<figure><img src="image_54.png" /><figcaption>FIG. 41. Selected tooth-bearing elements of *Terrestrisuchus gracilis*. A, part of left maxilla of NHMUK PV R 38290a in medial view. B, right dentary in lateral view and left dentary in medial view of NHMUK PV R 7557. *Abbreviations*: rt, replacement tooth; sr, serration. Scale bars represent 5 mm.</figcaption></figure>

Much of this surface forms the articulation facet for the angular (Figs 37D, 40A). This facet is slightly depressed relative to the rest of the surface, and it is demarcated on its posteromedial corner by a lip with a concave antero-lateral surface and a convex posteromedial surface. It appears that the surangular did not contact the articular. The retroarticular process tapers to a point distally at its upturned end, which is directed dorsally, medially and slightly anteriorly (Fig. 40A–C). The posteroventral end of the retroarticular process is formed by a short, knob-like keel (Fig. 40D).

## Dentition

The alveoli of both the maxilla (Fig. 14B) and dentary have an oval outline, being mesiodistally longer than labiolingually wide. They are closely packed together and are separated only by a narrow bony bridge. Although all

alveoli are filled with matrix, it can be inferred that they formed deep sockets and that tooth implantation was the-codont. As was also indicated by Crush (1984), 15 empty alveoli are preserved in the complete dental margin of the maxilla of NHMUK PV R 7557c, as well as an empty groove at the posterior end of the tooth-bearing margin that might have accommodated three additional teeth. Twelve teeth are visible on the right dentary of NHMUK PV R 7557 and 10 on the left dentary (Fig. 41B), whereas only two to four functional *in situ* teeth are present on the dentary of NHMUK PV R 38290b. The exact number of dentary teeth cannot be inferred. Crush (1980, 1984) reconstructed a total number of 24 teeth for the dentary but indicated that this number might be an overrepresentation.

All *in situ* preserved teeth of *Terrestrisuchus gracilis* are recurved with a sharply pointed apex. As in most non-crocodyliform crocodylomorphs, the posterior teeth on both the maxilla and dentary have a slightly convex distal
margin or carina (Fig. 41). The maxillary teeth are labio-
lingually compressed and bear clear serrations on their
distal carina and very poorly developed serrations on
their mesial carina (Fig. 41A). These serrations are present
along much of the distal carina, except for its apicalmost
portion. On the mesial carina, poorly developed serra-
tions are present only proximal to the tip, slightly distal
to the most curved region of the tooth in labial or lingual
view. The maxillary teeth of most non-crocodyliform cro-
codylomorphs bear serrations along both the mesial and
distal carinae and they extend further apically, almost
reaching the apex (e.g. *Saltoposuchus connectens*, Spiek-
man 2023; CM 29894, *Hesperosuchus agilis*, Colbert 1952;
*Dromicosuchus grallator*, Sues *et al.* 2003; *Redondavenator
quayensis*, Nesbitt *et al.* 2005; *Sphenosuchus acutus*,
Walker 1990; *Junggarsuchus sloani*, Ruebenstahl *et al.*
2022), although both the premaxillary and maxillary teeth
of *Kayentasuchus walkeri* lack serrations (Clark &
Sues 2002). In *Dibothrosuchus elaphros* the premaxillary
teeth lack serrations and most maxillary teeth bear poorly
developed serrations on both carinae, except for the
enlarged caniniform maxillary teeth, which bear serrations
only on the distal carina (Wu & Chatterjee 1993). In the
anterior part of the jaw the dentary teeth are less labiolin-
gually compressed and more slender than the maxillary
teeth (Fig. 41B). The teeth become gradually more labio-
lingually compressed and mesodistally wider posteriorly.
The distalmost teeth are the widest and have a slightly
convex distal margin. There is no enlarged anterior
fang-like tooth, in contrast to *Saltoposuchus connectens*,
*Hesperosuchus agilis*, CM 29894, *Dibothrosuchus elaphros*
and *Junggarsuchus sloani* (Colbert 1952; Clark *et al.* 2000;
Ruebenstahl *et al.* 2022; Spiekman 2023). Instead, there is
little difference in tooth height along the preserved tooth-
bearing margin in the dentary, and the homogeneous size
of the alveoli suggests a similar configuration for the
maxilla. Most of the dentary teeth are insufficiently
preserved to determine confidently whether they have
well-developed carinae and bear serrations. However, ser-
rations can be observed on the basal portion of the pos-
terior margin of the seventh tooth counted from anterior
on the right dentary of NHMUK PV R 7557. The two
dentary teeth of NHMUK PV R 38290b are very well pre-
served and located on the anterior half of the tooth-
bearing margin, and correspondingly are slender and have
little labiolingual compression. These teeth possess less
well-developed carinae than the more posterior teeth and
no serrations can be discerned on either margin.

A mesiodistally directed shallow groove is positioned
lingual to the alveoli in the maxilla (Fig. 14B). It is widest
lateral or labial to the palatal process of the maxilla and
terminates at the third alveolus counted from posterior,
excluding the groove that might have housed three fur-
ther distalmost teeth. At certain tooth positions small

resorption pits can be observed on the labial wall of this
groove that are positioned directly lingual to the alveolus
and which are confluent with it. This is most clearly visi-
ble at the second alveolus counted from anterior in
NHMUK PV R 7572b (Fig. 14D). The crown of a small
replacement tooth is present lingual to one of the func-
tional teeth in the left maxilla of NHMUK PV R 38290a
(Fig. 41A).

Up to six interdental plates can be observed in the
right dentary of NHMUK PV R 38290b (Fig. 38C). They
have a triangular outline in medial view and extend dor-
sal to the dental margin of the dentary. They can be dis-
tinguished from the teeth by the lack of an enamel layer.
Interdental plates are known for *Carnufex carolinensis*,
*Sphenosuchus acutus* and *Hesperosuchus agilis* among non-
crocodyliform crocodylomorphs (Colbert 1952; Walker
1990; Clark *et al.* 2000; Drymala & Zanno 2016) but have
not previously been identified in *Terrestrisuchus gracilis*
(Crush 1980; 1984).

Hyobranchial apparatus

In medial or ventral view the remains of two elongate
hyoid elements, representing the paired ceratobranchial I,
are preserved in NHMUK PV R 7591 (Fig. 10B). The
right ceratobranchial I is virtually intact, whereas the left
element is broken in several places and considerably dis-
placed. The anterior portion of the left ceratobranchial I
can also be observed in right lateral view inside the orbit
(Fig. 10A). Two plate-like elements are preserved at the
anterior end of the left ceratobranchial I, which are over-
lying the anterior end of the right jugal. These plates rep-
resent an ossified basihyal (Li & Clarke 2015).

