Osteohistology and paleobiological inferences of proterochampsids (Eucrocopoda: Proterochampsia) from the Chañares Formation (late Ladinian–early Carnian), La Rioja, Argentina

ABSTRACT

Proterochampsids were a group of stem archosaurs from the Middle–Late Triassic of South America. Using for the first time in proterochampsids quantitative microanatomical and morphological data and an inference model, we analyze the microstructure of postcranial bones of proterochampsids to infer life-history traits dealing with growth dynamics, ontogenetic changes, dermal armor histogenesis and lifestyle. We studied proterochampsid individuals from the Middle–Late Triassic Chañares Formation: three specimens of Chanaresuchus bonapartei: PVL 4575 (osteoderms and femur), CRILAR-Pv 80 (osteoderm) and CRILAR-Pv 81 (humerus), and a femur of a non-identified Rhadinosuchinae specimen (CRILAR-Pv 488). The osteoderms show a compact-uniform composition of parallel-fibered bone and lines of arrested growth (LAGs). The long bones are composed mainly of highly vascularized, parallel-fibered bone and isolated areas of woven-fibered bone in the innermost portion of cortex (only in C. bonapartei PVL 4575). The absence of an external fundamental system (EFS) indicates that all of the sampled individuals were somatically immature at time of death. The absence of mineralized bundles of collagenous fibers supports intramembranous ossification as the developmental origin for the osteoderms. With an inference model used for the first time in proterochampsids, the lifestyle inferred for C. bonapartei is terrestrial. Histological evidence indicates that sexual maturity is reached before both somatic and skeletal maturity. The growth patterns among proterochampsids exhibit some degree of variation even within a single species. Such variation can be due to different causes, including possibly phenotypic plasticity.

ACKNOWLEDGMENTS

We are thankful to M. Laurin (CR2P, Sorbonne Universités, Paris, France) for the helpful advice about the parameters and specific use of Bone Profiler and its application for the statistical model. We thank J. Cundiff (CMZ, Harvard University, U.S.A.) for the clarifying notes about the sectioned long bone of C. bonapartei (CMZ 4036) of the Ricqlès et al. (2008) article. We also thank A. Elbakyan, J. Bar and Wikipaleo Group for allowing free literature sharing. Finally, we thank C. Griffin (Virginia Tech, Blackburn, U.S.A.) and E. Amson (Museum für Naturkunde, Berlin, Germany) for their reviews and the suggestions made. Funding: PICT 2018-0717 (to JBD and IC), Universidad Nacional de San Juan grant (CICITCA, UNSJ-SECITI to MJT.).