A metriorhynchid crocodyliform braincase from northern Chile

ABSTRACT

A three-dimensionally preserved metriorhynchid braincase from the Oxfordian of northern Chile is described. The specimen is referred to the metriorhynchid Metriorhynchus cf. M. westermanni. The excellent preservation provides clear sutures and a detailed description, and X-ray computed tomographic (CT) scanning provides internal anatomical details. The general pattern of the orbitotemporal region is consistent with that of the basal thalattosuchian Pelagosaurus typus as described recently. The specimen from northern Chile shares with other metriorhynchids (e.g., Cricosaurus araucanensis, Metriorhynchus westermanni, M. casamiquelai, and Dakosaurus andiniensis) a dorsally exposed laterosphenoid, a laterosphenoid-prootic suture forming a blunt crest separating the supratemporal fenestra into two fossae for muscular attachment, and the quadrate incompletely sutured to the braincase. Thus, these features characterize not only basal but derived Thalattosuchia, as suggested by previous authors. The main difference in the orbitotemporal region is that in the specimen described herein, and in the other metriorhynchids examined, the trigeminal fossa is developed mainly caudal to the trigeminal foramen, whereas in P. typus the fossa is developed rostral and caudal to the trigeminal foramen. CT scanning indicates the presence of enlarged dorsal dural venous sinuses overlying the brain, as it has been described recently in Steneosaurus pictaviensis, and a well-developed sinus within the quadrate. The large foramen ventrolateral to the occipital condyle, which characterizes metriorhynchids, is confirmed as the entry for the internal carotid artery.

ACKNOWLEDGMENTS

We thank A. Hernández who found MDA 2. We are grateful to R. A. Alvarez del Rivero and O. Salvador of the Unidad Tomografía Computada at the Hospital Interzonal de Agudos de la Matanza ‘Dr. Diego Pairoissien’ (La Matanza, Argentina) for providing access to CT facilities and assistance with CT scanning, and to P. J. Currie (University of Alberta) for allowing the use of the software Mimics. J. Posik (Museo de La Plata, Argentina) mechanically prepared the specimen. We thank A. Garrido (Museo Olsacher, Zapala) and both S. Chapman and L. Steel (Natural History Museum, London) for their hospitality and access to collections. We thank H. C. Larsson (McGill University, Canada) for comments and English grammar corrections. We thank S. Pierce and an anonymous reviewer for their valuable comments. This research has been supported by the following grants: Agencia Nacional de Promociones Científicas y Tecnológicas de Argentina (PICT 25276, 0261), Consejo Nacional de Investigaciones Científicas y Tecnológicas (PIP 0426), and Programa de Incentivos UNLP N607 (Argentina).

Handling editor: Sean Modesto.