New information on the braincase of Sinraptor dongi Currie y Zhao 1993 (Theropoda: Allosauroidea).

PAULINA CARABAJAL, A.; CURRIE, P. J.

San Juan

Congreso; IV Congreso Latinoamericano de paleontología de vertebrados; 2011

Museo de Ciencias Naturales, Universidad Nacional de San Juan

The braincase of the basal theropod Sinraptor dongi (IVPP 10600) was described in detail when the name of the taxon was coined. Further mechanical preparation and new CT scanning of the material provides information about the endocranial morphology, inner ear and braincase pneumaticity of this taxon (Paulina Carabajal and Currie, in press). The ossified ethmoidal elements were found separated from the rest of the braincase. These are poorly known elements within dinosaurs, because remain cartilaginous in most subadult and adult theropods (Currie, 1997), reason why they were not recognized when the specimen was originally described (Currie and Zhao, 1993). The ethmoidal elements of Sinraptor are U-shaped, and the frontal roofs the olfactory tract and bulbs cavities. When the ethmoidal ossification is placed in anatomical position, an unossified space remains between the sphenethmoid and the orbitosphenoid, suggesting a cartilaginous septosphenoid when the animal was alive. Comparison with other theropods indicates four ossification patterns for the ethmoidal elements within this group (Paulina Carabajal, 2009). The braincase was CT scanned after removing the sediment filling the endocranium and pneumatic recesses. Particularly interesting is the presence of a developed caudal tympanic recess (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 95 ml. The cephalic and pontine flexures are similar to those in carcharodontosaurids (Larsson 2000; Paulina Carabajal and Canale, 2010), although the disposition of cranial nerves II-IV in the midbrain varies amongst these taxa. The encephalization quotient calculated for Sinraptor is within the range calculated for other basal theropods (Hopson, 1980).