The family of the chalicotheres (Chalico theriidae) has been described under ANCYLOPODA. Here it will suffice to note that the primitive hoofs grew into long curved claws which seem to have been used for scratching the ground, perhaps in search of water and succulent tubers. The earliest known chal icothere, Eomoropus of the Eocene of Wyoming, retained the primitive perissodactyl numbers of four toes in the forefeet and three in the hindfeet, while the upper molars approximated to the Eocene perissodactyl type.
The family of the rhinoceroses (see RHINO CEROS) is connected with that of the lophiodonts by such forms as Hyrachyus of the Eocene of North America. Hyrachyus has been called "the cursorial rhinoceros" because by comparison with its massive modern relatives it was distinctly light-limbed. Some what smaller than a South American tapir, it resembled that animal in the general configuration of its body and limbs. It did not, however, have a proboscis and, unlike a rhinoceros, it had no horn above the nose. All its front teeth of both jaws were un specialized, not yet like those of later rhinoceroses. Its upper molar teeth, however, had already begun to foreshadow the rhinoceros type in the obliquity and marked flattening of the outer surface of the hinder outer cusp, although the two outer main cusps were not yet submerged in the general outer wall. None of the known species of Hyrachyus were directly ancestral to the lines leading to the later rhinoceroses, some of the fore runners of which (named Prohyracodon and Eotrigonias) were already established during Eocene times. In Trigonias of the Lower Oligocene the true rhinoceros characters of the molars and premolars were well defined and the second lower incisors had begun to be enlarged as tusks. On the other hand, Trigonias was hornless and retained four toes on each forefoot and the upper canines, though small, were still retained. Most of the later Oligocene rhinoceroses had eliminated the fifth or outer digit of the forefoot and had also lost the upper and lower canines. In Diceratherium, a small rhinoceros abundant in the Lower Miocene of Nebraska, a small transversely-placed pair of oval horn-swell ings developed near the tip of the nasal bones. This group became extinct during the Miocene. Meanwhile the "aceratheres," or hornless rhinoceroses of Europe, were numerous during the Oligo cene and Miocene and some of them probably gave rise to the lines leading to the modern Sumatran and Indian rhinoceroses.
In addition to these central forms there were many peculiar side lines. In the Miocene and Lower Pliocene of North America and Europe were swarms of extremely short-f ooted hippopotamus like rhinoceroses (Teleoceras, etc.), with massive heads and very high-crowned molars and a single horn on the tip of the nose. The two existing species of African rhinoceroses and the related woolly rhinoceros of the Pleistocene of Europe are very highly speciJized; their exact derivation is unknown. The strangest of
all was the fossil Baluchitherium from the Upper Oligocene of Turkestan, Baluchistan and Mongolia, a titanic animal with a skull over four feet long and an estimated height at the shoulder of 13 feet.
Besides all these true rhinoceroses, there were two extinct groups to which some authors give the rank of distinct families, namely the hyracodonts and the amynodonts. The hyracodonts were small cursorial forms with rhinoceros-like molar teeth and unspecialized front teeth. Of these, the very small Trip/opus of the Middle and Upper Eocene had exceedingly long, slender, three toed feet. The Oligocene Hyracodon was much larger,—about the size of a calf,—with stockier feet. Both genera were exclusively North American. The amynodonts were larger forms, the oldest of which appears in the Upper Eocene of Utah. It was followed in the Lower Oligocene of the Big Badlands region of South Dakota by Metamynodon, a somewhat hippopotamus-like form with enlarged tusk-like upper and lower canine teeth and very elongated oblique outer walls of the upper molars; the stout fore feet retained four digits, the spreading hindfeet had the usual three toes of the rhinoceroses. Members of the same family have been found in the Lower Oligocene or Uppermost Eocene of Burma, Europe and Mongolia.
The Perissodactyla as an order prob ably originated in the northern hemisphere, perhaps in basal Eocene times, from some as yet undiscovered relatives of the Eocene condylarths or protoungulates. By Lower Eocene times the group was well represented in western North America and western Europe and was already beginning to diverge into numer ous families which have been classified by Osborn in five super families, namely the Hippoidea (containing the lines leading to the palaeotheres and horses), the Chalicotheroidea (or chalico theres), the Titanotheroidea (or titanotheres), the Tapiroidea (or tapirs and lophiodonts) and the Rhinocerotoidea (or rhinoceroses, hyracodonts and amynodonts). By Upper Eocene times the order was well represented in Burma and Mongolia. It was not until late Miocene or Pliocene times that the horses and tapirs pene trated into South America and the group never reached even the outposts of the Australian region. In Africa, which is now its greatest stronghold, it is conspicuously absent froth the Upper Eocene and Lower Oligocene of Egypt and is not known until the Miocene or Pliocene. On the whole the existing Perissodactyla represent a beaten order which has been gradually crowded out by the ruminant artiodactyls. Of its surviving families, the rhino ceroses and tapirs seem doomed to extinction unless preserved by stringent regulation. The zebras, however, seem to have a better chance of survival in certain districts.