LITOPTERN

(order Litopterna), any of various extinct hoofed mammals that first appeared in the Paleocene Epoch (which began approximately 66 million years ago) and became extinct during the Pleistocene Epoch (which ended about 10,000 years ago). The group's members were restricted to South America. In many ways the evolution of the litopterns paralleled that of hoofed mammals in the Northern Hemisphere; two distinct lines of litoptern evolution are discernible in the fossil record. One line, the proterotheres, strongly resembled the evolution of the horse. Their limbs were adapted to running and were similar in form and function to those of horses; the skull was long and low, with cheek teeth resembling those of horses. The proterotheres became extinct in the Pliocene Epoch (which ended approximately 1.6 million years ago), about the time that true horses appeared in South America. The other line of litoptern evolution is a group known as the macrauchenids, which resembled camels in many ways. The nasal opening of the skull was set far back and probably supported a short proboscis, or trunk. Some of the macrauchenids survived the intrusion of more advanced mammals from North America and persisted well into the Pleistocene Epoch, when they became extinct. It seems clear that the horselike litopterns succumbed to competition for similar resources by the true horses over the course of the Pliocene, but the macrauchenids were better adapted to their environment, probably swampy areas, and thus were able to compete for a while with the newly introduced North American forms.