New Insights Into the Brain, Braincase, and Ear Region of Tyrannosaurs (Dinosauria, Theropoda), with Implications for Sensory Organization and Behavior

Abstract

The braincase region of tyrannosaurs was investigated to provide insights on anatomical attributes relevant to inferences of sensory biology and behavior. CT scanning focused on three specimens of Tyrannosaurus rex, a juvenile Gorgosaurus, and the controversial Cleveland skull (CMNH 7541). Analysis shows that the cerebral hemispheres were enlarged, but conflicting information on the optic lobes suggests that brain conformation was not fully avian. Previous estimates of olfactory bulb size for T. rex were much too large, but even the corrected sizes are relatively larger than other theropods, suggesting that odor detection was indeed of particular importance to tyrannosaurs. The inner ears show a number of coelurosaurian traits, such as elongate and rounded and rostral, lateral semicircular canals, and incipient twisting of the common crus, which we interpret to be related to enhanced reflexes coordinating rapid eye and head movements. The cochlea is elongate, which, coupled with the finding of extensive tympanic pneumaticity, supports the inference of behavioral emphasis of low-frequency sounds. Three main groups of sinuses pneumatized the braincase, and there are a number of perhaps systematically relevant differences. Orientation of the endosseous labyrinth reveals that alert head postures of T. rex and Gorgosaurus were somewhat depressed below the horizontal, but the Cleveland skull had a very strongly down-turned posture. It is concluded that tyrannosaur sensory biology is consistent with their predatory coelurosaurian heritage, with emphasis on relatively quick, coordinated eye and head movements, and probably sensitive low-frequency hearing; tyrannosaurs apomorphically enhanced their olfactory apparatus. The taxonomic status of the Cleveland skull remains unresolved. Anat Rec, 292:1266Rec, 292: -1296Rec, 292: , 2009. V V C 2009 Wiley-Liss, Inc.

Skull Anatomy of the Bizarre Crocodylian Mourasuchus nativus (Alligatoridae, Caimaninae)

“…5), the endocast includes the caudal part of the forebrain and the hindbrain. As stated above, it seems to be dominated by large dura and venous sinuses that cover much of the brain structure, especially dorsally (Witmer et al, 2008;Witmer and Ridgely, 2009) and many of the brain details are not visible (Witmer et al, 2008). As in other crocodylians, the dorsal longitudinal sinus is expanded caudally in a central venous sinus, the torcular herophili (Sedlmayr, 2002), which in this specimen is huge with an ''hourglass'' outline diverging caudally (Fig.…”

“…As with many archosaurians, this specimen shows bony recesses from the cranial system of air-filled sinuses that pneumatized the skull (Witmer, 1990(Witmer, , 1995(Witmer, , 1997Witmer et al, 2008;Witmer and Ridgely, 2009) (Fig. 4).…”

“…It has been recognized in several archosaurs but its function remains unclear (e.g., Witmer and Ridgely, 2009). Contrary to C. crocodilus and A. mississippiensis, the median pharyngeal recess in M. nativus remains paired in almost all its extension and only connects its symmetrical tube near the external ventral opening.…”

“…4B,C,E). The median pharyngeal sinuses represent a poorly known group of sinuses (Witmer and Ridgely, 2009), which probably derive from a diverticulum of the pharynx separate from the middle ear sac (Witmer and Ridgely, 2009), producing two typical sinuses, both median in position: the subsellar and basisphenoid recesses. The dorsorostral projection that pneumatizes the basisphenoid corresponds to the basisphenoid recess which has no communication with the tympanic system.…”

Physical evidence of predatory behavior in Tyrannosaurus rex

“…The tooth is indistinguishable in morphology, size, and denticle character from known T. rex subadults (e.g., Los Angeles County Museum-23845 and Black Hills Institute-6439). An independent comparison of the ratio of the distance from crown tip (DCT) to the incremental crown length (ICL) for T. rex and Nanotyrannus, the only two contemporaneous large-bodied and large-toothed theropods (46)(47)(48)(49), with that of the embedded tooth places it firmly within the T. rex range (Fig. 3).…”