Endocranial anatomy of Rhynchocephalia (Diapsida, Lepidosauria): ontogenetic variation and allometric relationships in Clevosaurus brasiliensis (Upper Triassic, Brazil) and Sphenodon punctatus

Author(s)

Lívia Roese Miron

• Document type: Master's Dissertation
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC)
• Defense date: 2022-03-11

Examining board members

Annie Schmaltz Hsiou; Gabriel de Souza Ferreira; Rodrigo Temp Muller; Leonardo Rodrigo Kerber Tumeleiro

• Advisor: Annie Schmaltz Hsiou

Abstract

The endocranial cavities of living and fossil species, such as the brain and nasal cavities and endosseous labyrinth, can inform not only about the ecology and sensory capacities of vertebrates, but about the evolutionary history of their neuroanatomy. However, the correspondence between the brain and its cavity, and, therefore, its informative potential, varies taxonomically. Nowadays represented only by Sphenodon punctatus (the \"tuatara\"), endemic of New Zealand, Rhynchocephalia was the most diversified clade of Lepidosauria in the first half of Mesozoic. In the Upper Triassic of Rio Grande do Sul (Brazil), Clevosaurus brasiliensis is a rhynchocephalian particularly abundant (Caturrita Formation, Assemblage Zone of Riograndia, Norian). Despite the anatomy of the brain of S. punctatus being relatively well known, there is no description of its endocranial cavities so far, nor of any fossil rhynchocephalians. Therefore, this study aimed to: (i) describe the anatomy and ontogenetic variation of the endocranial cavities (endocasts) of S. punctatus; (ii) describe the endocast of C. brasiliensis and compare it to the living species; and (iii) investigate the correspondence between endocast and brain in S. punctatus. One specimen of C. brasiliensis and seven specimens of S. punctatus between hatchlings and adults were analysed. The endocasts were generated through Computed Tomography and digital segmentation. Linear regressions of their measurements were utilized to investigate their allometric growth. Also, The Reptilian Encephalization Quotient (REQ) was calculated for S. punctatus. The brain endocast of Rhynchocephalia presents a linearized organization, comprising narrow olfactory tract and expanded bulb; forebrain and midbrain expanded laterally and ventrally; and medulla oblongata with lateral surfaces practically parallel. The volume and general morphology of the brain cavity of S. punctatus reflect the brain poorly (only 30% of the brain cavity are occupied by the brain), but the measurements of total length, olfactory tract length and forebrain and midbrain length are very similar between models. Thus, when studying the correspondence between brain and endocast of the species, regional differences must be considered. The REQ of S. punctatus varied between 0.759 in a juvenile to 1.29 in a bigger adult when the brain mass was considered 30% of the endocast volume. However, it is likely that the brain fills more than that in juveniles, and, therefore, the brain mass and REQ of the juvenile are very likely underestimated. During its ontogeny, the brain cavity of S. punctatus becomes longer, narrower. and more dorsoventrally curved. The cerebral hemispheres expand laterally, and the ventral surface of the hindbrain goes from concave in the juveniles to convex in the adults. The endocast of C. brasiliensis presents characteristics from adults and juveniles of S. punctatus, being short as in the former and narrow as in the latter, but its dorsoventral angulation is lower and total volume is proportionally lower. Clevosaurus brasiliensis is a rhynchocephalian of very small size, and it presents cranial characteristics considered typical of juvenile S. punctatus. Therefore, the similarity between the endocast of C. brasiliensis and a juvenile of S. punctatus may be not only a consequence of allometry, but also a manifestation of the peramorphic morphology of the living species. (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