Insights into the shell microstructure of Bauruemys elegans and other pelomedusoids from the Cretaceous and Paleogene of Brazil, including first Testudines material from Jangada Roncador Village, Paraná Basin

Abstract

Pelomedusoides and Chelidae are groups of pleurodiran turtles that have evolved from the Cretaceous period to the present. In this study, we carried out thin sections in shell elements (including costal, neural, peripheral and xiphiplastron bones) from Bauruemys elegans, an undetermined Podocnemididae taxon, and shell fragments of other pelomedusoids from Upper Cretaceous and Paleogene sedimentary deposits. We explored and discussed shell microstructures and microanatomy data of these extinct pelomedusoids alongside two extant pleurodires, Podocnemis unifilis and Phrynops sp. Our study of costo-neural bones reinforces the hypothesis that the turtle carapace originated endoskeletally from ribs and vertebral arches. Our osteohistological examination of these shell bones suggests an aquatic to semi-aquatic lifestyle for the studied extinct Pelomedusoides. Notably, all examined bones show high bone compactness, a feature often associated with terrestrial life, although it is not exclusive to such a lifestyle. Additionally, our comparative analysis reveals greater shell compactness in Podocnemididae compared to the other specimens sampled. These findings contribute to our understanding of the environmental interactions of these pleurodires.