Contributions on the paleobiology of the largest “rauisuchian”, Fasolasuchus tenax (Archosauria, Pseudosuchia), from Los Colorados Formation (Late Triassic) from La Rioja, Argentina, based on microstructural analyses of the postcranium

PONCE, DENIS A.; CERDA, IGNACIO A.; DESOJO, JULIA B.

Salta

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

Fasolasuchus tenax Bonaparte, 1981 was the largest continental carnivore archosaur (up 10 m estimated body length) prior to the rise of large theropod dinosaurs, during the Late Triassic. Despite its important paleoecological role, the issues relative to its paleobiology are scarcely studied. This contribution provides an analyses of different bone elements of the postcranium of the holotype (PVL 3850) of F. tenax (i.e., right femur, dorsal rib and sacral osteoderm). From a microstructural perspective, we pretend to infer different paleobiological issues such as dynamic and growth rate, minimum age at death, maturity states and growth patterns. The cortical bone of the right femur (at diaphysis location) is formed by moderately defined fibrolamellar bone and possesses an extremely uniform well-developed vascularization arranged in a laminar to subplexiform pattern. The dorsal rib exhibits a high grade of secondary remodeling and preserves the external fundamental system (EFS) in the subperiosteal region, although it does not record any line of arrested growth rather than these. In the femur and in the osteoderm were registered 2 and 8 LAGs respectively. Since on the extant pseudosuchians, the Crocodylia, the osteoderms start to develop after the first year of life, it is estimated that F. tenax PVL 3850 reached a minimal age of 9 years old. Likewise, it is inferred that the individual attained somatic (presence of the EFS), sexual and skeletal maturity, the latter being supported by anatomical data (closed neurocentral sutures). The predominance of highly vascularized fibrolamellar tissue with a laminar-subplexiform vascularization reveals high rates of osteogenesis for the femur, which suggests a high rate of body growth for the individual. These rapid growth rates are features shared by several basal loricatans ("rauisuchians") studied to date (Batrachotomus, Effigia, Postosuchus) in their appendicular bones. Unlike what has been reported in other large pseudosuchian species (Desmatosuchus, Mystriosuchus), F. tenax reached its characteristic size in a relatively short period of time thanks to this accelerated growth rate. These results encourage a larger sampling of other skeletal elements to identify whether these features are autapomorphies and recognize this type of growth in close taxa (e.g., Saurosuchus, Sillosuchus).(Page R122)