Microsphenodon bonapartei Chambi-Trowell & Martinell & Whitesid & Romo de Viva & Soare & Schult & Gil & Bento & Rayfiel 2021, sp. nov.
Creators: name: Chambi-Trowell, Sofia A. V.
name: Martinell, Agust ́ ın G.
name: Whitesid, David I.
name: Romo de Viva, Paulo R.
name: Soare, Marina Bento
name: Schult, Cesar L.
name: Gil, Pamela G.
name: Bento, Michael J.
name: Rayfiel, Emily J.
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Rhynchocephalia
Microsphenodon
Microsphenodon bonapartei

description: Microsphenodon bonapartei sp. nov.   
(Figs 4, 6, 8–14; Supplemental Fig. 4)  
 Holotype. UFRGS-PV-0972- T (Fig. 9A, 10), a near-complete skull with some damage to its rostrum and missing the premaxillae and vomers. Much of the right side of the skull is damaged, but only the right maxilla and jugal are entirely missing. For the most part, the skull remains articulated and would have been ~ 19 mm in length.   
 Derivation of name. The specific term ‘ bonapartei ’ refers to the late Argentinian palaeontologist Jośe F. Bonaparte (1928–2020), who led the main collections of fossil vertebrates from the Linha S̃ao Luis outcrop until 2005, deposited at the UFRGS, including the discovery of the holotype specimen here described.   
 Additional referred specimens. UFRGS-PV-0613- T, UFRGS-PV-0848- T and UFRGS-PV-0827- T are referred to the new taxon, which were identified as C. brasiliensis in previous studies (Bonaparte & Sues 2006; Romo de Vivar Mart́ınez & Soares 2015; Romo-de-Vivar-Mart́ınez et al. 2021). UFRGS-PV-0613- T is a partial skull around 20 mm long, and is in similar condition to the holotype, with damage to its rostrum and the right side of the skull; it was described by Bonaparte & Sues (2006) as the paratype of C. brasiliensis, where they interpreted it to be a juvenile of the taxon. UFRGS-PV-0848- T is an isolated right mandible 24 mm long. UFRGS-PV-0827- T is an isolated right palatine.   
 Diagnosis. A small sphenodontian rhynchocephalian with a skull length of 19–22 mm, which has the following unique combination of features:   
1. A single caniniform tooth present on both maxilla and dentary.  
2. The maxillary and dentary dentition is acrodont and the largest teeth take the form of two enlarged mesiodistally elongated additional teeth at the posterior of the dentary and maxilla, posterior to a row of smaller non-hatchling teeth that alternate in size.  
3. A broad flat parietal table.  
4. Three to four small conical post-marginal teeth positioned behind the additional teeth of the maxilla.  
 5. A palatine with a single medially positioned tooth, in addition to a secondary row or ridge of two teeth that are positioned at 45 Ǫ to the long axes of the main lateral tooth row.   
6. Fused prearticular, articular and surangular bones.  
7. Paired frontals and parietals.  
8. Exoccipitals fused to basioccipital.  
9. Two rows of vomerine teeth, three rows of pterygoidal teeth and two rows of palatine teeth.  
 Description   
 Skull. Nearly all bones of the skull are represented, but there is no complete vomer, any premaxillae or verifiable stapes.   
