1911 Encyclopædia Britannica/Phenacodus
PHENACODUS, one of the earliest and most primitive of the ungulate mammals, typifying the family Phenacodontidae and the sub-order Condylarthra. The typical Phenacodus primaevus, of the Lower or Wasatch Eocene of North America, was a relatively small ungulate, of slight build, with straight limbs each terminating in five complete toes, and walking in the digitigrade fashion of the modern tapir. The middle toe was the largest, and the weight of the body was mainly supported on this and the two adjoining digits, which appear to have been encased in hoofs, thus foreshadowing the tridactyle type common in perissodactyle and certain extinct groups of ungulates. The skull was small, with proportionately minute brain; and the arched back, strong lumbar vertebrae, long and powerful tail, and comparatively feeble fore-quarters all proclaim kinship with the primitive creodont Carnivora (see Creodonta), from which Phenacodus and its allies, and through them the more typical Ungulata, are probably derived. All the bones of the limbs are separate, and those of the carpus and tarsus do not alternate; that is to say, each one in the upper row is placed immediately above the corresponding one in the row below. The full series of forty-four teeth was developed; and the upper molars were short-crowned, or brachyodont, with six low cones, two internal, two intermediate and two external, so that they were of the typical primitive bunodont structure. In habits the animal was cursorial and herbivorous, or possibly carnivorous.
In the Puerco, or Lowest Eocene of North America the place of the above species was taken by Euprotogonia puercensis, an animal only half the size of Phenacodus primaevus, with the terminal joints of the limbs intermediate between hoofs and claws, and the first and fifth toes taking their full share in the support of the weight of the body. These two genera may be regarded as forming the earliest stages in the evolution of the horse, coming below Hyracotherium (see Equidae).
As ancestors of the Artiodactyle section of the Ungulata, we may look to forms more or less closely related to the North American Lower Eocene genera Mioclaenus and Pantolestes, respectively typifying the families Mioclaenidae and Pantolestidae. They were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyle type in the structure of the feet. A third type of Condylarthra from the North American Lower Eocene is represented by the family Meniscotheriidae, including the genera Meniscotherium and Hyracops. These, it is suggested, may have been related to the ancestral Hyracoidea. Teeth and jaws probably referable to the Condylarthra have been obtained in European early Tertiary formations. All Ungulata probably originated from Condylarthra.
See H. F. Osborn, Skeleton of Phenacodus primaevus; comparison with Euprotogonia, Bull. Amer. Mus. x. 159. (R. L.*)