Because the hyoid elements of NHMUK PV R 7591 are
broken in several places but not distorted, it was possible
to digitally reassemble the apparatus in its original posi-
tion (Fig. 42). Ceratobranchial I is gracile and elongate,
being approximately as long as the entire parabasisphe-
noid (Fig. 10B; Table S2), which contrasts with the much
shorter ceratobranchial length in crocodyliforms (e.g.
*Simosuchus clarki*) and crown-crocodylians (Kley
*et al.* 2010; Li & Clarke 2015). The ceratobranchials curve
posterolaterally from the basihyal (Fig. 42A). At approxi-
mately half way along their anteroposterior length the
ceratobranchials no longer curve laterally, but rather
posterodorsally (Fig. 42B). The shaft is roughly circular in
cross-section. The basihyal is incomplete. Its left portion,
which is preserved in direct association with the left cera-
tobranchial I, is best-preserved (Fig. 42A). Its anterior
margin is distinctly convex in ventral view, and its lateral
and posterior margins are incomplete. No obvious open-
ings or notches are preserved on the basihyal body. Its
ventral surface is distinctly convex whereas its dorsal
<figure><img src="image_55.png" /><figcaption>**FIG. 42.** Digitally reconstructed and reassembled hyobranchial apparatus of NHMUK PV R 7591 in: A, ventral; B, left lateral; C, left anterodorsolateral view. *Abbreviations:* bshy, basihyal; crb, ceratobranchial; l, left; r, right. Scale bar represents 10 mm.</figcaption></figure>
surface is concave, as is the case in the cartilaginous basihyal of extant crocodylians (Li & Clarke 2015). The ossified basihyal is considerably smaller relative to the cer- atobranchials and to general skull size than in extant cro- codylians, and instead it is proportionally similar to the basihyals of birds and pterosaurs (Hill et al. 2015; Jiang et al. 2020). However, it cannot be excluded that the basihyal included a more extensive, cartilaginous portion in *Terrestrisuchus gracilis*.

## RESULTS OF THE SCLEROTIC RING ANALYSIS

The pFDA analysis with two activity bins had a mean accuracy of 92% for the classification of extant species as either nocturnal or non-nocturnal. The strongest support was found for *Terrestrisuchus gracilis* as non-nocturnal, with a mean posterior probability of 0.67 (range, 0.61–0.73). When an additional diel activity bin was added, the mean accuracy of classification was 84% and the strongest support was found for *Terrestrisuchus gracilis* as cathemeral, with a mean posterior probability of 0.54 (range, 0.40–0.60). The mean posterior probability that *Terrestrisuchus gracilis* was diurnal was 0.28 (range, 0.25–0.33). The mean posterior probability for

nocturnal activity in *Terrestrisuchus* was 0.18 (range, 0.15–0.24).

## DISCUSSION

*Inferred diel activity and palaeoecology of Terrestrisuchus gracilis*

This is the first time that diel patterns in an extinct cro- codylomorph have been assessed, and this is possible because of the unique occurrence of a largely intact scle- rotic ring in *Terrestrisuchus gracilis*. The results of our analyses indicate the greatest support for the skeletally immature individual of *Terrestrisuchus gracilis* that we have analysed here as being non-nocturnal and potentially cathemeral, with activity in a range of light levels (Fig. 43). *Terrestrisuchus gracilis* probably shared the pred- ator niche on the islands of the Bristol Channel archipel- ago with a small coelophysoid theropod, *Pendraig milnerae* (Spiekman et al. 2021), and it is conceivable that these two taxa were ecologically partitioned. The skull of *Pendraig milnerae* is not preserved, but a sclerotic ring is found in the closely related coelophysoid *Megapnosaurus rhodesiensis*. *Megapnosaurus rhodesiensis* is found to be nocturnal, with high (>0.99) posterior probabilities in

<figure><img src="image_56.png" /><figcaption>**FIG. 43.** Scatterplot showing the relationship between optic ratio (lens diameter-squared/(eyeball diameter × axial length)), sensu Schmitz & Motani 2010) and the geometric mean of orbit and sclerotic ring measurements for extant birds, squamates and selected fossil taxa, including *Terrestrisuchus*.</figcaption></figure>
both analyses 4 and 5 of Choiniere *et al.* (2021). Due to the general morphological similarity of coelophysoid theropods, it is possible that the whole clade was nocturnal and *Pendraig milnerae* was active during the night, while *Terrestrisuchus gracilis* hunted both day and night.

## Quadratojugal and the configuration of the infratemporal fenestra

The morphology of the quadratojugal (Fig. 24) and its relative position in the skull is remarkable, given that it occupies almost all of the space of the infratemporal fenestra in lateral view (Figs 11A, 13A). However, because the quadratojugal is oriented ventrolaterally to dorso- medially, similar to the orientation of the quadrate shaft, the quadratojugal is mostly placed in the space of the infratemporal fenestra rather than covering it laterally. The quadratojugal was also described as being positioned inside the infratemporal fenestra in *Dromicosuchus gralla- tor* (Sues *et al.* 2003), and a similar condition is also pre- sent in CM 29894 (Clark *et al.* 2000). Unfortunately, as for *Terrestrisuchus gracilis*, the exact extent of the quadra- tojugal is unknown for both taxa. Together with the anterodorsal flange of the quadrate, which articulated with the medial margin of the squamosal (= lateral mar- gin of the supratemporal fenestra), the quadratojugal would have formed a partial separation between the supratemporal fenestra and the lateral opening of the infratemporal fenestra. This potentially restricted the space occupied by the adductor chamber where the man- dibular adductor musculature is housed (Holliday & Witmer 2007; Holliday *et al.* 2013). However, it remains unknown to what extent the relative size of the mandibu- lar adductor musculature was restricted, given that this region is not adequately preserved in any known non- crocodyliform crocodylomorph.

## Pneumatic cavities

The presence of osteological correlates for pneumatic sinuses and diverticula in the skull is well established for archosaurs, and probably goes back to non-archosaurian archosauromorphs (Witmer 1997; Sobral & Müller 2019). Among pseudosuchians, some limited pneumaticity is present in the braincase among non-crocodylomorphs on the ventral side of the basioccipital and parabasisphenoid (Gower 2002; Leardi *et al.* 2020), but extant crocodylians possess an extensive system of paratympanic (i.e. relating to the middle ear) sinuses, which might be related to increased acoustic capabilities (Dufeu & Witmer 2015). The presence of extensive braincase pneumaticity was previously established in the non-crocodyliform

crocodylomorph *Sphenosuchus acutus* from the Lower Jurassic of South Africa (Walker 1990), and more derived crocodylomorphs such as *Macelognathus vagans*, *Almada- suchus figarii*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* also have extensive braincase pneumatization that additionally includes a highly pneumatized quadrate and parabasisphenoid (Leardi *et al.* 2017, 2020; Ruebenstahl *et al.* 2022). Some evidence for extensive pneumatization is also present in the poorly known *Kayentasuchus walkeri* (Clark & Sues 2002). However, the cranial morphology of the earliest crocodylomorphs was previously insufficiently documented to determine the extent of braincase pneu- maticity in these forms, and therefore the origin of this unique crocodylomorph sinus system remained poorly understood (Leardi *et al.* 2020). The skull of NHMUK PV R 7591 as reconstructed with the µCT data enables us to study braincase pneumaticity in *Terrestrisuchus gracilis* in detail for the first time.