 Maxilla. The maxillary facial process (Fig. 11A–C) is relatively high but not as much as in Clevosaurus brasiliensis and is broader. It bears a pronounced premaxillary process and a single caniniform (UFRGS-PV-0613- T, Romo de Vivar Mart́ınez & Soares 2015, fig. 3C, 4F; Fig. 9B, C), which is followed by an edentulous region where the likely hatchling teeth have been worn to the bone, followed by around three to four smaller additional teeth that appear to alternate in size (Romo de Vivar Mart́ınez & Soares 2015, fig. 4F), and ultimately two large additional teeth bearing flanges. All the teeth are acrodont (Fig. 4A–C, N). Posterior to the main tooth row are three to four small conical teeth. It is unclear whether the premaxillary process bore any successional teeth anterior to the caniniform, as this part of the maxilla is damaged. The posterior process is broad and only narrows abruptly close to its end. Unlike C. cambrica (Chambi-Trowell et al. 2019) and C. bairdi (Sues et al. 1994, fig. 1B), there is no posterior curvature of the maxillary dorsal process forming the antero-orbital margin; this shape is more akin to the maxilla of Diphydontosaurus (Whiteside 1986, fig. 4A). Unlike Diphydontosaurus, however, the suborbital process remains near uniform for much of its length rather than tapering. Posteriorly, the maxilla forms a ventrally positioned medial shelf that contacts the jugal and ectopterygoids. Dorsomedially, the maxilla is somewhat concave where it would have contacted the nasal and prefrontal.   
 Nasal. Only the left nasal (Fig. 11D–F) is preserved, and the anterior portion is damaged. The nasals are large and elongated compared to those in the short robust snouts of Clevosaurus and more like those of Diphydontosaurus, Gephyrosaurus or Planocephalosaurus (Evans 1980; Fraser 1982; Whiteside 1986). The nasals are less convex than those of C. brasiliensis, reflecting a long rostrum positioned at a shallower angle.   
 Prefrontal. The prefrontal (Fig. 11D–F) appears relatively typical for rhynchocephalians, with a roughly lunate shape and a broad lateral contact with the maxilla, and medially with the nasal. Ventrally it contacts the palatine, but probably not the jugal unlike in C. brasiliensis. It has sustained some damage on its dorsolateral surface.   
 Postfrontal. The anterior process of the postfrontal (Fig. 11G, H) is long and columnar, as in Planocephalosaurus and Diphydontosaurus (Fraser 1982; Whiteside 1986), while the posterior process appears to be very short to non-existent, but may simply be damaged, and the ventral process is bifurcated, with a deep groove into which the postorbital would have articulated. This ‘four-cornered’ postfrontal with a deep groove is very similar in shape to that of C. minor (Fraser 1988, fig. 39).   
 Frontal. The frontals (Fig. 11I, J) are not fused, unlike those of basal rhynchocephalians (Evans 1980; Fraser 1982; Whiteside 1986). Rather, they are paired, forming an hourglass shape, with two long posterior flattened processes that overlap the parietals dorsally. Anterodorsally there are facets for the prefrontals and nasals.   
 Parietal. The parietals (Fig. 11I, J) form a broad and relatively short flat parietal table, and they are paired, unlike the fused elements of some basal rhynchocephalians (Evans 1980; Fraser 1982; Whiteside 1986); the fusion is late in ontogeny in Diphydontosaurus according to Whiteside (1986). The posterior process on each parietal is short and tapers rapidly, with a distinctive slot on its dorsal surface for the supratemporal where it overlay both the parietal and quadrate.   
 Jugal. The jugal (Fig. 11K, L) formed a broad contact with the ectopterygoid medioventrally and with the maxilla laterally. It contacted the palatine anteriorly. The bone formed a broad contact with the postorbital along the lateral surface of its dorsal process, which also overlay the squamosal ventrally. The jugal did not interdigitate with the postorbital unlike C. brasiliensis. The posterior process is missing in all specimens.   
 Postorbital. The postorbital (Fig. 11M, N) is a triradiate bone that laterally overlapped the squamosal and jugal. The ventral process would have partially wrapped around the jugal along its lateral surface, a condition shared with European Clevosaurus (Fraser 1988; Chambi-Trowell et al. 2019) but differing from the interdigitating condition seen in the jugal in C. brasiliensis.   
 Squamosal. The squamosal (Fig. 11O, P) is generally typical of Clevosaurus (see Fraser 1988). There is some damage to the ventral process, but most of the bone is preserved. The ventral process is not curved or as broad as in C. brasiliensis. The lateral surfaces of both squamosals are damaged, so it is unclear whether there was a similar lateroventral depression as in C. brasiliensis.   
