A BRAIN THE SIZE OF A WALNUT: NEW STUDIES OF BRAIN AND INNER EAR STRUCTURE IN STEGOSAURUS (DINOSAURIA: ORNITHISCHIA) BASED ON CT SCANNING AND 3D VISUALIZATION

The Jurassic dinosaur Stegosaurus has been emblematic of the small-brained dinosaur since it was remarked that its brain was the size of walnut. Brain structure in Stegosaurus has been studied previously using traditional methods (i.e., generating latex and/or plaster casts of the endocranial cavity). Virtual endocasts of four Stegosaurus (Dinosauria: Ornithischia) specimens were examined for the first time using computed tomography (CT) and three-dimensional visualization software (Avizo 6.3). This technique differs from those involving physical endocasts in that it allows the virtual endocast to be viewed in isolation as well as in situ within the surrounding translucent braincase. This unique perspective (“crystal skull” view) permits the tracing of cranial nerves through their corresponding canals and foramina within the anatomical context of the cranial bones, allowing unambiguous identification of external foramina. Additionally, general brain regions that as of yet have only been latex-injected and artistically illustrated for Stegosaurus were analyzed using this technique. Hypotheses relating to the presence, placement, and size of osteological correlates for various intracranial structures (e.g., cranial nerves, the pituitary fossa, major brain regions, vasculature) were tested, providing new information on the disposition of cephalic soft tissues that will shed light on the discrimination of generalized brain regions. General form and relative size of the brain and its components, coupled with the structure of the endosseous labyrinth of the inner ear, provide new neurological insights that largely affirm the perception of Stegosaurus as having had modest cognitive and sensory capabilities.