The recesses on the ventral side of the basioccipital and parabasisphenoid of *Terrestrisuchus gracilis* represent a rel- atively basal condition among Crocodylomorpha. As in all other known non-crocodyliform crocodylomorphs, the basioccipital recess (s.s. following Leardi *et al.* 2020) excav-ates the ventral surface of the basioccipital. However, compared with other taxa (e.g. *Sphenosuchus acutus*, *Almadasuchus figarii*; Walker 1990; Leardi *et al.* 2020) this recess is relatively shallow and it lacks the distinct longi- tudinal ridge that divides the recess into two separate chambers (Fig. 31C, I). The recess is anteriorly framed by the anterior margin of the basioccipital, which forms a transverse lamina that is directed mostly ventrally (Fig. 31H). On the parabasisphenoid, the sub- basisphenoid recess forms a shallow longitudinal trough across the ventral surface of the parabasisphenoid body, posterior to the basipterygoid processes (Fig. 35D, F). A similar morphology is common among many archosaurs, including non-crocodylomorphs pseudosuchians (e.g. *Postosuchus alisonae* and *Batrachotomus kupferzellensis*; Gower 2002; Nesbitt 2011, described as the median pha- ryngeal recess or parabasisphenoid recess therein), and also occurs in *Saltoposuchus connectens*, *Sphenosuchus acu- tus* and thalattosuchians (Walker 1990; Herrera *et al.* 2018; Spiekman 2023). However, such a trough is lacking in derived non-crocodyliform crocodylomorphs. In contrast, in *Almadasuchus figarii* and *Junggarsuchus sloani* a recess is formed on the posterior surface of the parabasisphe- noid that is directly connected to the recess on the basioccipital posterior to it; the recess on the parabasi- sphenoid is not visible in ventral view in these taxa (Leardi *et al.* 2020; Ruebenstahl *et al.* 2022). Although the posterior end of the parabasisphenoid is incomplete in NHMUK PV R 7591, such a posterior opening was almost certainly absent in *Terrestrisuchus gracilis*. In *Sphe- nosuchus acutus* this opening was also absent, but instead
the connection between the basioccipital recess s.s. and the trough-like sub-basisphenoid recess was formed by a triangular articulation between the basioccipital and parabasisphenoid in ventral view (Walker 1990, fig. 20). In this configuration the posterolateral ends of the parabasisphenoid articulate onto the basal tubera of the basioccipital, thus creating a space on the ventral surface between the basioccipital and parabasisphenoid that can be considered a basioccipital recess s.l. following Leardi et al. (2020), also known as the medial Eustachian foramen. Although the parabasisphenoid and basioccipital articulation is not preserved in *Terrestrisuchus gracilis* in NHMUK PV R 7591 (Fig. 10B), the condition of *Terrestrisuchus gracilis* appears similar to that seen in *Sphenosuchus acutus* (Fig. 30B). However, the basioccipital recess s.s. in *Terrestrisuchus gracilis* is not as deeply excavated, and the angle formed between the basioccipital and parabasisphenoid is more obtuse than in *Sphenosuchus acutus* (Fig. 29B-D). Consequently, the basioccipital recess s.l., formed by the recess on the basioccipital and the sub-basisphenoid recess, appears to be much shallower in *Terrestrisuchus gracilis*, representing a condition that is intermediate between that of *Sphenosuchus acutus* and that seen in non-crocodylomorph pseudosuchians such as *Batrachotomus kupferzellensis* (Gower 2002). As in *Sphenosuchus acutus* (Walker 1990), there is also no connection between the basioccipital recess and the pneumatic cavities of the precarotid and postcarotid recesses of the parabasisphenoid in *Terrestrisuchus gracilis*, whereas such a connection has been established for *Almadasuchus figarii* and possibly *Junggarsuchus sloani* (Leardi et al. 2020; Ruebenstahl et al. 2022). The Eustachian system therefore did not reach through the basioccipital recess into the paratympanic sinus system.

The pneumatization in the body of the parabasisphenoid is extensive, with remarkably large precarotid and postcarotid recesses that are partially separated by the carotid pillar (Fig. 35A, B, E). The precarotid recess invades the large basipterygoid processes, which are completely hollow, as well as the base of the cultriform process, where it forms the rostral recess. A similar extension of the pneumatization into the basipterygoid processes and cultriform process is also present in other non-crocodyliform crocodylomorphs, whereas the latter is absent in non-crocodylian crocodyliforms and *Almadasuchus figarii* (Leardi et al. 2020).

The lateral surface of the prootic bears a single facial recess and two inter tympanic recesses ventral to the crista prootica (Fig. 34A, C). Both these recesses are also present in other early crocodylomorphs, with the inter tympanic recess also being subdivided into two fossae in *Almadasuchus figarii* (Leardi et al. 2020). These facial and inter tympanic recesses would have housed separate diver-

ticula of the pharyngotympanic sinus system based on comparisons with extant crocodylians (DufEAU & Witmer 2015; Kuzmin et al. 2021). In addition, *Terrestrisuchus gracilis* also possesses precochlear cavities, which were also reported for *Sphenosuchus acutus* and which were ascribed a pneumatic function by Walker (1990). No homologous structure to these cavities is present in extant crocodylians nor in other extinct crocodylomorphs, and therefore the exact relation to the pharyngotympanic sinus system is difficult to establish. Based on their relative position and association with the facial nerve, they probably facilitated the prootic diverticulum, like the facial recess directly dorsal to the precochlear cavities (Fig. 34A). Because of the strong deformation of the laterosphenoid it is not possible to assess the presence of pneumatization in this element.

