A new basal turtle from the Triassic of Argentina challenges the idea of homology in peripheral plates in turtles.

JULIANA STERLI; RICARDO MARTÍNEZ; IGNACIO A. CERDA; CECILIA APALDETTI

Jornada; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2019

Microanatomical and histological studies showed that the bony plates of all known turtles have a structure formed by external and internal cortices surrounding an internal core of cancellous bone. A new species of turtle from the Late Triassic from the Provicne of San Juan shows an unexpected structure in its peripheral plates that challenges the homology of these plates in turtles. Peripheral plates of this new Triassic turtle display an internal cavity surrounded by a thin layer of bone, which has a trilaminar structure formed by two cortices framing cancellous bone. Following Remane?s probabilistic criteria to define homology, this new Triassic turtle only fulfills the positional criterion (peripheral plates encircle the carapace in all known turtles). On the contrary, the structural (the microanatomy and histology) and the transitional (lack of ?transitional? / ?intermediate? forms) criteria are not fulfilled. Nevertheless, as the position and general morphology of peripheral plates are very similar, it could be interpreted as a case of ?deep? homology, where a particular phenotypic manifestation may vary but the common ability to express such feature is established. Besides the peculiar microanatomy and histology of this new turtle from San Juan, in this contribution we also present its general cranial, shell, and appendicular anatomy as well as phylogenetic analysis exploring its relationships. The general anatomy (PVSJ 903, 904, 1092, and 1093) allows us recognizing it as a new species, closely related to Palaeochersis talampayensis (Late Triassic from the Province of La Rioja, Argentina) and Australochelys africanus (Early Jurassic, South Africa).