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| Clevosaurus hadroprodon
Hsiou, Nydam, Simões, Pretto, Onary, Martinelli, Liparini, Martínez, Soares, Schultz & Caldwell, 2019
Reconstruction: Jorge Blanco.
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Abstract
The early evolution of lepidosaurs is marked by an extremely scarce fossil record during the Triassic. Importantly, most Triassic lepidosaur specimens are represented by disarticulated individuals from high energy accretion deposits in Laurasia, thus greatly hampering our understanding of the initial stages of lepidosaur evolution. Here, we describe the fragmentary remains of an associated skull and mandible of Clevosaurus hadroprodon sp. nov., a new taxon of sphenodontian lepidosaur from the Late Triassic (Carnian; 237–228 Mya) of Brazil. Referral to Sphenodontia is supported by the combined presence of a marginal dentition ankylosed to the apex of the dentary, maxilla, and premaxilla; the presence of ‘secondary bone’ at the bases of the marginal dentition; and a ventrally directed mental process at the symphysis of the dentary. Our phylogenetic analyses recover Clevosaurus hadroprodon as a clevosaurid, either in a polytomy with the Late Triassic to Early Jurassic Clevosaurus and Brachyrhinodon (under Bayesian inference), or nested among different species of Clevosaurus (under maximum parsimony). Clevosaurus hadroprodon represents the oldest known sphenodontian from Gondwana, and its clevosaurid relationships indicates that these sphenodontians achieved a widespread biogeographic distribution much earlier than previously thought.
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| Figure 2 Type and referred specimens of Clevosaurus hadroprodon sp. nov. Photograph (a) and outline (b) of holotype partial skull (MMACR PV-027-T). Photograph (c) and outline (d) of referred partial left dentary (MMACR PV-028-T). Photograph (e) and outline (f) of posterior view of section in box in c showing cross sectional view of tooth attachement.
Abbreviations: Lt Den, left dentary; Lt Mx, left maxilla; MeG, Meckel’s groove; Pmx, right premaxilla; Pmx Fac, premaxillary facet in maxilla; Rt Den, right dentary; Rt Mx, right maxilla; SB, secondary bone; D.V.Cr., dentary ventral crest; Sym, symphyseal region of dentary.
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Systematic Palaeontology
Lepidosauria Dumeril and Bibron, 1839 sensu Evans, 1984
Rhynchocephalia Günther, 1867 sensu Gauthier et al., 1988
Sphenodontia Williston, 1925 sensu Benton, 1985
Clevosauridae Bonaparte and Sues, 2006 (sensu this study)
Clevosaurus hadroprodon sp. nov.
Etymology: Species epithet comes from a combination of ‘hadroprodon’ (from the Greek “hadros”-meaning large) and “protos”, meaning first (Greek), and “odous”, meaning tooth (Greek); in reference to the “big first tooth”, i.e. the large tusk-like tooth of the premaxilla and the anteriormost/first dentary tooth position.
Locality and horizon: Linha Bernardino locality, Candelária municipality, Rio Grande do Sul State, Southern Brazil (Fig. 1); Santa Maria Formation (Santa Maria Supersequence, Candelária Sequence), Rosário do Sul Group, Paraná Basin; Carnian, Late Triassic. .....
Diagnosis: Small sphenodontian rhynchocephalian differing from all other known sphenodontians in possessing the following combination of features: (1) a large, blunt, tusk-like tooth in both premaxilla and first tooth position of dentary; (2) in having an angled, but nearly vertical mandibular symphysis and relatively deep dentary; (3) absence of an edentulous gap between the tusk-like tooth and the remaining dentition; (4) absence of a posterodorsal process of the premaxilla; (5) in lacking well-developed medial-posteromedial expansion of the posterior dentition; (6) and lacking flanges on the teeth, and (7) lacking labially expanded teeth. Feature 4 makes this new taxon different from many sphenodontians, including other species of Clevosaurus, Rebbanasaurus, Godavariasaurus, Priosphenodon, Sphenotitan, Paminzisaurus and Ankylosphenodon. Features 5 and 6 make this taxon different from all other described species of Clevosaurus [Clevosaurus hudsoni, C. bairdi, C. latidens, C. sectumsemper, C. cambrica, and Clevosaurus sp. (China)]
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| Figure 3 Phylogenetic analyses evidencing the position of Clevosaurus hadroprodon sp. nov. within Rhynchocephalia using two distinct optimality criteria. (a) Resultant topology of the strict consensus of eight most parsimonious tree (249 steps each) (CI = 0.43; RI = 0.69); (b) Majority-rule consensus tree derived from the Bayesian analysis. Nodes values represent posterior probabilities. (Note that Clevosauridae sensu Bonaparte and Sues, was not recovered as a monophyletic group in both analyses and the genus Clevosaurus was not recovered as monophyletic in the Bayesian topology).
Abbreviations: “Cle”: Clevosauridae; Hom: Homeosauridae; Pleu: Pleurosauridae; Opi: Ophistodontia.
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| Figure 5 Life reconstruction of Clevosaurus hadroprodon.
Reconstruction created by Jorge Blanco.
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Annie S. Hsiou, Randall L. Nydam, Tiago R. Simões, Flávio A. Pretto, Silvio Onary, Agustín G. Martinelli, Alexandre Liparini, Paulo R. Romo de Vivar Martínez, Marina B. Soares, Cesar L. Schultz and Michael W. Caldwell. 2019. A New Clevosaurid from the Triassic (Carnian) of Brazil and the Rise of Sphenodontians in Gondwana. Scientific Reports. 9, 11821. nature.com/articles/s41598-019-48297-9
In the shadow of the dinosaurs eurekalert.org/e/9Q6y via @EurekAlert
