HIERONYMUS, T.L.*; WITMER, L.M.; Ohio University, Athens; Ohio University, Athens: From armor to anchor: Character evolution of the skin and horns of rhinos

Previous reconstructions of extinct rhinocerotid taxa have invoked three separate patterns of nasal horns, but without specifying an explicit and mechanistic relationship between unpreserved soft tissues and available osteological correlates. New data on the anatomy and histology of rhinoceros horn attachment allows a reevaluation of previous reconstructions and an assessment of character evolution in the dermis and epidermis that make up these unique horns. Numerous osteological specimens of extant rhinoceros and taxa with similar dermal histology were examined to test a mechanistic hypothesis of osteological correlate formation in relation to an epidermal horn. Sixteen extinct rhinocerotoid taxa were then examined for osteological correlates indicative of derived dermal architecture and keratinous horns. The resulting character scores were optimized onto a composite phylogeny of Rhinocerotoidea. Based on the co-occurrence of these osteological correlates with other morphological features commonly associated with agonistic behaviors, a phylogenetically independent pairwise comparison was conducted to determine the extent of character correlation among a diverse array of extant mammalian taxa. This study found a consistent osteological correlate of horn attachment related to mechanical epigenetic control of bone growth. This correlate is separate from the correlate of derived dermal architecture that occurs elsewhere on the skull. The latter first appears at the base of Rhinocerotidae in the Middle Eocene, while true horns do not appear until the Late Oligocene � Early Miocene, and only within the crown group Rhinocerotini. Character correlations between the relevant soft tissues and other features in many extant mammalian taxa provide a basis for reconstructions of behavior and ecology in extinct rhinocerotids.