Braincase, neuroanatomy, and neck posture of Amargasaurus cazaui (Sauropoda, Dicraeosauridae) and its implications for understanding head posture in sauropods

ABSTRACT

The braincase of Amargasaurus cazaui from the Lower Cretaceous of Argentina represents the only dicraeosaurid sauropod neurocranial material known from South America. It has been computer tomographically (CT) scanned and three-dimensional digital reconstructions of the endocranium and inner ear have been made. The cranial endocast is complete, with a volume of approximately 94–98 ml, excluding the dorsal sinuses. The labyrinth of the inner ear is dorsoventrally taller than the lagena, which is conical, and relatively short. The anterior semicircular canal is longer than the posterior and lateral semicircular canals, as in most non-titanosaurid sauropods. When the braincase is oriented with the lateral semicircular canal positioned horizontally, the occipital condyle is oriented posteroventrally, suggesting that the head was held with the muzzle pointing downward. The morphology of the atlas and axis, together with the reconstruction of the osteological neutral pose of the neck, supports this neck and head position, and also indicates the presence of the proatlas in this taxon. The evidence presented here for the skull and neck position of Amargasaurus fits with a midheight food-gathering strategy. The presence of titanosauriforms and rebbachisaurids, together with Amargasaurus, supports the niche partitioning hypothesis for the La Amarga Formation sauropods.

ACKNOWLEDGMENTS

We thank A. Kramarz (MACN) for permitting the CT scanning and study of the specimen, and M. Zamboni and his colleagues at the Alejandro Posadas Hospital (Buenos Aires) for their time and kind help realizing the CT scans. I. Cerda (MPCA) helped during the CT scanning process in the transportation of the material. R. Romero is thanked for the atlas-axis drawings. V. Arbour (University of Alberta) instructed one of the authors (A.P.C.) in the use of the software Mimics and Geomagic. Photographs of the braincase and endocast of Dicraeosaurus were shared by F. Knoll (Museo Nacional de Ciencias Naturales–CSIC, Spain). We are grateful to the reviewers J. Whitlock (Mount Aloysius College) and F. Knoll (MNCN-CSIS) and the editor H. You, whose comments have greatly improved this paper. This study was primarily developed during a CONICET postdoctoral scholarship (A.P.C. and J.L.C.), and was partially funded by a Dinosaur Research Institute travel grant (to A.P.C.) and a PICT 0378 (to J. L. C.).

Handling editor: Hailu You