK PV R 7591 prior to the damage incurred by this specimen. *Abbreviations*: an, angular; ar, articular; arf, facet for the articular; de, dentary; def, dentary facet; emf, external mandibular fenestra; gr, groove; j, jugal; pra, prearticular; praf, prearticulation facet; sa, surangular. Scale bars represent: 10 mm (A–C); 5 mm (D, E).</figcaption></figure>
<figure><img src="image_51.png" /><figcaption>**FIG. 38.** Selected dentaries of *Terrestrisuchus gracilis*. A, right dentary in lateral view and left dentary in medial view of NHMUK PV R 7557. B, right dentary of NHMUK PV R 7561 in ventrolateral view. C, right dentary of NHMUK PV R 38290b in medial view. *Abbreviations*: al c, alveolar canal; for, foramen; fr, frontal; idp, interdental plate; lr, longitudinal ridge; Mk g, Meckelian groove. Scale bars represent 10 mm.</figcaption></figure>

most of the anteroposterior length of the splenial, but it gradually diminishes anteriorly to a poorly developed ridge in the anterior portion of the element. Posteriorly the ventral margin is not completely preserved, but the folding also appears less well pronounced in this region of the bone. The dorsolateral surface of the splenial within this fold is concave, forming a shallow groove along the ventral margin of the bone.

The posterior margin of the splenial is bifurcated. The posteroventral process is considerably larger than the

posterodorsal process and it is curved posterodorsally, as in CM 29894 (Clark *et al.* 2000, fig. 2). This process is forked at its posterior end, with the posterodorsal projection reaching further posteriorly than the posteroventral projection. Here, the splenial articulated with the angular. The posterodorsal process is poorly preserved but appears to terminate in a sharp point. This process articulated with the surangular and possibly the coronoid (Dollman *et al.* 2019). The notch formed between both posterior processes was considered to form the articulation with
<figure><img src="image_52.png" /><figcaption>**FIG. 39.** Right splenial of NHMUK PV R 37734 in lateral view. Note that the element identified as con? was identified as a coronoid by Crush (1980), but this interpretation is not corroborated here. *Abbreviations:* con, coronoid; pdp, posterodorsal process; pvp, posteriorventral process; vf, ventral fold. Scale bar represents 10 mm.</figcaption></figure>

the coronoid according to Crush (1980, 1984), although it seems more likely that this notch mostly formed the anterior margin of the internal mandibular fenestra, as is the case in CM 29894, *Junggarsuchus sloani* and *Protosuchus haughtoni* (Clark et al. 2000; Dollman et al. 2019; Ruebenstahl et al. 2022), as was also suggested by Allen (2010).

**Surangular.** The preserved section of the right surangular of NHMUK PV R 7591 forms an elongate plate-like bone (Fig. 37A, D, E). Its ventral margin formed the dorsal margin of the external mandibular fenestra. The preserved ventral margin is almost certainly incomplete. The dorsal margin of the surangular is very slightly convex in lateral view (Fig. 37D), which contrasts with the dorsally widely rounded surangulars seen in other early crocodylomorphs such as *Saltoposuchus connectens*, *Trialestes romeri*, *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and, in particular, *Junggarsuchus sloani* (Walker 1990; Lecuona et al. 2016; Ruebenstahl et al. 2022; Spiekman 2023). The lateral surface is smooth and virtually completely flat; a surangular foramen, as seen in *Junggarsuchus sloani* (Ruebenstahl et al. 2022), is absent. The medial surface is gently concave and it is characterized by a distinct thickening of its dorsal margin near the anterior end of the preserved bone (Fig. 37E). This thickening is positioned posterior to the ectopterygoid in medial or ventral view, and it therefore almost certainly represents the coronoid process of the surangular. The surangular coronoid process is not visible in lateral view, as is typical for crocodylomorphs with the possible exception of *Sphenosuchus acutus* (Walker 1990). The cross-section of this thickening is subcircular in outline. The concave medial surface

ventral to the thickening formed part of the lateral surface of the adductor fossa of the mandible. According to Crush (1980), the surangular was overlapped by the dentary near its anterior end in lateral view, and this condition can still be observed in the remaining anterior portion of the mandible. In *Junggarsuchus sloani*, in contrast, the surangular laterally overlaps the dentary and its anterior end (Ruebenstahl et al. 2022). The posterior end of the surangular is convex and somewhat rectangular in outline. It articulates with the dorsal shelf of the angular and is slightly overlapped by it in lateral view (Fig. 37D).

