Well preserved skulls of Triceratops are extremely abundant in Maastrictian rocks from western North America. Although ossification obscures sutural and structural derails in many skulls, others reveal much previously undescribed morphology. The circumnarial area includes enlarged nares and a complex narial fossa. A nasal horn of varying size, augmented by an epinasal ossification, caps the snout. The anterodorsal orbital margin is formed by the lacrimal and the supraorbital, which has fused to the lateral side of the prefrontal. This thick and laterally extensive orbital rim restricted forward vision in Triceratops. The ossified braincase completely surrounds the brain cavity, and is overlain by a deep frontal and cornual sinus complex. These sinuses probably acted as shock absorbers to counteract stresses placed on the postorbital horns during intraspecific interactions. The frill is heavily vascularized, lacks fenestrae, and is well buttressed by expansive paroccipital processes. Endocasts reflect all twelve cranial nerves, major structures of the brain, and some arterial and venous drainage systems.
