CONICET | Buscador de Institutos y Recursos Humanos

Metriorhynchids were a group of Mesozoic crocodyliforms completely adapted to a marine lifestyle. Although there are a large number of metriorhynchid skulls, mainly from the Middle and Upper Jurassic from the European margins of the Western Tethys, most of them have collapsed into a single layer. This has confounded accurate braincase descriptions. A three-dimensional metriorhynchid braincase of Metriorhynchus cf. westermanni from the Oxfordian of northern Chile has excellent preservation and shows details of the sutures. X-ray computed tomographic (CT) scanning provides internal anatomical details. The general pattern of the orbitotemporal region is consistent with that described by other authors in the basal thalattosuchian Pelagosaurus typus. The specimen from northern Chile shares with other metriorhynchids (Cricosaurus araucanensis, Metriorhynchus westermanni, M. casamiquelai and Dakosaurus andiniensis) a dorsally exposed laterosphenoid, a laterosphenoid-prootic suture that forms a blunt crest separating the dorsotemporal fenestra into two muscular fossae, and a quadrate that is incompletely sutured to the braincase. The main difference in the orbitotemporal region is that in the Chilean specimen, and the other metriorhynchids examined, the trigeminal fossa is mainly positioned caudal to the trigeminal foramen, whereas in Pelagosaurus typus it is rostral and caudal to the trigeminal foramen. The CT scans reveal the presence of enlarged dorsal dural venous sinuses overlying the brain, a paired tube-like cavities connecting the cranioquadrate passage to the dorsal dural venous sinus, and a well developed sinus within the quadrate. The large foramen ventrolateral to the occipital condyle, characteristic for metriorhynchids, is confirmed as the opening for the internal carotid artery.

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