Body-Size Diversity and Community Structure of Fossil Hyracoids

Hyracoidea is a mammalian order that shows remarkable diversity in body mass through time in contrast to the narrow range of body mass that occurs among extant species of the order. Despite growing knowledge of taxonomic, locomotor, and dietary diversity among Tertiary hyracoids, estimates of body mass for extinct forms have been unavailable. Fourteen regression equations derived from dental measurements of a wide range of ungulates were used to estimate body mass in extinct hyracoids. Significant differences in estimates of mass for individual species occurred among different regressions, among different tooth measurements, and between upper and lower molars, raising the question of how regression equations should be selected. Despite the problems inherent in estimation of body mass, it is clear that extinct hyracoids ranged from small taxa, the size of rabbits, to large ones, the size of Sumatran rhinos. Early Tertiary communities of hyracoids in Africa prior to the arrival of groups of northern ungulates demonstrated a range of body mass comparable to that of modern communities of ungulates (3-2,000 kg). Later communities of hyracoids showed reduced diversity in size associated with greater diversity among sympatric nonhyracoid ungulates. Pliocene to Recent hyracoids showed limited diversity in body size within a single community, with Eurasian pliohyracines being restricted to a large size range (800-1,400 kg) and modern procaviids being restricted to a small size range (1.0-4.5 kg).