Osteohistology and palaeobiological inferences of proterochampsids (Eucocropoda: Proterochampsia) from Chañares Formation (late Ladinian?early Carnian), La Rioja, Argentina

PONCE, D.; TROTTEYN, M. J.; CERDA, IGNACIO A.; FIORELLI, LUCAS E.; DESOJO, J. B.

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Lugar: Lawrence; Año: 2021

0272-4634

ABSTRACT?Proterochampsids were a group of stem archosaurs from the Middle?LateTriassic of South America. Using for the first time in proterochampsids quantitativemicroanatomical and morphological data and an inference model, we analyze themicrostructure of postcranial bones of proterochampsids to infer life-history traits dealingwith growth dynamics, ontogenetic changes, dermal armour histogenesis and lifestyle. Westudied 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-identifiedRhadinosuchinae specimen (CRILAR-Pv 488). The osteoderms show a compact-uniformcomposition of parallel-fibered bone and lines of arrested growth (LAGs). The long bones arecomposed 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). Theabsence of an external fundamental system (EFS) indicates that all of the sampled individualswere somatically immature at time of death. The absence of mineralized bundles ofcollagenous fibers supports intramembranous ossification as the developmental origin for theosteoderms. With an inference model used for the first time in proterochampsids, the lifestyleinferred for C. bonapartei is terrestrial. Histological evidence indicates that sexual maturity isreached before both somatic and skeletal maturity. The growth patterns amongproterochampsids exhibit some degree of variation even within a single species. Suchvariation can be due to different causes, including possibly phenotypic plasticity.