As in *Sphenosuchus acutus*, a separate, true rhomboidal recess is not present on the otoccipital of NHMUK PV R 7591, in contrast to the condition in more derived crocodylomorphs such as *Almadasuchus figarii* and *Junggarsuchus sloani*, and crocodyliforms (Walker 1990; Leardi et al. 2020; Kuzmin et al. 2021; Ruebenstahl et al. 2022). In these more derived taxa the rhomboidal recess forms part of the surface for the lateral pharyngotympanic canal or (lateral) Eustachian canal, through which the median pharyngeal system (i.e. the pneumatized space connecting to the pharynx) is connected to the pharyngotympanic sinus system (i.e. the enlarged and pneumatized space that includes the middle ear region) (Colbert 1946; DufEAU & Witmer 2015; Kuzmin et al. 2021). In the reconstructed braincase of NHMUK PV R 7591 a space is enclosed medially by the basioccipital, laterally and ventrally by the cochlear flange of the parabasisphenoid, and dorsally by the ventral margin of the perilymphatic loop of the otoccipital, as is visible in posterior view (Fig. 29B). This space might have formed the pharyngotympanic canal in *Terrestrisuchus gracilis*, representing a similar condition as that described for *Sphenosuchus acutus*, which also lacks a true rhomboidal recess (Walker 1990). The excavated anterior surface of the paroccipital process of the otoccipital represents the posterior tympanic recess (Fig. 33A, E), although this excavation is not as well developed as in more derived crocodylomorphs, except for *Sphenosuchus acutus* (Walker 1990; Leardi et al. 2017; Leardi et al. 2020).

As in extant crocodylians, the pharyngotympanic sinus system of *Terrestrisuchus gracilis* extends onto the quadrate through the infundibular and quadrate diverticula and, subsequently, through the siphonium, onto the articular of the mandible (DufEAU & Witmer 2015). *Terrestrisuchus gracilis* possesses a large and deeply excavated internal quadrate fenestra on the posterior surface of its shaft. However, it lacks an extensively pneumatized body that is largely hollow internally, as is the case in *Dibothrosuchus elaphros*, *Maeclagnathus vagans*, *Almadasuchus*
figarii, *Junggarsuchus sloani*, and crocodyliforms, and it is thus more similar to the condition in *Sphenosuchus acutus*, *Saltoposuchus connectens* and *Hesperosuchus agilis*, which also lack internal pneumatization of the quadrate (Walker 1990; Leardi et al. 2017; Leardi et al. 2020; Kuzmin et al. 2021; Ruebenstahl et al. 2022; Spiekman 2023). The articular of *Terrestrisuchus gracilis* possesses very large pneumatic spaces, which is remarkable given that articular pneumaticity has previously not been established in non-crocodyliform crocodylomorphs, despite relatively well-preserved articulars being known in *Sphenosuchus acutus*, *Dibothrosuchus elaphros* and *Junggarsuchus sloani* (Walker 1990; Ruebenstahl et al. 2022). Pneumatic cavities in the articular are present in extant crocodylians, where they decrease in relative size through ontogeny. In *Alligator mississippiensis* the articular diverticulum occupies c. 50% of the bone volume in perinatal specimens and only 2% in large individuals (Dufeu & Witter 2015). The presence of such a large articular diverticulum could therefore indicate an immature ontogenetic state for NHMUK PV R 7591, although this cannot be stated confidently given that the pharyngotympanic sinus system in non-crocodyliform crocodylomorphs is markedly different from that of extant crocodylians in many respects (Walker 1990; Leardi et al. 2020; Kuzmin et al. 2021).

Walker (1990, p. 75) provided a list of the cranial pneumatic cavities of *Sphenosuchus acutus*. Most pneumatic features for the braincase listed there are also present in *Terrestrisuchus gracilis*. Only the ‘postfenestral depression’, a fossa positioned on the otoccipital and prootic posterior to the fenestra ovalis, could also not be observed, but this feature has also not been reported in any other crocodylomorph taxon. In contrast, *Terrestrisuchus gracilis* has considerably more pneumaticity in the articular than reported for other crocodylomorphs. As such, braincase pneumaticity was already well established among Crocodylomorpha during the Late Triassic and represents an early feature in crocodylomorph evolution.

### *Hyoid apparatus*

In crocodylians the tongue is firmly attached to the bucal floor by connective tissue and muscles, thus severely limiting tongue movement (Li et al. 2018). This is thought to be related to their hypercarnivorous diet, and consequently the hyoid of crocodylians is relatively simple and poorly ossified, generally having only a single pair of relatively short rod-like ceratobranchials, with a cartilaginous mid-line hyobranchial body (Hill et al. 2015; Li & Clarke 2015; Li et al. 2018). In fact, no ossified hyobranchial body has previously been described in extinct and extant Crocodylomorpha (Li et al. 2018). Among early

crocodylomorphs a curved ceratobranchial I is present in *Protosuchus richardsoni* (Colbert & Mook 1952). The ceratobranchial I of *Protosuchus richardsoni* is similarly slender and curved as in NHMUK PV R 7591, but it is comparatively less elongate. A small, gracile fragment of a ceratobranchial is also present in *Dromicosuchus grallator* but its relative length cannot be established (Sues et al. 2003). Among archosauromorphs ossified basihyals were previously known to occur only in avemetatarsalians (Hill et al. 2015; Li et al. 2018). In other known non-avemetatarsalian archosauromorphs, the ossified hyoid apparatus consists exclusively of simple rod-like ceratobranchials I that can be of considerable length in both pseudosuchians and non-archosaurian archosauromorphs (e.g. Li et al. 2006; De França et al. 2013; Butler et al. 2015; Li & Clarke 2015; Spiekman et al. 2020). Thus, *Terrestrisuchus gracilis* represents the first known occurrence of an ossified basihyal in Archosauromorpha outside Avemetatarsalia. The presence of elongated hyoids and an ossified mid-line hyobranchial body in *Terrestrisuchus gracilis* could suggest that the tongue was more flexible and less restricted than that of extant crocodylians (Li et al. 2018).

### CONCLUSION

We have provided a detailed revision of the taxonomy and the cranial anatomy of *Terrestrisuchus gracilis*, a small-bodied and highly gracile early crocodylomorph. *Terrestrisuchus gracilis* is restricted to the Upper Triassic fissure fill locality of Pant-y-Ffynnon in southern Wales. Crocodylomorph remains from surrounding fissure fill localities are either unidentifiable (Crocodylomorpha indet., Ruthin, Durdham Down and Tytherington quarries) or are likely to represent at least one new taxon (Cromhall Quarry) that will be described elsewhere. The complete hypodigm of *Terrestrisuchus gracilis* is provided in Table S1, a revised diagnosis is formulated, and the most complete specimens are figured (Figs 2–9). The morphological findings presented in this study were also recently integrated into a phylogenetic analysis of non-crocodyliform crocodylomorphs (Spiekman 2023; matrix modified from Leardi et al. 2017), which included updated scorings for *Terrestrisuchus gracilis* and *Saltoposuchus connectens*.

*Terrestrisuchus gracilis* possesses a narrow skull that lacks certain features present in other non-crocodyliform crocodylomorphs such as enlarged anterior maxillary teeth, a sagittal crest, and prominent longitudinal crests on the frontal and squamosal. A µCT scan of the well-preserved partial skull of NHMUK PV R 7591 yielded much additional information on the braincase anatomy of *Terrestrisuchus gracilis*. It showed that the posterior
skull region was extensively pneumatized, suggesting that complex braincase pneumatization occurred earlier in crocodylomorph evolution than previously considered (Leardi et al. 2020).

