Crania 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 carnivores
Creodonta. The skull bore a saggital crest. All the bones of the limbs are separate, and those of the carpus and tarsus do not alternate - 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
cusps, two internal, two intermediate and two external, so that they were of the typical primitive
bunodont structure. The dental formula is . The typical
Phenacodus primaevus was a relatively small ungulate about long and weighed up to , of slight build, with straight limbs each terminating in five complete toes, and walking in the
digitigrade fashion of the modern
horse. 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 hooves, foreshadowing the
tridactyl type common in
perissodactyls and certain extinct groups of ungulates.
Paleoecology In habits, the animal was cursorial and herbivorous, or possibly carnivorous. In the early Paleocene of North America, the place of the above species was taken by
Tetraclaenodon puercensis, an animal only half the size of
P. 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
artiodactyl section of the Ungulata, we may look to forms more or less closely related to the North American Lower Eocene genus
Mioclaenus, typifying the family
Mioclaenidae. The species of
Mioclaenus were five-toed, bunodont
Condylarthra, with a decided approximation to the perissodactyl type in the structure of the feet. A second type of Condylarthra from the North American Lower Eocene is represented by the family
Meniscotheriidae, including the genus
Meniscotherium. A 2014 cladistic analysis places both
Phenacodus and
Meniscotherium within stem perissodactyls. ==Phylogeny==