Microsphenodon bonapartei, Chambi-Trowell & Martinell & Whitesid & Romo de Viva & Soare & Schult & Gil & Bento & Rayfiel, 2021

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, Journal of Systematic Palaeontology 19 (11), pp. 787-820 : 813-814

Microsphenodon bonapartei

Originally considered to be the juvenile form of Clevosaurus brasiliensis ( Bonaparte & Sues 2006; Romo de Vivar Mart́ınez & Soares 2015), we note the following morphological differences that indicate it represents a different genus and species: fusion of articular, surangular and prearticular; fusion of exoccipitals and basioccipital; deep diagonal wear facets on dentary; lack of an enlarged posterior-most tooth on the dentary; two rows of teeth on the palatine (albeit one is rudimentary); elongated antorbital region of the skull; broad parietal table; four-cornered postfrontal; typical acrodonty (teeth sit on the jaw crest and do not extend deeply into the jaw bones); elongated premaxillary process on maxilla; short posterior process on the parietal; relatively gracile jaw; non-interdigitating facets between jugal-postorbital and maxilla-prefrontal; and presence of (small) caniniforms. All four specimens of M. bonapartei suggest an animal with a skull around 20 mm long, which is only slightly smaller than C. brasiliensis (with specimens averaging around 25 mm).

Our phylogenetic analysis ( Fig. 16 ) recovered Microsphenodon as the earliest diverging eusphenodontian after Polysphenodon . Plesiomorphic features include its relatively elongated snout, fusion of the exoccipitals to the basioccipital, broad flat parietal table, low gracile dentary, and its multiple rows of palatal teeth, while apomorphies include its fully acrodont dentition, a high coronoid process, differentiated dentition, and a large parietal opening. The last character-state could be interpreted as evidence the specimen is juvenile, perhaps based on the wider, flatter parietals in juvenile Sphenodon compared to adults, but some cranial sutures in Microsphenodon appear to be well fused, and the parietal opening size varies substantially among extinct rhynchocephalians, being small in Gephyrosaurus and outgroups, and large in clevosaurs and most eusphenodontians.

The palate of Microsphenodon is its key diagnostic feature, bearing multiple rows of teeth on the vomers, palatines and pterygoids. Although there are more tooth rows than in most eusphenodontians, the number of rows is still fewer than in earlier diverging forms such as Gephyrosaurus , Diphydontosaurus and Planocephalosaurus . The palatine of Microsphenodon is similar to that of Rebbanasaurus from the Early Jurassic of India ( Evans et al. 2001), both of which bear two rows of palatine teeth, one reduced to just two to three teeth and positioned at roughly 45 Ǫ to the main row of teeth. Medially, the palatine bears a single tooth that is otherwise an apomorphy (a single tooth or cluster of teeth placed medially on the palatine) known only within clevosaurs, suggesting this feature was plesiomorphic to Clevosaurus , though it is also arguably known in Sphenotitan from the Rhaetian of Argentina (Mart́ınez et al. 2013). Whether this single tooth is present in Rebbanasaurus is unknown because this part of the palatine is missing.

The marginal teeth of Microsphenodon show some resemblances to those of derived rhynchocephalians. For example, the dentary is like that of Sphenocondor from the Middle Jurassic of Argentina (Apestegúıa et al. 2012), with a high but elongated and blunt coronoid process, a gracile elongated dentary ramus and similar complex tooth differentiation. There are a few large, mesiodistally elongated additional teeth, and anterior to this many smaller teeth alternating in size, and successional teeth located most anteriorly, including a caniniform. Microsphenodon differs from Sphenocondor in showing no evidence of any other successional teeth anterior to the caniniform ( Fig. 3C ). The only known specimen of Sphenocondor has been identified as a juvenile because some of its dentary teeth alternate in size, but it is roughly the same size as all known specimens of Microsphenodon , and adult specimens of Diphydontosaurus similarly display alternation in tooth size, suggesting that this type of dentition is not restricted to juveniles. Likewise, adult specimens of Sphenodon also have additional teeth that alternate in size ( Maisano 2001). Further, Microsphenodon , like Sphenocondor , also has caniniform teeth, a tooth form not known in any other sphenodontians before the Jurassic, and not outside Neosphenodontia. However, the dentary of Sphenocondor also differs from Microsphenodon in seemingly possessing a large incisiform tooth and lacking the pronounced gap between the coronoid process and teeth (see below for further explanation).

