Tedford, Richard H. Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York.
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Hoofed herbivores confined to the Cenozoic of South America. The order Litopterna was well represented from the Paleocene to the Pleistocene, and apparently arose on that continent from a condylarth ancestry. By later Paleocene time, two main lines of descent were clearly demarcated. The Proterotheriidae (Fig. 1) displayed a remarkable evolutionary convergence with the horses in their dentition and in reduction of the lateral digits of their feet. In one group, the foot was reduced to a single median toe by early Miocene time. The members of the Macraucheniidae (Fig. 2) were proportioned much as in the camels and by late Tertiary time had similarly lost the vertebral arterial canal of the cervical vertebrae. They early developed a tendency toward retraction of the nasal bones; by late Tertiary time, the nasal opening was situated above the eyes on top of the skull. This specialization has been thought to indicate the presence of a trunk or perhaps dorsally situated nostrils adapted for a submerged amphibious existence. Three functional digits remain in the feet of even the most advanced macraucheniids. See also: Archaic ungulate
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