Ungulata

UNGULATA, an order or super-order of placental mam mals including the hoofed herbivorous quadrupeds. Aristotle in his work on The Parts of Animals, in describing the extremities of the viviparous quadrupeds, says that ". . . some are bifid and have hoofs instead of nails, as the sheep, the goat, the elephant, the hippopotamus; and some have undivided feet, as the solid hoofed animals, the horse and the ass. . . ." After the Renaissance Wotton (1552), following Aristotle, di vided the viviparous quadrupeds into the many-toed, double hoofed and single-hoofed. In 1693 John Ray divided the vivipa rous quadrupeds into two grand divisions, the Ungulata, or hoofed, and the Unguiculata, or clawed, forms. The former were subdi vided into: (a) the Monochela, or Solidipeda, with solid hoofs, including the horse, the ass and the zebra; (b) the Dichela, or Bisulca, with cloven hoofs; and (c) the Tetrachela or Quadrisulca, including the rhinoceros and hippopotamus. The Dichela were again subdivided into the Ruminantia, or ruminants, and the Non ruminantia, or swine ; the Ruminantia were finally divided into those with permanent horns, namely the cattle, sheep and goats, and those with deciduous horns, of the deer kind. Here then was a usable and nearly correct classification of the ungulates before the beginning of the i8th century.

Subsequent discoveries have added to the Ungulata a great many extinct and some recent groups which were wholly unknown to Ray. Thus in Osborn's Age of Mammals ( o) the "Cohort Ungulata" includes no less than 13 "orders" of hoofed mammals. While each of these is a more or less natural group of animals re lated by descent from a common ancestral stock, the derivation and interrelationships of the or ders themselves are still far from clear. It is well established that the typical ungulates, namely the Perissodactyla (horses, tapirs, rhinoceroses, etc.) and the Arti odactyla (ruminants, swine, etc.) were wholly distinct from each other in the Lower Eocene, some 5o to 6o million years ago. It is indeed not improbable that the ungulate, or hoofed herbivorous type, was evolved several different times from different families of placental mammals of the Cretaceous period, or in other words, that many of the resemblances between ungulate orders are exam ples of either parallel or convergent evolution.

Order Condylarthra.

In the Basal Eocene formations of New Mexico and in the Lower Eocene of Wyoming have been found the fragmentary jaws and teeth, and very rarely more or less entire skeletons, of small hoofed mammals which on the whole were structurally intermediate between the oldest creodonts or flesh-eaters, and the true ungulates. Of these the most primi tive was the genus Mioclaenus, known chiefly from the conical cusped upper molars, which recall those of such primitive car nivores (creodonts) as Tricentes and Claenodon. Somewhat higher in the scale was the group of species of the genus Hyop sodus ; these little animals were about as large as a hedgehog, the skull being of remarkably generalized type. The dental formula, (Incisors Canines i Premolars 4 Molars was that of other very primitive placental mammals. The crown-patterns of the molar teeth were advancing ever further away from the prim itive tritubercular type toward the more typical condylarth types presently to be described.

By far the most famous condylarth was the Lower Eocene Phenacodus primaevus, an animal about as large as a Newfound land dog, which has figured in many textbooks as the "five-toed ancestor of the horse." But W. D. Matthew has advanced decisive evidence against this view. For in Phenacodus (q.v.) each quad- ' rangular upper molar had four conical main cusps and two very small intermediate cusps or conules; whereas in the contemporary ancestors of the horse family the oblique crests of the molars were progressively developed. Even in the Lower Eocene genus Ectocion, which was related to Phenacodus, the detailed patterns of the upper and lower molars were not quite right to be struc turally ancestral to the molars of the primitive horses. Another Eocene condylarth, the genus Meniscotherium, had more complex upper molar patterns of the type known as buno-lopho-selenodont (i.e., with cones, ridges and crescents), which in some respects foreshadow the general molar types of such later ungulates as the hyracoids, the chalicotheres and the litopterns.