Cranial endocast of the Jurassic theropod Sinraptor dongi (Currie & Zhao)

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

Calgary

Simposio; The Alberta Paleontology Asociation annual symposium; 2010

The Alberta Paleontology Asociation- University of Calgary

The endocranium corresponds to the space inside the braincase occupied by the brain andother soft tissues. An endocast represents the morphology of those soft tissues notpreserved but which left impressions on the medial walls of the braincase. Some authorsdefine it as the cast of the dura mater of the brain. A complete endocast of the holotype ofSinraptor dongi (IVPP 10600) was made in latex after removing mecanically thesediment filling the cerebral cavity of the braincase. Thus, allows the description of theendocranial anatomy of the specimen, which includes “brain” morphology and positionof the cranial nerve roots (Fig. 1b,c). The endocast was compared with the other knownallosauroid theropod endocasts, including Carcharodontosaurus saharicus and C.idiguensis, the generally consider as a carcharodontosaurid Acrocanthosaurus, andAllosaurus. As in Carcharodontosaurus and Giganotosaurus the olfactory bulbs inSinraptor are aligned with the forebrain, the midbrain is posteroventrally inclined and thehindbrain is parallel but ventral to the forebrain, showing similarly angled cephalic andpontine flexures. The dorsalmost section, the dural peak, is aligned with cranial nerve IIin Sinraptor, but is anterior to it in Giganotosaurus. Unlikely, in Acrocanthosaurus, theedocast is shorter and the midbrain is strongly posteroventrally inclined. The length of theendocast of Sinraptor, including the olfactory bulbs, is 20 mm, and has a maximum widthof 55 mm at the cerebral hemispheres. This length is approximately 30% shorter than theendocast in Giganotosaurus carolinii. The volume of the endocast delimited between thenarrowest transverse constriction of the olfactory tracts anteriorly and the exit of thecranial nerve XII (hypoglossal) posteriorly is approximately 95 ml. This volume waspreliminary calculated by water displacement. There are no sutures on the surface of the2endocast reaffirming that belongs to an adult specimen. Small and diffuse vascular marksare present only in the dorsal surface of the midbrain. The cranial nerves and bloodvessels have a similar disposition as in Carcharodontosaurus and Giganotosaurus. Theolfactory tract is relatively short, and the olfactory bulbs were enclosed by ossifiedethmoidal elements (which are preserved not articulated with the braincase). The cranialnerves II and III are located dorsolaterally to cranial nerve II. The Cranial nerve V isaligned with the flocculus of the cerebellum. Unlike the mentioned carcharodontosaurids,the flocculus is constricted, conforming two projections; and the dorsal and smaller partcorresponding probably to a vascular element (Fig. 1c). The Cranial nerves VI areseparated transversally by 10 mm in the ventral part of the medulla oblongata. Thecranial nerves XI-XI conform a single root in half-moon shape. There is a single root forthe branches of cranial nerve XII. The caudal middle cerebral vein (called before thecapitis dorsalis vein) is dorsal to the cranial nerve V and close to the midline and itscounterpart. This preliminary study suggests that the endocast of Sinraptor shares moresimilarities with carcharodontosaurids than with Acrocanthosaurus.