COMPARATIVE ENDOCRANIAL MORPHOLOGY OF TWO THALATTOSUCHIAN (CROCODYLOMORPHA) SPECIES: STENEOSAURUS BOLLENSIS AND CRICOSAURUS ARAUCANENSIS

HERRERA YANINA

Haarlem

Congreso; XIV Annual Meeting of the European Association of Vertebrate Palaeontologists; 2016

Thalattosuchians are Mesozoic crocodylomorphs mainly known from marine deposits from the Lower Jurassic till the Lower Cretaceous. Thalattosuchia comprises two main lineages, Teleosauridae and Metriorhynchoidea, with the latter encompassing the fully pelagic forms, the Metriorhynchidae clade. Here, I present data on virtual endocasts obtained after CT scanning of two Thalattosuchia specimens: Steneosaurusbollensis (BSPG 1984 I 258) and Cricosaurus araucanensis (MLP 72-IV-7-1), and provide a comparative analysis. Three-dimensional reconstructions of the brain, inner ear, carotid canal, and paratympanic sinus system were made. Both endocasts show that the brain is sub-horizontally displaced, with not well-marked pontine and cerebral flexures. A conspicuous tube-like cavity that connects with the dorsal longitudinal venous sinus has also been identified in both taxa. In S. bollensis the carotid foramen pierces the exoccipital with a posterolateral direction, differing from the condition in metriorhynchids, on which the foramen has a strictly posterior direction. In the S. bollensis the carotid canals are incompletely ossified in the middle region, being impossible to differentiate the carotid canals from the pneumatic cavity, whereas in C. araucanensis the canals are completely ossified. The semicircular canals of the inner ear of both specimens are aligned in approximately orthogonal planes in three-dimensional space. The canals in S. bollensis are thinner than the ones of C. araucanensis (narrower in diameter relative to overall labyrinth size). In both taxa the anterior semicircular canal is longer than the posterior, which in turns is similar in size to the lateral canal. Several interconnected diverticular expansions from the pharyngotympanic sinus and median pharyngeal sinus related to the paratympanic sinus system have been identified in S. bollensis. In C. araucanensis only a few of them were identified, likely more related to the low resolution of the CT scan than to an anatomical feature of this specimen. The endocast morphology of Steneosaurusbollensis indicates the presence of several features previously suggested to be exclusive of the Metriorhynchidae clade. These findings highlight the limited knowledge about the neuroanatomy of Thalattosuchia, indicating that it is a topic that still needs to be further explored.