 Supratemporal. The supratemporal (Fig. 12A, B) is a small, convex, plate-like bone that connects the squamosal to the parietal, with a ridge on its ventral side that articulates with both. It is roughly rectangular in profile, and midway along its long axes there is a parallel indentation, possibly indicating a site for attachment of the M. depressor mandibulae. Supratemporals have not been confirmed previously in any rhynchocephalians except clevosaurs (Jones 2006; Chambi-Trowell et al. 2020). The supratemporal may be damaged posteriorly, and it is unclear whether it articulated with the paroccipital process as in C. brasiliensis.   
 Quadrate. The overall shape of the quadrate (Fig. 12C–E) shares some similarity with C. brasiliensis, but it is far less robust, with a pronounced curved ventral condyle that is angled at roughly 45 Ǫ to the vertical strut in medial view, resulting in a more posteriorly positioned dorsal condyle. The medially positioned shelf along the ventral edge of the quadrate flange is pronounced but flat and not curved dorsally as in C. brasiliensis. The bone is very porous around the condyles. A quadratojugal cannot be distinguished from the quadrate, and the quadratojugal foramen is small compared to that of Sphenodon punctatus and positioned ventrolaterally.   
 Vomer. No complete vomer is known. Only a fragment of the left vomer, articulated with the palatine, is preserved in the holotype (Fig. 12F). It bears two rows of teeth, one central row with three teeth preserved and one lateral with two teeth preserved. Other disarticulated teeth are found separately in the matrix. Numerous teeth on the vomer are a character typical of basal rhynchocephalians such as Gephyrosaurus, Planocephalosaurus and Diphydontosaurus (Evans 1980; Fraser 1982; Whiteside 1986), but two rows are also observed in C. brasiliensis.   
 Palatine. The palatine (Figs 4C, 6E–G, 12G, H) is distinct, bearing a slightly curved lateral row of six teeth (Fig. 12G), a single medially placed tooth, and a fused row of two teeth (Fig. 12H) that are angled at roughly 45 Ǫ to the lateral row (Figs 4C, 6E–G). This condition of a short, angled secondary row of teeth is seen also in Rebbanasaurus (Evans et al. 2001) though that taxon has three rather than two teeth. The isolated medially positioned tooth is characteristic of Clevosaurus (Fraser 1988; Klein et al. 2015; Chambi-Trowell et al. 2019), but not Clevosaurus brasiliensis. The six teeth in the lateral row are significantly fewer than observed in Gephyrosaurus, Planocephalosaurus and Diphydontosaurus (Evans 1980; Fraser 1982; Whiteside 1986) and more similar to C. cambrica, C. hudsoni and C. sectumsemper (Fraser 1988; Klein et al. 2015; Chambi-Trowell et al. 2019). There is a lateral groove on the palatine between the lateral tooth row and the maxillary process, as previously noted (Romo de Vivar Mart́ınez & Soares 2015; Romo-de-Vivar-Mart́ınez et al. 2019). The maxillary process would likely have had a broad contact with the maxilla in life but the anterior part of the maxillary process has broken off. Posteriorly, the maxillary process contacts the ectopterygoid and jugal. The palatine articulates posteriorly on its dorsal surface with the ectopterygoid, and with the pterygoid along its medial ventral surface, excluding the pterygoid from the suborbital fenestra. This differs from the condition in non-eusphenodontians (Evans 1980; Fraser 1982; Whiteside 1986); however, the pterygoid has only the slightest contact with the suborbital fenestra in the reconstruction of Diphydontosaurus by Whiteside (1986, fig. 3b).   
