Meeting Abstract
Fully ossified, hollow nasal crests occur rarely in vertebrates, & even less often interact directly with the airway. Hadrosaur dinosaurs are the best known example of such craniofacial morphology. Until a mass death assemblage of the alcelaphine bovid Rusingoryx atopocranion was unearthed from Kenyan Pleistocene deposits, osseous nasal crests were unknown outside of Archosauria. Adult Rusingoryx reveal anatomical & functional analogs with lambeosaurine hadrosaurs, suggesting deep homoplasy between these distantly related groups. An understanding of how this bizarre morphology evolves & why it is so rare remains incomplete when only adults are considered. The presence of juveniles in the Rusingoryx assemblage presents a unique opportunity to examine evolution of nasal crest ontogeny, in which crest development strengthens bovid-hadrosaur parallels. In this case, analogous cranial elements are rearranged in a surprisingly similar sequence. In both taxa, elevation of the incipient crest is accomplished by a dorsal expansion of the (pre)frontal bones. The caudal border of the crest migrates from anterior to posterior relative to the orbit with strong caudal flexion of the frontal bones. Outgroup comparisons uncover similar shifts in both external & internal cranial morphology, including dorsal rotation of the nasal passages prior to crest inflation. Although hadrosaurs & bovids have each achieved osseous nasal crests from non-homologous cranial architecture, the ontogenetic & evolutionary antecedents to crest formation are largely similar. This suggests that, in order for terrestrial vertebrates to evolve hollow circumnarial domes, a suite of coordinated and highly specific developmental-evolutionary shifts must occur, perhaps in response to a limited set of environmental factors.
