Abstract

Skeletal pathologies in the fossil record are commonly considered indicators of an individual animal’s life history and development, and not reflections of processes related to all or part of a lineage’s history. Individual elements of many extinct and extant rhinocerotids display arthritis-like osteopathologies. The proportion of severe osteopathology increases over the last 50 million years from about 30% of all elements in a taxon from the Eocene showing some form of osteopathology to 100% of all elements in modern taxa. The Rhinocerotidae preserved in the fossil record do not represent a direct lineage, but a group of closely related organisms that present a study system for pathological evolution and development. Osteopathology is examined here in six extinct taxa spanning 50 Ma (Hyrachyus eximus, Diceratherium sp., Trigonias osborni, Menoceras arikarensus, Aphelops sp., and Teleoceras sp.) as well as the five living species of rhinoceroses. Seven pathological indicators (overgrowth, lipping, remodeling, erosions, worn articular surface, and variable foramen shape and size) were scored for each element on a scale of 1-4 and mapped on a phylogenetic tree using Mesquite to establish ancestral and lineage expression of osteopathology (i.e., that present in the common ancestor and also that present as successive points along the tree). We estimated mass from femur length and compared resulting values with the literature. We measured cursoriality using the established proxies of metatarsal length over femur length (MT/F), and total hind-leg length (femur + tibia + metatarsal). Estimated mass increases from 50 Ma to the present and correlates with the increased expression of osteopathology while cursoriality, the other trait examined as a possible contributing factor, did not. This degree of osteopatholgical expression may be an example of evolutionary compromise.