Furthermore, the µCT data also showed the presence of an ossified sclerotic ring and hyoid apparatus. The hyoid apparatus is composed of elongate, paired ceratobranchials I and an ossified basihyal. The latter represents the first identification of this structure among non-avemetatarsalian archosaurs. Similarly, the sclerotic ring represents one of only two occurrences of this structure in a non-marine crocodylomorph (also present in *Junggarsuchus sloani*, Nesbitt et al. 2013), and it enabled us to investigate the diel activity patterns in an extinct crocodylomorph for the first time. Our quantitative sclerotic ring analyses show greatest support for non-nocturnal, possibly cathemeral activity patterns for *Terrestrisuchus gracilis*. These new findings add considerable information to the morphology and palaeobiology of non-crocodyliform crocodylomorphs.

*Acknowledgements.* The morphological description of *Terrestrisuchus gracilis* was aided by the unpublished PhD thesis of Peter Crush, who described the crocodylomorph material from Panty-Ffynnon in excellent detail. Additional information was also derived from the unpublished PhD thesis of Rachael Allen, who provided additional observations on the material. David Gower (NHMUK) is thanked for insightful discussions, and Roger Benson kindly provided the R script used for part of the analyses of the sclerotic ring. We thank Lu Allington-Jones (NHMUK) for conservation work and Kevin Webb and Aimee McArdle (NHMUK) for specimen photography. We thank reviewers Alexander Ruebenstahl and Juan Martín Leardi, as well as editor Philip Mannion for their constructive suggestions that helped improve the manuscript. Stephan Spiekman was funded by a Swiss National Science Foundation Early Postdoc Mobility Fellowship (P2ZHP2_195162 to SNFS) and the Deutsche Forschungs-meinschaft (grant no. SCHO 791/7-1 to Rainer Schoch, SMNS). Open Access funding enabled and organized by Projekt DEAL.

*Author contributions.* Conceptualization SNF Spiekman (SNFS), RJ Butler (RJB), SCR Maidment (SCRM); Data Curation SNFS, V Fernandez (VF), SCRM; Formal Analysis SNFS, VF, RJB, SCRM; Funding Acquisition SNFS, RJB, SCRM; Investigation SNFS, SCRM; Methodology SNFS, VF, RJB, SCRM; Project Administration SNFS, SCRM; Resources SNFS, VF, KN Dollman (KND), SCRM; Supervision RJB, SCRM; Validation SNFS, VF, RJB, KND, SCRM; Visualization SNFS, VF, SCRM; Writing—Original Draft Preparation SNFS, VF, RJB, SCRM; Writing—Review & Editing SNFS, VF, RJB, KND, SCRM.

DATA ARCHIVING STATEMENT

The data generated for this study, comprising the raw 16-bit tomograms and the resulting digital 3D mesh model of

the skull, both in the *in situ* and ‘reassembled’ *in vivo* configurations, as well as each segmented bone individually, are available at Morphosource: https://www.morphosource.org/projects/000518675 (raw 16-bit tomograms, https://doi.org/10.17602/M2/M518680; ‘reassembled’ *in vivo* mesh model of the skull, https://doi.org/10.17602/M2/M545401; *in situ* mesh model of the skull, https://doi.org/10.17602/M2/M545398; links to *in vivo* mesh models of individual segmented bones available in Table S3.

Editor. Philip Mannion

SUPPORTING INFORMATION

Additional Supporting Information can be found online (https://doi.org/10.1002/spp2.1534):

**Table S1.** Complete hypodigm (specimen list) of *Terrestrisuchus gracilis*.

**Table S2.** Cranial measurements of *Terrestrisuchus gracilis*.

**Table S3.** Links to *in vivo* mesh models of individual segmented bones.

**Appendix S1.** Files and script used to conduct the sclerotic ring analysis.

**Appendix S2.** Overview of the cranial elements of *Terrestrisuchus gracilis* in the NHMUK collections.

REFERENCES

ALLEN, D. J. 2003a. Ontogenetic determination of a new specimen confirms *Terrestrisuchus* to be a junior synonym of *Saltoposuchus*. *The Palaeontological Association Newsletter*, **53**, 72–74.

ALLEN, D. J. 2003b. When *Terrestrisuchus gracilis* reaches puberty it becomes *Saltoposuchus connectens!* *Journal of Vertebrate Paleontology*, **23**(3), 29A.

ALLEN, R. C. 2010. The anatomy and systematics of *Terrestrisuchus gracilis* (Archosauria, Crocodylomorpha). PhD thesis, Northern Illinois University, USA.

ATKINS, J. B. and FRANZ-ODENDAAL, T. A. 2016. The sclerotic ring of squamates: an evo-devo-eco perspective. *Journal of Anatomy*, **229**, 503–513.

BAILLEUL, A. M., SCANNELLA, J. B., HORNER, J. R. and EVANS, D. C. 2016. Fusion patterns in the skulls of modern archosaurs reveal that sutures are ambiguous maturity indicators for the Dinosauria. *PLoS One*, **11**, e0147687.

BARRIOS, F., BONA, P., PAULINA-CARABAJAL, A. and GASPARINI, Z. 2018. Re-description of the cranio-mandibular anatomy of *Notosuchus terrestris* (Crocodyliformes, Mesoeucrocodylia) from the Upper Cretaceous of Patagonia. *Cretaceous Research*, **83**, 3–39.

BENSON, R. B. J., SCHMITZ, L., CHOINIERE, J. N., NEENAN, J. M. and CHAPELLE, K. E. 2021. Supplementary models, data and scripts for Choiniere et al. 2021. OSF Storage. https://doi.org/10.17605/OSF.IO/TEQ73

BENTON, M. J. and CLARK, J. M. 1988. Archosaur phylogeny and the relationships of the Crocodylia. 295–338. In
BENTON, M. J. (ed.) *The phylogeny and classification of the tetrapods. Vol. 1. Amphibians, reptiles, birds*. Clarendon Press, Oxford.

BONAPARTE, J. F. 1972. Los tetrápodos del sector superior de la Formación Los Colorados, La Rioja, Argentina (Triásico Superior). 1 Parte. *Opera Lilloana*, **22**, 1-183.

BROCHU, C. A. 1996. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. *Journal of Vertebrate Paleontology*, **16**, 49-62.

de BUFFRÉNIL, V., CLARAC, F., FAU, M., MARTIN, S., MARTIN, B., PELLÉ, E. and LAURIN, M. 2015. Differentiation and growth of bone ornamentation in vertebrates: a comparative histological study among the Crocodylomorpha. *Journal of Morphology*, **276**, 425-445.

BUSBEY, A. B. III and GOW, C. 1984. A new protosuchian crocodile from the Upper Triassic Elliot Formation of South Africa. *Palaeontologia Africana*, **25**, 127-149.

