Postcranial anatomy of Sebecus icaeorhinus (Crocodyliformes, Sebecidae) from the Eocene of Patagonia

ABSTRACT

We describe postcranial remains of new specimens referred to Sebecus icaeorhinus found in the lower section of the Sarmiento Formation at Cañadón Hondo (central Patagonia, Argentina), commonly regarded as part of the Casamayoran South American Land Mammal Age (middle Eocene). The new specimens include a partially articulated postcranium associated with teeth and fragmentary remains of the mandible that allows their identification as S. icaeorhinus. This taxon was almost exclusively known from skull remains from the same stratigraphic unit and was characterized by unique cranial features such as a long, high, and narrow rostrum bearing serrated teeth. The new material reveals numerous details on the postcranial anatomy of this crocodyliform, including the presence of proportionately long limbs and 10 autapomorphies in the vertebrae, forelimb, and pelvic girdle (some of which are interpreted as adaptations to terrestriality and an erect limb posture). These features depict a highly modified postcranial anatomy for S. icaeorhinus in comparison with that of neosuchian crocodyliforms, paralleling the uniqueness of its skull anatomy. The new information is also phylogenetically informative and incorporated into a cladistic analysis that corroborates not only the close affinities of Sebecidae with Baurusuchidae (sebecosuchian monophyly), but also the deeply nested position of this clade within Notosuchia. The incorporation of postcranial characters to the phylogenetic analysis also results in a novel arrangement of the basal mesoeucrocodylians recorded in the Cretaceous–Cenozoic of Gondwana, clustering all of these species into a large monophyletic clade.

ACKNOWLEDGMENTS

The specimens described were found during fieldwork led by A. López-Arbarello (MPEF-PV 1776) and by M. de la Fuente (PIP-CONICET 00795). Research for this contribution was possible due to the grant PICT 0736 (Agencia Nacional de Promoción de Ciencia y Técnica). This is JML's R-50 contribution to the Instituto de Estudios Andinos Don Pablo Groeber. J. González completed the drawings. We thank Aluar Aluminio Argentino and Mr. J. Groizard for access to the SEM lab. We thank F. Ortega for comments on the manuscript and access to specimens of Iberosuchus. We also thank the participants of the field expeditions: P. Puerta, M. Cardenas, J. Sterli, I. Maniel, B. von Baczko, M. Ramírez, and D. Castro. The specimens were prepared by L. Reiner. TNT is a free program made available by the Willi Hennig Society.

Handling editor: You Hailu