Although it is convenient to follow the custom of grouping the ungulates, that is hoofed mammals, together, they are a disparate group of Orders with little in common beyond the fact that the claws of the primitive mammals evolved into hooves as part of a remarkably effective swift-running mechanism. It seems highly likely that hooves evolved by parallel evolution in a variety of forms although it is also possible that there was some ultimate common Tertiary ancestor. In other respects, however, the ungulate Orders show large fundamental differences, particularly in respect of their dentitions and gut morphology. The consensus view was expressed succinctly by Romer (1962) as ‘A cow is not improbably as closely related to a lion as to a horse’.

Even the most typically ungulate Orders, the Perissodactyla (odd-toed) and Artiodactyla (even-toed), comprise notably disparate forms, a principal disparity being between their dentitions. In most genera in the artiodactyl Sub-order Ruminantia, the upper incisors are replaced by a firm mucosal pad against which the lower incisors bite this incisor mechanism is associated with the complex stomach typical of ruminants. The incisor mechanism in the sheep is described in Chapter 24 (p. 552).

The equids (Perissodactyla) and the ruminants have very hypsodont cheek teeth of basically similar strucure adapted for triturating a strictly herbivorous diet however, the equids have not lost their upper incisors.

The pigs and peccaries (Sub-order Suina), although also artiodactyls, have dentitions that are totally different from those of the ruminant artiodactyls, and suited to their entirely different, omnivorous habit. There is an incisor-to-incisor mechanism, the canines form tusks and the cheek teeth are brachydont and bunodont.