BUTLER, R. J., EZCURRA, M. D., MONTEFELTRO, F. C., SAMATHI, A. and SOBRAL, G. 2015. A new species of basal rhynchosaur (Diapsida: Archosauromorpha) from the early Middle Triassic of South Africa, and the early evolution of Rhynchosauria. *Zoological Journal of the Linnean Society*, **174**, 571-588.

CHOINIERE, J. N., NEENAN, J. M., SCHMITZ, L., FORD, D. P., CHAPELLE, K. E., BALANOFF, A. M., SIPLA, J. S., GEORGI, J. A., WALSH, S. A., NORELL, M. A., XING, X., CLARK, J. M. and BENSON, R. B. J. 2021. Evolution of vision and hearing modalities in theropod dinosaurs. *Science*, **372**, 610-613.

CLARK, J. M. 1986. Phylogenetic relationships of the crocodylomorphs archosaurs. PhD thesis, University of Chicago, USA.

CLARK, J. M. and SUES, H.-D. 2002. Two new basal crocodylomorph archosaurs from the Lower Jurassic and the monophyly of the Sphenosuchia. *Zoological Journal of the Linnean Society*, **136**, 77-95.

CLARK, J. M., SUES, H.-D. and BERMAN, D. S. 2000. A new specimen of *Hesperosuchus agilis* from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. *Journal of Vertebrate Paleontology*, **20**, 683-704.

CLARK, J. M., XU, X., FORSTER, C. A. and WANG, Y. 2004. A Middle Jurassic 'sphenosuchian' from China and the origin of the crocodylian skull. *Nature*, **430**, 1021-1024.

COLBERT, E. H. 1946. The Eustachian tubes in the Crocodilia. *Copeia*, **1946**, 12-14.

COLBERT, E. H. 1952. A pseudosuchian reptile from Arizona. *Bulletin of the American Museum of Natural History*, **99**, 561-592.

COLBERT, E. H. 1989. The Triassic dinosaur *Coelophysis*. *Museum of Northern Arizona Bulletin*, **57**, 1-160.

COLBERT, E. H. and MOOK, C. C. 1952. The ancestral crocodilian *Protosuchus*. *Bulletin of the American Museum of Natural History*, **97**, 143-182.

COPE, E. D. 1869-1870. Synopsis of the extinct Batrachia, Reptilia and Aves of North America. *Transactions of the American Philosophical Society*, **14**, 1-252.

CRUSH, P. J. 1980. An early, terrestrial, crocodile from South Wales. PhD thesis, University College London, UK.

CRUSH, P. J. 1984. A late Upper Triassic sphenosuchid crocodilian from Wales. *Palaeontology*, **27**, 131-157.

DE FRANÇA, M. A., LANGER, M. C. and FERIGOLO, J. 2013. The skull anatomy of *Decuriasuchus quartacolonia* (Pseudosuchia: Suchia: Loricata) from the middle Triassic of Brazil. *Geological Society, London, Special Publications*, **379**, 469-501.

DOLLMAN, K. N. and CHOINIERE, J. N. 2022. Palate evolution in early-branching crocodylomorphs: implications for homology, systematics, and ecomorphology. *The Anatomical Record*, **305**, 2766-2790.

DOLLMAN, K. N., CLARK, J. M., NORELL, M. A., XING, X. and CHOINIERE, J. N. 2018. Convergent evolution of a eusuchian-type secondary palate within Shartegosuchidae. *American Museum Novitates*, **3901**, 1-23.

DOLLMAN, K. N., VIGLIETTI, P. A. and CHOINIERE, J. N. 2019. A new specimen of *Orthosuchus stormbergi* (Nash 1968) and a review of the distribution of Southern African Lower Jurassic crocodylomorphs. *Historical Biology*, **31**, 653-664.

DRYMALA, S. M. and ZANNO, L. E. 2016. Osteology of *Carnufex carolinensis* (Archosauria: Pseudosuchia) from the Pekin Formation of North Carolina and its implications for early crocodylomorph evolution. *PLoS One*, **11**, e0157528.

DUFEAU, D. L. and WITMER, L. M. 2015. Ontogeny of the middle-ear air-sinus system in *Alligator mississippiensis* (Archosauria: Crocodylia). *PLoS One*, **10**, e0137060.

EZCURRA, M. D. 2016. The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. *PeerJ*, **4**, e1778.

FERREIRA-CARDOSO, S., ARAÚJO, R., MARTINS, N. E., MARTINS, G. G., WALSH, S., MARTINS, R. M. S., KARDJILOV, N., MANKE, I., HILGER, A. and CAS-TANHINHA, R. 2017. Floccular fossa size is not a reliable proxy of ecology and behaviour in vertebrates. *Scientific Reports*, **7**, 2005.

FOFFA, D., WHITESIDE, D. I., VIEGAS, P. A. and BENTON, M. J. 2014. Vertebrates from the Late Triassic *Thecodontosaurus*-bearing rocks of Durdham Down, Clifton (Bristol, UK). *Proceedings of the Geologists' Association*, **125**, 317-328.

FRASER, N. C., PADIAN, K., WALKDEN, G. M. and DAVIS, A. L. M. 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. *Palaeontology*, **45**, 79-95.

GAUTHIER, J. A. and PADIAN, K. 2020. Archosauria. 1187-1193. *In de QUEIROZ, K., CANTINO, P. D. and GAUTHIER, J. A. (eds) *Phylonyms: A companion to the PhyloCode*. CRC Press.*

GOW, C. E. 2000. The skull of *Protosuchus haughtoni*, an Early Jurassic crocodyliform from southern Africa. *Journal of Vertebrate Paleontology*, **20**, 49-56.

GOWER, D. J. 1997. The braincase of the early archosaurian reptile *Erythrosuchus africanus*. *Journal of Zoology*, **242**, 557-576.

GOWER, D. J. 2002. Braincase evolution in suchian archosaurs (Reptilia: Diapsida): evidence from the rauisuchian *Batrachotomus kupferzellensis*. *Zoological Journal of the Linnean Society*, **136**, 49-76.

GOWER, D. J. and WALKER, A. D. 2002. New data on the braincase of the aetosaurian archosaur (Reptilia: Diapsida) *Stagonolepis robertsoni* Agassiz. *Zoological Journal of the Linnean Society*, **136**, 7-23.
GRIFFIN, C. T., STOCKER, M. R., COLLEARY, C., STEFANIC, C. M., LESSNER, E. J., RIEGLER, M., FORMOSO, K., KOELLER, K. and NESBITT, S. J. 2021. Assessing ontogenetic maturity in extinct saurian rep- tiles. *Biological Reviews*, **96**, 470–525.

HARRIS, J. D., LUCAS, S. G., ESTEP, J. W. and LI, J. 2000. A new and unusual sphenosuchian (Archosauria: Croco- dylomorpha) from the Lower Jurassic Lufeng Formation, Peo- ple's Republic of China. *Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen*, **215**, 47–68.

