THE EVOLUTIONARY STRATEGY OF THE EQUIDAE AND THE ORIGINS OF RUMEN AND CECAL DIGESTION

There are two main groups of ungulates or hoofed mammals living today, perissodactyls and artiodactyls. They are distinguished by a basic difference in foot morphology, which indicates that the two groups arose independently from a basal "proto-ungulate" stock (the order Condylarthra) in the early Tertiary. This difference involves the axis of symmetry of the foot. In perissodactyls it passes through the third metapodial, while in artiodactyls it passes between the third and fourth metapodials. The perissodactyls surviving today are horses, rhinos and tapirs. The recent artiodactyls are a much larger and more diverse group, broadly covering all "cloven-hoofed" animals such as pigs, hippos, camels, deer, giraffes, antelope, cattle and sheep. The perissodactyls must have evolved by the late Paleocene, as by the early Eocene four out of five of the known superfamilies are already distinguishable (Romer, 1966). Artiodactyls are also seen in the early Eocene (Romer, 1966), but the two major groups of ruminant artiodactyls, the tylopods and the pecorans, do not appear in the fossil record until the late Eocene (Gazin, 1955) (see Fig. 1). The perissodactyls were the dominant medium to largesized herbivores during the Eocene, and in the early Oligocene a reduction in their diversity coincided with the beginning radiations of the ruminant groups of artiodactyls. These latter animals were the dominant medium-sized herbivores throughout the remainder of the Tertiary, at least in terms of species diversity. The perissodactyls originated in North America; the origin of the artiodactyls, whether in North America or in Eurasia, is still in question (Olson, 1971). As the rise of the artiodactyls at the Eocene-Oligocene boundary coincides with the decrease in the diversity of the perissodactyls, theories have been advanced to explain this coincidence in terms of perissodactyls being competitively inferior to artiodactyls. One obvious difference between the two groups is the difference in limb morphology. Although the axis of symmetry of the foot seems an unlikely candidate for conferring a selective advantage, artiodactyls do possess a unique double trochleared tarsal or ankle joint. This structure has been regarded as a key feature of the group and contributory to their evolutionary success, on the grounds that it has enabled artiodactyls to achieve rapid acceleration for predator escape (e.g., Schaeffer, 1947; Romer, 1968; Colbert, 1969). While there is little doubt that this particular adaptation was of value to the artiodactyls themselves, I do not think, however, that it had a great deal to do with the relative competitive efficiencies of these two ungulate groups. For a start, this tarsus appeared in a highly advanced condition in the first artiodactyls in the early Eocene (Schaeffer, 1947) long before their radiation and diversification, and is found in noncursorial as well as cursorial forms. In addition, modern ecological work demonstrates that predation is not necessarily detrimental