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

PONCE, DENIS; M. JIMENA TROTTEYN; CERDA, IGNACIO; LUCAS E. FIORELLI; DESOJO, JULIA BRENDA

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Lugar: Lawrence; Año: 2021

0272-4634

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