HAY, O. P. 1930. *Second bibliography and catalogue of the fossil vertebrata of North America*. Carnegie Institution of Washing- ton, 1074 pp.

HERRERA, Y., LEARDI, J. M. and FERNÁNDEZ, M. S. 2018. Braincase and endocranial anatomy of two thalattosu- chian crocodylomorphs and their relevance in understanding their adaptations to the marine environment. *PeerJ*, **6**, e5686.

HILL, R. V., D'EMIC, M. D., BEVER, G. S. and NOR- ELL, M. A. 2015. A complex hyobranchial apparatus in a Cretaceous dinosaur and the antiquity of avian paraglossalia. *Zoological Journal of the Linnean Society*, **175**, 892–909.

HOLLIDAY, C. M. and WITMER, L. M. 2007. Archosaur adductor chamber evolution: integration of musculoskeletal and topological criteria in jaw muscle homology. *Journal of Morphology*, **268**, 457–484.

HOLLIDAY, C. M., TSAI, H. P., SKILJAN, R. J., GEORGE, I. D. and PATHAN, S. 2013. A 3D interactive model and atlas of the jaw musculature of *Alligator mississip- piensis*. *PLoS One*, **8**, e62806.

von HUENE, F. 1921. Neue Pseudosuchier und Coelurosaur- ier aus dem Württembergischen Keuper. *Acta Zoologica*, **2**, 329–403.

IRMIS, R. B., NESBITT, S. J. and SUES, H.-D. 2013. Early Crocodylomorpha. *Geological Society, London, Special Publica- tions*, **379**, 275–302.

JIANG, S., LI, Z., CHENG, X. and WANG, X. 2020. The first pterosaur basihyal, shedding light on the evolution and function of pterosaur hyoid apparatuses. *PeerJ*, **8**, e8292.

KLEY, N. J., SERTICH, J. J., TURNER, A. H., KRAUSE, D. W., O'CONNOR, P. M. and GEORGI, J. A. 2010. Cra- niofacial morphology of *Simosuchus clarki* (Crocodyliformes: Notosuchia) from the late Cretaceous of Madagascar. *Journal of Vertebrate Paleontology*, **30**, 13–98.

KUZMIN, I. T., BOITSOVA, E. A., GOMBOLEVSKIY, V. A., MAZUR, E. V., MOROZOV, S. P., SENNIKOV, A. G., SKUTSCHAS, P. P. and SUES, H. D. 2021. Brain- case anatomy of extant Crocodylia, with new insights into the development and evolution of the neurocranium in crocodylo- morphs. *Journal of Anatomy*, **239**, 983–1038.

LEARDI, J. M., POL, D. and CLARK, J. M. 2017. Detailed anatomy of the braincase of *Macelognathus vagans* Marsh, 1884 (Archosauria, Crocodylomorpha) using high resolution tomography and new insights on basal crocodylomorph phylogeny. *PeerJ*, **5**, e2801.

LEARDI, J. M., POL, D. and CLARK, J. M. 2020. Braincase anatomy of *Almadasuchus figarii* (Archosauria, Crocodylomor- pha) and a review of the cranial pneumaticity in the origins of Crocodylomorpha. *Journal of Anatomy*, **237**, 48–73.

LECUONA, A., EZCURRA, M. D. and IRMIS, R. B. 2016. Revision of the early crocodylomorph *Trialestes romeri* (Arch- osauria, Suchia) from the lower Upper Triassic Ischigualasto Formation of Argentina: one of the oldest-known crocodylo- morphs. *Papers in Palaeontology*, **2**, 585–622.

LI, Z. and CLARKE, J. A. 2015. New insight into the anatomy of the hyolingual apparatus of *Alligator mississippiensis* and implications for reconstructing feeding in extinct archosaurs. *Journal of Anatomy*, **227**, 45–61.

LI, C., WU, X.-C., CHENG, Y.-N., SATO, T. and WANG, L. 2006. An unusual archosaurian from the marine Triassic of China. *Naturwissenschaften*, **93**, 200–206.

LI, Z., ZHOU, Z. and CLARKE, J. A. 2018. Convergent evo- lution of a mobile bony tongue in flighted dinosaurs and pterosaurs. *PLoS One*, **13**, e0198078.

MELSTROM, K. M., TURNER, A. H. and IRMIS, R. B. 2021. Reevaluation of the cranial osteology and phylogenetic position of the early crocodyliform *Eopneumatosuchus colberti*, with an emphasis on its endocranial anatomy. *The Anatomical Record*, **305**, 2557–2582.

MUSSINI, G., WHITESIDE, D. I., HILDEBRANDT, C. and BENTON, M. J. 2020. Anatomy of a Late Triassic Bris- tol fissure: Tytherington fissure 2. *Proceedings of the Geologists' Association*, **131**, 73–93.

NESBITT, S. J. 2011. The early evolution of archosaurs: relationships and the origin of major clades. *Bulletin of the American Museum of Natural History*, **352**, 1–292.

NESBITT, S. J. and SUES, H.-D. 2021. The osteology of the early-diverging dinosaur *Daemonosaurus chauliodus* (Archo- sauria: Dinosauria) from the *Coelophysis* Quarry (Triassic: Rhaetian) of New Mexico and its relationships to other early dinosaurs. *Zoological Journal of the Linnean Society*, **191**, 150– 179.

NESBITT, S. J., IRMIS, R. B., LUCAS, S. G. and HUNT, A. P. 2005. A giant crocodylomorph from the Upper Triassic of New Mexico. *Paläontologische Zeitschrift*, **79**, 471–478.

NESBITT, S. J., TURNER, A. H. and WEINBAUM, J. C. 2013. A survey of skeletal elements in the orbit of Pseudosu- chia and the origin of the crocodylian palpebral. *Earth & Environmental Science Transactions of the Royal Society of Edin- burgh*, **103**, 365–381.

NESBITT, S. J., LANGER, M. C. and EZCURRA, M. D. 2020. The anatomy of *Asilisaurus kongwe*, a dinosauriform from the Lifua Member of the Manda Beds (~Middle Triassic) of Africa. *The Anatomical Record*, **303**, 813–873.

POL, D., RAUHUT, O. W. M., LECUONA, A., LEARDI, J. M., XU, X. and CLARK, J. M. 2013. A new fossil from the Jurassic of Patagonia reveals the early basicranial evolution and the origins of Crocodyliformes. *Biological Reviews*, **88**, 862–872.

RUEBENSTAHL, A. A., KLEIN, M. D., YI, H., XU, X. and CLARK, J. M. 2022. Anatomy and relationships of the early diverging Crocodylomorphs *Junggarsuchus sloani* and *Dibothro- suchus elaphros*. *The Anatomical Record*, **305**, 2463–2556.

