New information on the braincase of <i>Sinraptor dongi</i> Currie and Zhao (Theropoda, Allosauroidea): ethmoidal region, endocranial anatomy and pneumaticity

Abstract

The ossified ethmoidal elements of Sinraptor dongi Currie and Zhao (IVPP 10600) are U-shaped, and the frontal roofs the olfactory tract and cavities. When the ethmoidal ossification is placed in anatomical position in the braincase, an unossified space remains between the sphenethmoid and the orbitosphenoid. This suggests there was a cartilaginous septosphenoid when the animal was alive. The braincase was CT scanned after removing the sediment filling the endocranium and pneumatic recesses, and this led to the recognition of some traits that have not been previously described. Particularly interesting is the presence of a well developed caudal tympanic recess (generally considered as a trait characteristic of coelurosaurs), and the internal morphology of the basisphenoidal recess and associated pneumatic cavities. There is a longitudinal passage connecting the lateral tympanic recess with the basisphenoid recess that probably had both pneumatic and vascular functions. Endocranially, there is no medullar eminence, and the floccular recess has an hour-glass shape. The volume of the endocranial cavity, excluding the olfactory tract and bulbs, is 95ml. The cephalic and pontine flexures are similar to those in carcharodontosaurids (Carcharodontosaurus and Giganotosaurus), although the disposition of cranial nerves II-IV in the midbrain varies amongst these taxa. The CT scans revealed a small second branch for cranial nerve XII. The encephalization quotient calculated for Sinraptor is within the range calculated for other carnosaurs. The comparison within Allosauroidea indicates that the endocast of Sinraptor shares more similarities with carcharodontosaurines and Allosaurus than with Acrocanthosaurus, which has an endocast that is strongly different from other carcharodontosaurids.