Pachycephalosaur paleoneurology (Archosauria: Ornithischia)

ABSTRACT

Pachycephalosaur endocranial casts display a distinctive suite of traits that include large divergent olfactory bulbs, large olfactory nerves, short thick olfactory tracts, a moderately expanded cerebrum not separated from optic lobes and cerebellum dorsally, steep cranial flexure, and reduced pontine flexure. The cerebrum, olfactory tract, and olfactory lobes are located in the broad interorbital space between widely set eyes. Endocast length displays negative allometry relative to frontoparietal length, both in ontogeny and between genera of different size within the group. Reduction of the pontine flexure is the result of ventral rotation of the occiput and is most likely associated with reorganization of the skull for head butting. The partial endocast of Yaverlandia does not display typical pachycephalosaur endocranial anatomy, making its membership in the group questionable.