Information on the structure of the brain, cranial nerves, encephalic vasculature, and inner ear of the centrosaurine ceratopsid dinosaur Pachyrhinosaurus lakustai (Upper Cretaceous, Wapiti Formation, Alberta, Canada) is presented based on computed tomographic scanning of an isolated braincase followed by 3D visualization. The resulting digital cranial endocast and endosseous labyrinth are the most complete for any ceratopsid, to date, and are compared with physical and digital endocasts of other ceratopsians. In general, the structure of the brain of P. lakustai, as inferred from the endocast, was relatively primitive and much more like extant nonavian diapsid endocasts than like endocasts of archosaur groups with derived brains, such as pterosaurs, hadrosaurs, or coelurosaurs (including extant birds). Total brain size is relatively small, and none of the externally discernable brain regions (e.g., olfactory bulbs, cerebrum, cerebellum, optic lobes) are expanded. Currently, there are insufficient data to determine whether the simple structure and small size of the brain of P. lakustai are truly plesiomorphic or represent apomorphic reduction (evolutionary reversal). The endosseous labyrinth reveals a short cochlea, suggesting that airborne sounds were not particularly important behaviorally. The semicircular canals of the labyrinth are actually somewhat more elongate compared with what little is known of other neoceratopsian labyrinths. The fossil specimen is slightly compressed transversely, and this plastic deformation was subsequently ameliorated by a simple digital retrodeformation that provides a realistic view of brain size and shape. The most significant biological result of the project is that, even accounting for any deformation, the sensorineural and cognitive capabilities of Pachyrhinosaurus were modest, certainly in comparison with some other dinosaur clades, such as hadrosaurids and coelurosaurian theropods.

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An animation of the 3D visualization of the digital endocast is available on NRC Research Press’ Website at http://pubs.nrc-cnrc.gc.calenglbooks/books/9780660198194.html

Key words: Dinosauria, Ceratopsia, Pachyrhinosaurus, brain, behavior, endocast, CT scanning.