Virtual endocasts of Clevosaurus brasiliensis and the tuatara: Rhynchocephalian neuroanatomy and the oldest endocranial record for Lepidosauria

Author(s)

Roese-Miron, Livia ; Jones, Marc Emyr Huw ; Ferreira, Jose Darival ; Hsiou, Annie Schmaltz

• Document type: Journal article
• Source: Anatomical Record-Advances in Integrative Anatomy and Evolutionary Biology; v. N/A, p. 24-pg., 2023-03-23.

Abstract

Understanding the origins of the vertebrate brain is fundamental for uncovering evolutionary patterns in neuroanatomy. Regarding extinct species, the anatomy of the brain and other soft tissues housed in endocranial spaces can be approximated by casts of these cavities (endocasts). The neuroanatomical knowledge of Rhynchocephalia, a reptilian clade exceptionally diverse in the early Mesozoic, is restricted to the brain of its only living relative, Sphenodon punctatus, and unknown for fossil species. Here, we describe the endocast and the reptilian encephalization quotient (REQ) of the Triassic rhynchocephalian Clevosaurus brasiliensis and compare it with an ontogenetic series of S. punctatus. To better understand the informative potential of endocasts in Rhynchocephalia, we also examine the brain-endocast relationship in S. punctatus. We found that the brain occupies 30% of its cavity, but the latter recovers the general shape and length of the brain. The REQ of C. brasiliensis (0.27) is much lower than S. punctatus (0.84-1.16), with the tuatara being close to the mean for non-avian reptiles. The endocast of S. punctatus is dorsoventrally flexed and becomes more elongated throughout ontogeny. The endocast of C. brasiliensis is mostly unflexed and tubular, possibly representing a more plesiomorphic anatomy in relation to S. punctatus. Given the small size of C. brasiliensis, the main differences may result from allometric and heterochronic phenomena, consistent with suggestions that S. punctatus shows peramorphic anatomy compared to Mesozoic rhynchocephalians. Our results highlight a previously undocumented anatomical diversity among rhynchocephalians and provide a framework for future neuroanatomical comparisons among lepidosaurs. (AU)

• FAPESP's process: 20/06819-5 - New materials of Clevosaurus (Rhynchocephalia, Sphenodontia) from the Triassic of Southern Brazil: approaches about ontogenetic variation and endocranial anatomy
• Grantee: Lívia Roese Miron
• Support Opportunities: Scholarships in Brazil - Master
• FAPESP's process: 19/14153-0 - Neogene vertebrate microremains of the Brazilian Amazon region, with emphasis on the temporal resolution of the Solimões Formation, Miocene of the Acre Basin
• Grantee: Annie Schmaltz Hsiou
• Support Opportunities: Regular Research Grants