SCHMITZ, L. and MOTANI, R. 2010. Morphological differ- ences between the eyeballs of nocturnal and diurnal amniotes revisited from optical perspectives of visual environments. *Vision Research*, **50**, 936–946.
SCHMITZ, L. and MOTANI, R. 2011. Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. *Science*, **332**, 705–708.

SERENO, P. C. and WILD, R. 1992. *Procompsognathus*: the-ropod, “thecodont” or both? *Journal of Vertebrate Paleon-tology*, **12**, 435–458.

SKINNER, M., WHITESIDE, D. I. and BENTON, M. J. 2020. Late Triassic island dwarfs? Terrestrial tetrapods of the Ruthin fissure (South Wales, UK) including a new genus of procolophonid. *Proceedings of the Geologists’ Association*, **131**, 535–561.

SOBRAL, G. and MÜLLER, J. 2019. The braincase of *Mesosuchus browni* (Reptilia, Archosauromorpha) with information on the inner ear and description of a pneumatic sinus. *PeerJ*, **7**, e6798.

SOBRAL, G., SOOKIAS, R. B., BHULLAR, B.-A. S., SMITH, R. M. H., BUTLER, R. J. and MÜLLER, J. 2016. New information on the braincase and inner ear of *Euparkeria capensis* Broom: implications for diapsid and archosaur evolution. *Royal Society Open Science*, **3**, 160072.

SPIEKMAN, S. N. F. 2023. A revision and histological investigation of *Saltoposuchus connectens* (Archosauria: Crocodylomorpha) from the Norian (Late Triassic) of southwestern Germany. *Zoological Journal of the Linnean Society*, **199**, 354–391.

SPIEKMAN, S. N. F., NEENAN, J. M., FRASER, N. C., FERNANDEZ, V., RIEPPEL, O., NOSOTTI, S. and SCHEYER, T. M. 2020. The cranial morphology of *Tanystropheus hydroides* (Tanystropheidae, Archosauromorpha) as revealed by synchrotron microtomography. *PeerJ*, **8**, e10299.

SPIEKMAN, S. N. F., EZCURRA, M. D., BUTLER, R. J., FRASER, N. C. and MAIDMENT, S. C. R. 2021. *Pendraig milnerae*, a new small-sized coelophysoid theropod from the Late Triassic of Wales. *Royal Society Open Science*, **8**, 210915.

SPIEKMAN, S. N. F., FERNANDEZ, V., BUTLER, R. J., DOLLMAN, K. N. and MAIDMENT, S. C. R. 2023. Data from: A taxonomic revision and cranial description of *Terres-trisuchus gracilis* (Archosauria, Crocodylomorpha) from the Upper Triassic of Pant-y-Ffynnon Quarry (southern Wales). MorphoSource. https://www.morphosource.org/projects/000518675

SUES, H.-D., OLSEN, P. E., CARTER, J. G. and SCOTT, D. M. 2003. A new crocodylomorph archosaur from the Upper Triassic of North Carolina. *Journal of Vertebrate Paleontology*, **23**, 329–343.

TOLJAGIČ, O. and BUTLER, R. J. 2013. Triassic–Jurassic mass extinction as trigger for the Mesozoic radiation of crocodylomorphs. *Biology Letters*, **9**, 20130095.

VICKARYOUS, M. K. and HALL, B. K. 2008. Development of the dermal skeleton in *Alligator mississippiensis* (Archosauria, Crocodylia) with comments on the homology of osteoderms. *Journal of Morphology*, **269**, 398–422.

VOOGD, J. and WYLIE, D. R. 2004. Functional and anatomical organization of floccular zones: a preserved feature in vertebrates. *Journal of Comparative Neurology*, **470**, 107–112.

WALKDEN, G. M., FRASER, N. C. and SIMMS, M. J. 2021. Reply to Walkden, Fraser and Simms (2021): The age and formation mechanisms of Late Triassic fissure deposits, Gloucestershire, England: Comments on Mussini, G., Whiteside, D. I., Hildebrandt C. and Benton M. J. *Proceedings of the Geologists’ Association*, **132**, 127–137.

WALKER, A. D. 1970. A revision of the Jurassic reptile *Hallopus victor* (Marsh), with remarks on the classification of crocodiles. *Philosophical Transactions of the Royal Society B*, **257**, 323–372.

WALKER, A. D. 1985. The braincase of *Archaeopteryx*. 123–134. *In* HECHT, M. K., OSTROM, J. H., VIOHL, G. and WELLNHOFER, P. (eds) *The beginnings of birds*. Freunde des Jura-Museums Eichstätt.

WALKER, A. D. 1990. A revision of *Sphenosuchus acutus* Haughton, a crocodylomorph reptile from the Elliot Formation (Late Triassic or Early Jurassic) of South Africa. *Philosophical Transactions of the Royal Society B*, **330**, 1–120.

WHITESIDE, D. I. and BENTON, M. J. 2021. Reply to Walkden, Fraser and Simms (2021): The age and formation mechanisms of Late Triassic fissure deposits, Gloucestershire, England: Comments on Mussini, G., Whiteside, D. I., Hildebrandt C. and Benton M. J. *Proceedings of the Geologists’ Association*, **132**, 138–141.

WHITESIDE, D. I. and MARSHALL, J. E. A. 2008. The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK. *Geological Magazine*, **145**, 105–147.

WHITESIDE, D. I., DUFFIN, C. J., GILL, P. G., MARSHALL, J. E. A. and BENTON, M. J. 2016. The Late Triassic and Early Jurassic fissure faunas from Bristol and South Wales: stratigraphy and setting. *Palaeontologia Polonica*, **67**, 257–287.

WITMER, L. M. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. *Journal of Vertebrate Paleontology*, **17**, 1–76.

WU, X.-C. and CHATTERJEE, S. 1993. *Dibothrosuchus elaphros*, a crocodylomorph from the Lower Jurassic of China and the phylogeny of the Sphenosuchia. *Journal of Vertebrate Paleontology*, **13**, 58–89.

YATES, A. M. 2003. A new species of the primitive dinosaur *Thecodontosaurus* (Saurischia: Sauropodomorpha) and its implications for the systematics of early dinosaurs. *Journal of Systematic Palaeontology*, **1**, 1–42.

YOUNG, M. T., BRUSATTE, S. L., RUTA, M. and DE ANDRADE, M. B. 2010. The evolution of Metriorhynchoidea (Mesoeucrocodylia, Thalattosuchia): an integrated approach using geometric morphometrics, analysis of disparity, and biomechanics. *Zoological Journal of the Linnean Society*, **158**, 801–859.

von ZITTEL, K. A. 1890. *Handbuch der Palaeontologie. 1. Abteilung Palaeozoologie. Band 3. Vertebrata*. R. Oldenbourg, Munich & Leipzig.