OSTEODERM MICROSTRUCTURE OF PHYTOSAURS AND AETOSAURS (EUREPTILIA, ARCHOSAURIFORMES)

SCHEYER, T.; DESOJO, J.B; CERDA, I.

Congreso; 10th International Congress of vertebrate morphology; 2013

We analysed the osteoderm microstructure of several archosauriforms. The phytosaur sample consists of osteoderms of Leptosuchussp., Paleorhinus sp., Pseudopalatus sp. and Phytosauria indet. (all from North America) and cf. Mystriosuchus sp. (Germany). The aetosaur sample included osteoderms of Aetosaurus ferratus andParatypothorax andressorum (Germany), Stagonolepis olenkae (Poland), and Adamanasuchus eisenhardtae, Calyptosuchus cf. wellesi, Desmatosuchus smalli and D. spurensis, Paratypothorax sp., , Tecovasuchus chatterjeei, Typothorax cf. T. coccinarum and Typothorax sp. and of a new stagonolepidid (North America). Both phytosaur and aetosaur osteoderms are composed of a diploe structure with external and basal compact bone layers framing an internal cancellous bone core. The external cortex of phytosaur osteorderms is mostly composed of parallel fibered bone. In contrast, in aetosaurs the external cortex mainly consists of lamellar bone, with lines of resorption within the primary bone that indicate successive cycles of bone erosion and deposition. The basal cortex in both groups is composed of parallel fibered bone. The cancellous bone of the internal core is more developed in aetosaurs than in phytosaurs. Woven or fibrolamellar boneoccur in both taxa. Structural fibers only occur in the internal core of two phytosaurs. Growth marks (annuli and/or LAGs) are developed in both external and basal cortices of aetosaurs and phytosaurs. We conclude: 1- Given the presence of a well developed core of cancellous bone tissue in aetosaurian osteoderms, the estimation of the absolute age of the individuals from growth mark counting alone is not possible, but may be feasible in phytosaurs. 2- Bone microstructures are more uniform in phytosaur osteoderms and show a higher level of disparity among aetosaur osteoderms. 3- Some aetosaur osteoderms express histological features that are potentially apomorphic for a species/genus.