It is important to compare Microsphenodon with Lanceirosphenodon (Romo de Vivar et al. 2020a) , also from the Riograndia AZ (Candeĺaria Sequence) of the Linha S̃ao Luiz outcrop in southern Brazil. Lanceirosphenodon resembles Microsphenodon , but there are several apomorphies that distinguish the two taxa. While the holotype and associated specimens of Microsphenodon are believed to represent adult individuals of a similar size, it is probable that Lanceirosphenodon is based on a juvenile holotype, with the distance between the symphysis to the coronoid process being ~ 7.1 mm, making it approximately twothirds the size of Microsphenodon . The pronounced gap between the ultimate tooth and the coronoid process of Microsphenodon , seen in other early sphenodontians including Diphydontosaurus , Planocephalosaurus and Clevosaurus spp. ( Fraser 1982, pl. 70 (2); Whiteside 1986, fig. 4B; Fraser 1988, fig. 19), is absent in Lanceirosphenodon . This gap does not relate to ontogeny as it is present throughout life in Clevosaurus hudsoni (Fraser 1988, fig. 23). There also appear to be two series of alternating teeth on the dentary of Lanceirosphenodon (Romo de Vivar et al. 2020a, fig. 4B), one apparently representing the hatchling teeth and the other the larger additional teeth, but there is only one such series in Microsphenodon . Lanceirosphenodon also lacks pronounced diagonal wear facets on its dentary, but this could reflect its young age. Further, though both Lanceirosphenodon and Microsphenodon share the trait of two large additional teeth most posteriorly on the dentary following a row of additional teeth that alternate in size, their teeth differ in shape. The two largest posterior teeth of Microsphenodon are mesiodistally elongated with an equidistant triangular profile, while those of Lanceirosphenodon are more conical (Romo de Vivar et al. 2020a, fig. 4). If Lanceirosphenodon is a juvenile, then it likely would have had more of these larger additional teeth as an adult. The maxilla of Clevosaurus brasiliensis possesses two large, additional teeth ( Fig. 3D ), so the possession of two large posterior-most teeth in the maxilla and dentary might have been a plesiomorphic trait for the rhynchocephalians of this Assemblage Zone.

Figures

Figure 3. Digital segmentation of selected cranial material of Clevosaurus brasiliensis (A, B, F–H, R, S, UFSM-PV-0131; C–E, I–O, UFRGS-PV-0748-T; P, Q, UFRGS-PV-0974-T). A, B, right premaxilla in A, lateral; and B, anterior views. C–E, right maxilla in C, lateral; D, ventral; and E, medial views. F, G, nasals in F, left lateral; and G, anterior views. H, rostrum in left lateral view. I, right prefrontal in lateral view. J, right postfrontal in lateral view. K, left postfrontal in medial view. L, parietals and frontals in dorsal view. M, frontals in ventral view. N, O, left jugal in N, lateral; and O, medial views. P, Q, left postorbital in P, lateral; and Q, medial views. R, S, left squamosal in R, lateral; and S, medial views. Abbreviations: add, additional; ant, anterior; con, conical; ept, ectopterygoid; f, frontal; fac, facet; ht, hatchling; j, jugal; m, maxilla; n, nasal; p, parietal; pal, palatine; pinf, pineal foramen; pm, premaxilla; po, postorbital; pof, postfrontal; post, posterior; prf, prefrontal; proc, process; q, quadrate; sq, squamosal; st, supratemporal; th, tooth; ven, ventral. Scale bars for C–E, L, M equal 10 mm; F, G, I, N–S equal 5 mm; and A, B, H, J, K equal 1 mm.

Figure 16. Phylogeny of Rhynchocephalia, recovered from: A, strict consensus tree from the maximum parsimony analysis with Bremer values above 1 labelled beside the nodes; and B, 50% majority rule consensus tree from the Bayesian-inference analysis with clade credibility values (decimal proportions) labelled beside nodes.