**Angular.** The angular is an elongate, thin bone that formed the ventral portion of the mandible posterior to the dentary in lateral view (Fig. 37). The anterior end of the angular was probably laterally overlapped by the posterior end of the dentary (Fig. 37C, F). Although accurate observations are hampered because this region is damaged, this interpretation is corroborated by Crush (1980, fig. 153), who studied this specimen prior to the damage. Furthermore, this corresponds with the articulation seen in *Carnufex carolinensis*, *Litargosuchus leptorhynchus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Drymala & Zanno 2016; Dollman et al. 2019; Ruebenstahl et al. 2022). Much of the dorsal margin of the angular forms the ventral margin of the external mandibular fenestra in *Terrestrisuchus gracilis*, as is the case in crocodylomorphs generally. The angular extends far posteriorly beyond the glenoid fossa, similar to the condition in other crocodylomorphs (*Dromicosuchus grallator*, *Sphenosuchus acutus* and *Junggarsuchus sloani*), with the possible exception of *Dibothrosuchus elaphros*, in which it might not have reached posteriorly beyond the glenoid fossa
(Walker 1990; Sues *et al.* 2003; Ruebenstahl *et al.* 2022). In lateral view the angular articulates dorsally with the surangular posterior to the external mandibular fenestra (Fig. 37D, F). Posterior to this, the angular forms a long articulation with the articular dorsal to it until approximately the anteroposterior halfway point of the retroarticular process on the latter, where the angular terminates. Medially, the angular forms an articulation with the splenial anteriorly, and directly posterior to this with the prearticular (Fig. 37E).

The angular maintains its dorsoventral height for most of its length, with both the ventral and dorsal margins being straight. However, the posterior portion of the angular is distinctly expanded, with its distal end being directed posterodorsally (Fig. 37B), as is also the case in other crocodylomorphs (e.g. Walker 1990; Ruebenstahl et al. 2022). This expanded section forms the articular surface for the articular. Here, the angular is lateromedially very thin and the medial surface is predominantly dorsally facing. The posterior end of the angular tapers into a sharp point (Fig. 37B, C). The medial surface of the angular is characterized by a thickened ventral margin, which has a roughly oval cross-section. It extends along the entire length of the preserved right angular, except for the expanded posterior portion of the bone. The medial surface of the angular directly dorsal to this thickened ventral margin is distinctly concave. This concave surface formed the articulation facet for the prearticular, although near the anterior end of the element it might also have formed part of the Meckelian groove.

The lateral surface is convex along its entire anteroposterior length, with this convexity being most prominent ventrally, where it forms the ventral margin of the mandible (Fig. 37C). A faint groove is present on the lateral surface of the angular. It is located in the middle portion of the element and it is oriented anteroventrally to posterodorsally. This groove probably represents the articular surface for the dentary. The anterior end of the angular gradually tapers anteriorly, forming a sharp point. A distinct longitudinal ridge on the lateral surface of the angular, as seen in *Carnufex carolinensis* and *Junggarsuchus sloani* (Drymala & Zanno 2016; Ruebenstahl et al. 2022), is absent in *Terrestrisuchus gracilis*.

**Prearticular.** Like much of the angular and all of the dentary, the prearticular of NHMUK PV R 7591 was broken following the descriptions of Crush (1980) and Allen (2010), and although it has subsequently been repaired, the preserved anterior portion of the bone is badly damaged. As in *Junggarsuchus sloani* and *Dibothrosuchus elaphros* (Ruebenstahl *et al.* 2022), the suture between the prearticular and articular posterior to it is difficult to establish even with the aid of µCT data, suggesting a tight articulation between these elements. The

prearticular is covered ventrally by the angular and laterally by the surangular (Fig. 37E). The lateral surface of the prearticular is strongly concave and the medial surface is strongly convex. Correspondingly, the right prearticular has an inverted C-shaped cross-section in anterior view. Near the posterior end of the prearticular, the medioventral end of the bone bears a distinct, plate-like dorsally directed process of unknown function. Based on the description of Crush (1980), the prearticular decreases in dorsoventral height anteriorly. Anteriorly, the ventral part of the prearticular is covered by the angular in lateral view, and further anteriorly, it is completely covered medially by the splenial. Posteriorly, the prearticular dorsally overlaps the articular.

