Witmer, L., Ridgely, R. and Sampson, S.: THE EAR REGION, CEREBRAL ENDOCAST, AND CEPHALIC SINUSES OF THE ABELISAURID THEROPOD DINOSAUR MAJUNGATHOLUS

WITMER, Lawrence, RIDGELY, Ryan, Ohio Univ., Athens, OH; SAMPSON, Scott, Univ. Utah, Salt Lake City, UT

Detailed study of the skull of the abelisaurid theropod dinosaur Majungatholus, based on CT scanning and 3D visualization of specimens discovered from the Late Cretaceous of Madagascar, allows new insights into cephalic soft tissues. Separate elements of a disarticulated skull were scanned, and anatomical structures of interest (e.g., osseous labyrinth, cerebral endocast, pneumatic sinuses) were extracted (segmented) and rendered in 3D. These datasets were then registered with a 3D surface model of an articulated cast, allowing the segmented soft tissues to be viewed in place in the whole skull. The virtual endocast reveals, as perhaps expected given the taxon’s position as a basal neotheropod, that the brain did not fill the endocranial cavity and that the general organization was primitive; the cerebellar flocculus is small. Likewise, the labyrinth of the inner ear is fairly typical for non-coelurosaurian theropods. The orientation of the lateral semicircular canal suggests that the alert head posture was basically horizontal, corresponding to the position of maximal binocular overlap. Identification of anatomical domains (e.g., adductor, tympanic, ocular, oropharyngeal), coupled to comparisons with extant outgroups, clarifies soft-tissue reconstruction. For example, the boundaries of the epithelial middle ear sac can be identified, and show that the median pharyngeal pneumatic system probably communicated with the middle ear. The dorsal and caudal tympanic recesses present in many higher theropods are absent, but the rostral tympanic recess is present and has an unusual caudal expansion within the braincase. The frontal bones have a sinus (perhaps unique among theropods) that is almost certainly of pneumatic origin, deriving probably from the antorbital diverticulum in the lacrimal bone. The frontal sinus is variable in extent and is nearly absent in the individual with the largest cornual process (‘horn’). The lacrimals and particularly the nasals are extensively pneumatized by more typical antorbital diverticula such that the volume of the antorbital sinus and its diverticula exceeded the volume of the main nasal cavity itself.