Palaeohistology and external microanatomy of Triassic rauisuchian osteoderms (Archosauria: Pseudosuchia) and a preliminary comparison with selected outgroup taxa (i.e., aetosaurs and Revueltosaurus)

SCHEYER, TORSTEN; DESOJO, J. B.; CERDA, IGNACIO

Simposio; First International Symposium of Paleohistology; 2011

Postcranial osteoderms are plesiomorphic for Archosauria and can be found in all major pseudosuchians lineages, including e.g. aetosaurs, rauisuchians, or crocodylomorphs. Here we survey the external microanatomy and histology of postcranial osteoderms (i.e. dorsal paramedian and caudal osteoderms) in rauisuchians, a widely distributed assemblage of extinct carnivorous pseudosuchian taxa. Osteoderms of eight rauisuchians (incl. Batrachotomus kupferzellensis from Germany, Prestosuchus chiniquensis, “P.” loricatus and Rauisuchus tiradentes from South America, Ticinosuchus ferox and possible juvenile rauisuchian from Switzerland, Tikisuchus romeri and Yarasuchus deccanensis from India) were found to be rather compact bones, which usually lack significant bone remodelling or large areas of cancellous bone, thus presenting good growth records. Highly vascularised woven- or fibrolamellar bone tissue deposited in the core areas indicates higher growth rates during earlier life stages, whereas a more compact parallel-fibered bone matrix indicates reduced growth rates later in development. The predominant mode of osteoderm formation appears to be compatible with intramembraneous ossification, although a clear diagnosis remains tentative in fossils, due to the lack of stainable soft-tissue. The B. kupferzellensis osteoderm showed bone tissue of possible metaplastic origin, indicating that there may have been more than one mode of skeletogenesis active in this group. For outgroup comparison, paramedian osteoderms of aetosaurs (incl. material of Aetosaurus ferratus and Paratypothorax andressorum from Germany, Stagonolepis sp. nov. from Poland, and Calyptosuchus wellesi, Paratypothorax sp., and Typothorax coccinarum from North America) and of the Upper Triassic pseudosuchian Revueltosaurus callenderi (North America) were sampled. The aetosaur samples showed well developed diploe structures in which the cancellous part can be quite extensive. This is in contrast to previously sampled Aetosaurinae osteoderms from South America (incl. Aetosauroides scagliai), which generally lacked a larger area of interior cancellous bone (Cerda and Desojo, 2011). Woven- or fibrolamellar bone tissue was found in the Aetosaurinae osteoderms and in Typothorax coccinrum, whereas the other aetosaur samples were predominantly composed of parallel-fibred or lamellar-zonal bone. In R. callenderi, the interior core consisted of dense medullar bone, with dense trabecular bone being present only in the thick central region. Otherwise the specimen was composed of (often densely remodelled) parallel-fibred bone tissue.