 Pterygoid. The pterygoid (Fig. 12I, J, L) bears three rows of teeth, the third row being very short and consisting of just three teeth, as in C. brasiliensis, Brachyrhinodon and Planocephalosaurus (Fraser 1982). There is an interdigitating facet between the pterygoids anteriorly, and dorsally there is a raised ridge of bone on either side of this facet, but this is not as pronounced as in C. brasiliensis. The ramus between the start of the pterygoid flange and the lateral process is shorter than that of Sphenodon (Evans 2008) but is relatively longer and narrower than in C. brasiliensis. The pterygoid lateral processes widen laterally and articulate ventrally with the ectopterygoids. The basipterygoid facets are cup-like.   
 Ectopterygoid. The ectopterygoid (Fig. 12K) closely resembles that of C. brasiliensis but has a relatively longer, more gracile maxillary process and a shorter ventral process.   
 Epipterygoid. The epipterygoid (Fig. 12L) is rod-like dorsally and wider and more flattened ventrally where it articulates with the pterygoid flange. It is nearly identical to that of C. brasiliensis and other clevosaurs where this has been described, as well as Gephyrosaurus and Diphydontosaurus (Evans 1980; Whiteside 1986; Sues et al. 1994; Chambi-Trowell et al. 2019, fig. 8C).   
 Parabasisphenoid. The parasphenoid and basisphenoid are fused (Fig. 13A–F), forming the floor of the braincase, which is much narrower than in C. brasiliensis. Anteriorly, there are two elongate basipterygoid processes that widen anteriorly and articulate with the pterygoids. The cultriform process is elongate and reaches at least as far as the end of the interpterygoid vacuity. There are two carotid foramina, one on each side of the base of the cultriform process (Fig. 13F).   
 Basioccipital-exoccipitals. The basioccipitals and exoccipitals are fused (Fig. 13A–F), as in Gephyrosaurus, Planocephalosaurus and Diphydontosaurus (Evans 1980; Fraser 1982; Whiteside 1986) and differing from the condition in C. brasiliensis and other clevosaurs. The exoccipitals would have formed more than 50% of the foramen magnum. The basal tubera are short and project posteroventrally. The occipital condyle is lunate in shape, with a slightly raised medial ridge positioned just anteriorly of the condyle, as in Diphydontosaurus (Whiteside 1986, fig. 26a). The basioccipital articulates with the dorsal surface of the parabasisphenoid.   
 Opisthotics, prootics and supraoccipital. These bones (Fig. 13A–F) cannot be distinguished in the scans, and it is possible all three were fused, forming the roof of the braincase, as observed in C. hudsoni (Fraser 1988, p. 140; O’ Brien et al. 2018; Chambi-Trowell et al. 2019) and C. brasiliensis. The roof of the braincase appears to have a much lower bone density than the rest of the skull and as a result is hard to distinguish from the matrix, a feature in common with C. cambrica and C. hudsoni (O’ Brien et al. 2018; Chambi-Trowell et al. 2019) and C. brasiliensis. The supraoccipital forms part of the dorsal margin of the foramen magnum, and its dorsal surface is broad and relatively flat, with no dorsal crest present. The pila antotica is similar in shape to that of Clevosaurus, lateromedially flattened and curving dorsally, widening to a squared-off head.   
 Dentary. The dentaries (Fig. 8F–H, L) are relatively long and narrow, with a high coronoid process and they have the pronounced edentulous region between the ultimate tooth and coronoid process often seen in clevosaurs and in Diphydontosaurus and Planocephalosaurus (Fraser 1982; Whiteside 1986; Fraser 1988; Fraser & Benton 1989; Sues et al. 1994; Chambi-Trowell et al. 2019). As in most rhynchocephalians, there is a raised lip of secondary bone on the lateral surface of the dentary. The symphysis is long and slants anterodorsally at a low angle, and seemingly lacks the medially positioned post-symphyseal lamina seen in Lanceirosphenodon (Romo de Vivar et al. 2020a, fig. 5). The dentary has pronounced diagonal wear facets from the maxillary teeth, as often seen in Clevosaurus (Fraser 1988; Klein et al. 2015; Chambi-Trowell et al. 2019), as well as Diphydontosaurus, but not present in C. brasiliensis. The anterior-most region of the dentary appears to be edentulous (but as the scanned holotype has damage in this region of the jaw, we cannot verify this). There appears to be a single caniniform, followed by 4–5 hatchling teeth, then 4–6 teeth that alternate in size, increasing in size posteriorly (Fig. 8F, G; UFRGS-PV-0613- T, Romo de Vivar Mart́ınez & Soares 2015, fig. 3C, 4F), and finally there are two large flanged mesiodistally elongated teeth with roughly triangular lateral profile. There are no escape structures, and rather like Lanceirosphenodon (Romo de Vivar et al. 2020a), the teeth have a convex lingual surface with the cusp deflected lingually. Like C. convallis, the hatchling teeth remain visible in adult specimens.   
