Microestructura ósea de osteodermos de arcosauriformes basales

Abstract

The occurrence of osteoderms is common in the most Archosauriformes lineage, including basal forms as doswelliids and proterochampsids. In this survey, osteoderms of doswelliids Doswellia kaltenbachi, Archeopelta arborensis, Tarjadia ruthae and Vancleavea campi; and proterochampsids Pseudochampsa ischigualastensis and Chanaresuchus bonapartei were examined in order to find out several aspects of their paleobiology, such as histogenesis, age estimation, origin of external sculpturing, paleoecology, relation with soft tissue and systematics. Also, dorsal ribs of Vancleavea campi and Chanaresuchus bonapartei were compared for age estimation. Doswelliids osteoderms show a trilaminar structure: two cortices of compact bone (external and basal) that enclosing a reduced internal core of cancellous bone. In contrast, proterochampsids osteoderms are entirely compact. Doswellia kaltenbachi differs from Archeopelta arborensis and Tarjadia ruthae in the type of his ornamentation development. While in the first, the ornamentation is formed and maintained by preferential bone growth, a complex pattern of resorption and redepositation is inferred in the former. Vancleavea campi exhibits the high degree of variation among doswelliids. Due to the occurrence of structural fibers and the lack of ornamentation, is likely that this specie belongs to another lineage. When compared with dorsal ribs, osteoderms appears to be better elements for skeletochronological studies. The high degree of compactness in the osteoderms of all the examined taxa is congruent with an aquatic or semi-aquatic life style. In general, osteoderms histology of doswelliids bears a closer resemblance to that of phytosaurs and pseudosuchians than the proterochampsid osteoderms microstructure.