Osteoderm microstructure of the Late Triassic Argentinean “rauisuchian” (Archosauria: Pseudosuchia)

IGNACIO A. CERDA; JULIA B. DESOJO; TORSTEN M. SCHEYER; CESAR M. SCHULTZ

General Roca, Río Negro.

Jornada; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina; 2012

We analyse and discuss the microanatomy and histology of osteoderms of the “rauisuchian” Saurosuchus galilei Reig and Fasolasuchus tenax Bonaparte. This is the first histological description of “rauisuchian” osteoderms from Argentina. We studied transversal sections of a cervical osteoderm of S. galilei (PVSJ 32) from the Ischigualasto Formation (Carnian-Norian) and a caudal osteoderm of F. tenax (PVL 3850) from Los Colorados Formation (Late Norian). Whereas the osteoderm of S. galilei has a rather compact structure, F. tenax shows a diploe architecture, with a central cancellous core bordered by two compact cortices. Both specimens show the external and basal cortices composed mainly of poorly vascularised, parallel-fibred bone tissue. Sharpey’s fibres are commonly observed in the compact bone. In S. galilei, the internal region consists of a well vascularised core in which the intrinsic fibres exhibit important variation, ranging from coarse, parallel-fibred to woven-fibred bone tissues. Cyclical growth marks are well recorded in the basal and external cortices of both specimens. The observed variation in the bone microanatomy (compact vs. diploe) has been previously reported in the osteoderms of other “rauisuchian” taxa (e.g. Prestosuchus chiniquensis Huene) and appear to be related to the age and/or sex and reproductive status of specimens (osteoderms of older individuals and/or reproductive females are more remodelled). Also, the presence of highly vascularized woven bone tissue deposited in the core area of S. galilei has been recorded in other “rauisuchians” and suggests higher growth rates during earlier ontogenic stages.