 Coronoid. The coronoid (Fig. 8F–H) is a small, thin bone; it is unclear whether it projected above the dentary.   
 Articular complex (prearticular, surangular and articular) and angular. The articular complex (Fig. 8D–F) appears to be fused, as the facets between the bones are indistinguishable from one another. The angular is flattened and reaches as far anteriorly as the posterior-most tooth of the dentary. The mandibular foramen is formed between the surangular and dentary and is large, unlike that of C. brasiliensis. The dorsal surface of the surangular is also very wide and robust compared to other rhynchocephalians, and the articular is robust, with a remarkably long retroarticular process that forms a spoon-like structure that tapers posteriorly and is deepest laterally, as in C. brasiliensis. The articular condyle is full of porous bone. The fusion of the prearticular, articular and surangular in Microsphenodon is an important difference from Clevosaurus brasiliensis but a feature shared with early diverging rhynchocephalians such as Gephyrosaurus, Diphydontosaurus and Planocephalosaurus.   
 Additional and postcranial material. Among the holotype material, long rod-like elements are observed beneath the skull (Fig. 10D), inferred to be remnants of the hyoid apparatus. Likewise, we also found very thin broken fragments of bone behind both orbits (Fig. 10A, C), which we suggest are fragments of the sclerotic ossicles. The axis and atlas are present (Fig. 13G–K), but the former has sustained considerable damage dorsally so that much of the neural arch is missing and only the centrum of the axis remains intact. The centrum of the atlas is fused to that of the axis, forming the odontoid process which articulates dorsally to the atlas intercentrum. The axis intercentrum is possibly preserved, but the suture lines between it and the atlas intercentrum cannot be distinguished in the scans and only the atlas intercentrum is prominent. The atlas/axis complex has a similar configuration to that in basal rhynchocephalians such as Gephyrosaurus (Evans 1981, fig. 2) and eusphenodontians like Sphenodon.   
 Reconstructions   
 The anatomy of the two Brazilian taxa has been presented in some detail based on M CT scans, and we offer standardized skull drawings of both (Fig. 14). The detail is based entirely on the fossils, and, because of their completeness, we had to make very few assumptions or restorations of missing parts. In the case of Clevosaurus brasiliensis (Fig. 14A–D), it lacks a complete vomer, as well as the stapes. In the case of Microsphenodon bonapartei (Fig. 14E–H), the posterior process of the jugal is broken off, only a small portion of the left vomer is preserved and it is missing the premaxillae and stapes. The whole-body life drawings (Fig. 15) represent the head shapes according to the fossil data, but the body shapes are speculative.
description: Published as part of Chambi-Trowell, Sofia A. V., Martinell, Agust ́ ın G., Whitesid, David I., Romo de Viva, Paulo R., Soare, Marina Bento, Schult, Cesar L., Gil, Pamela G., Bento, Michael J. & Rayfiel, Emily J., 2021, The diversity of Triassic South American sphenodontians: a new basal form, clevosaurs, and a revision of rhynchocephalian phylogeny, pp. 787-820 in Journal of Systematic Palaeontology 19 (11) on pages 803-810, DOI: 10.1080/14772019.2021.1976292, http://zenodo.org/record/10959831