**Articular.** The articular is characterized by a pronounced retroarticular process that is strongly upturned dorsally and medially at its posterior end, and by the presence of a large canal perforating the body of the articular (Figs 37, 40). This canal is positioned on the medial side of the articular, just posterior to the glenoid fossa, and it opens on the dorsal and ventromedial sides of the articular (Fig. 40B-D). This undoubtedly represents the canal for the articular diverticulum, which is also present in extant crocodylians and which connects to the extensive pneumatic network of the posterior end of the skull (DufEAU & WITMER 2015). Interestingly, openings in the articular for the articular diverticulum are absent in all other known non-crocodyliform crocodylomorphs for which the articular is well preserved (*Sphenosuchus acutus*, *Dibothrosuchus elaphros*, *Junggarsuchus sloani*; WALKER 1990; Ruebenstahl *et al.* 2022). The dorsal opening of the articular diverticulum is roughly circular in dorsal view (Fig. 40C) whereas the ventromedial opening is oval, being anteroposteriorly longer than transversely wide in ventral view (Fig. 40D). The two openings are separated by a thin bony bridge that is anteroposteriorly oriented directly posterior to the glenoid fossa in medial view (Fig. 40B). This bridge is gently curved, and it consequently has a concave dorsal margin and convex ventral margin in medial view.

The glenoid fossa occupies the dorsal surface of the anterior end of the articular and it is trapezoidal in outline in dorsal view, being widest at its anterior end (Fig. 40C). Its surface is roughened and slightly concave, except for its lateral and anteromedial corners, which are slightly convex. The posterior margin of the glenoid fossa is demarcated by an elevated posterior lip, which is most prominent directly anterolateral to the dorsal opening for the articular diverticulum. Here, the lip faces prominently dorsally, with a concave anterior face and convex posterior surface in lateral view (Fig. 40A). The posterior portion of the glenoid fossa forms the roof of the articular diverticulum (Fig. 40B). The anterior surface of the
<figure><img src="image_53.png" /><figcaption>FIG. 40. Digitally reconstructed left articular of NHMUK PV R 7591 in: A, lateral; B, medial; C, dorsal; D, ventral view. *Abbreviations*: anf, angular facet; cri, central ridge; dfad, dorsal foramen for the articular diverticulum; glf, glenoid fossa; kl, keel; lri, lateral ridge; mli, medial ridge; praf, prearticulation facet; rap, retroarticular process; uerap, upturned end of retroarticular process; vmfad, ventromedial foramen for the articular diverticulum. Scale bar represents 5 mm.</figcaption></figure>

articular, which articulated with the posterior part of the prearticular (Fig. 37E), is rounded and triangular in anterior view.

The expanded region posterior to the glenoid fossa is the retroarticular process. The dorsal surface of the retroarticular process is characterized by three ridges. Two ridges form the lateral and medial margins of the articular, respectively, whereas another longitudinal ridge is positioned on the central portion of the dorsal surface of the retroarticular process (Fig. 40C). Near its anterior end this ridge frames the lateral side of the dorsal foramen of the articular diverticulum. The portion of the retroarticular process positioned medial to the central ridge forms a thin flange that has a strongly concave dorsal surface and correspondingly a convex ventral surface in its anterior portion. The dorsal foramen of the articular diverticulum is placed at the anterior end of this flange. Posteriorly, this ventral surface transitions into the smooth face of the upturned posterior end of the retroarticular process, which is distinctly concave at its base and roughly flat near its apex (Fig. 40B). A strongly

upturned retroarticular process is also present in *Carnufex carolinensis*, *Litargosuchus leporhynchus*, *Sphenosuchus acutus* and *Dibothrosuchus elaphros* among non-crocodyliforms (Walker 1990; Drymala & Zanno 2016; Ruebenstahl et al. 2022; SNFS pers. obs., BP/I/5237), whereas the retroarticular process is only marginally upturned in *Trialestes romeri*, *Dromicosuchus grallator* and *Junggarsuchus sloani* (Sues et al. 2003; Lecuona et al. 2016; Ruebenstahl et al. 2022). The medial margin of the retroarticular process forms a distinct and dorsoventrally thin ridge (Fig. 40B). This medial ridge is distinctly sinusoidal in dorsal view, with a convex anterior portion and a concave posterior portion that contributes to the upturned posterior end of the process (Fig. 40C). The portion of the retroarticular process lateral to the central ridge is comparatively much thicker (Fig. 40A). Its dorsal surface is wide, smooth and concave (Fig. 40C). This surface is particularly concave and deeply excavated at its posterior end, where it forms the base of the upturned portion of the retroarticular process. The ventral surface of this part of the retroarticular process is directed ventrolaterally.