Osteoderm microstructure of phytosaur and aetosaur, other archosauriform (Eureptilia, Archosaurifomes)

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

Congreso; 10th International Congress of Vertebrate Morphology; 2013

In the present contribution, we analyse and discuss 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 bone was recorded in both taxa. Structural fibers were only recorded 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. Based on these results, 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 appears to be not possible.Such estimation seems to be more reliable 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 seem to express histological features that are potentially apomorphic for a species/genus.