Osteoderms of Simosuchus clarki (Crocodyliformes: Notosuchia) from the Late Cretaceous of Madagascar

ABSTRACT

The pug-nosed crocodyliform Simosuchus clarki is known from several specimens that collectively preserve, in addition to the skull and postcranial endoskeletal elements, a complete complement of osteoderms covering every major body region. This comprehensive sample of osteoderms facilitates the reconstruction, from multiple sources, of the entire postcranial dermal skeleton of this crocodyliform. In this contribution, I describe the anatomy, arrangement, and histology of the osteoderms of S. clarki, and provide comments on the phylogenetic and functional importance of these structures. As in other crocodyliforms, the osteoderms of Simosuchus were organized into discrete ‘shields’ covering the dorsal and ventral regions of the trunk and tail. Simosuchus also exhibited extensive osteodermal coverage of the limbs, a feature that characterizes very few crocodyliforms. The osteoderms from different shields possess distinctive morphological features, allowing isolated osteoderms to be assigned accurately to their respective shields. Paleohistological investigations reveal porous, lightly built osteoderms overlying dorsal and appendicular regions, and gastral osteoderms with a flat, diploe-like structure. Details about growth and longevity in Simosuchus are equivocal when assessed using data from osteoderms alone. Simosuchus possessed a tetraserial paravertebral shield, with at least four accessory parasagittal rows of osteoderms on either side. Sagittal segmentation probably enhanced flexibility with respect to ancestral crocodyliforms, but the presence of numerous, tightly sutured accessory rows suggests that lateral undulation, and therefore swimming, was likely restricted. Several new morphological characters are described here, and it is proposed that these will be informative in future studies of crocodyliform phylogeny.