Ontogeny of cranial epi-ossifications in Triceratops

Abstract

ABSTRACT Historically, the scarcity of non-adult Triceratops fossils collected from Upper Cretaceous sediments of North America limited our understanding and promoted controversy with regard to the morphology, and presence or absence of cranial epi-ossifications in this widely known horned dinosaur. The recent discovery of several exceptionally well preserved juvenile and subadult Triceratops skulls and numerous juvenile, subadult, and adult cranial elements, from the Hell Creek Formation of eastern Montana, confirms the ontogeny and morphology of epi-ossifications in this study. We propose to standardize the terminology for these four cranial epi-ossifications: epinasal, epijugal, epiparietal, and episquamosal. We describe the ontogeny and timing of the fusion of each of these epi-ossifications and the rostral from a cranial growth series. Although the timing is variable, the epinasal fuses first, followed by the rostal, the epijugals, the episquamosals, and lastly by the epiparietals. Co-ossification of the epinasal, rostral and epijugals unites several of the anterior (rostral-nasal-premaxillae) and lateral (jugal-quadratojugal) skull elements. In combination with forward directed postorbital horns and a massive fan-shaped frill, cranial epi-ossifications may have enhanced visual display and species communication in Triceratops.