Temporal range: Late Paleocene – Middle Eocene
An artist's illustration of Phenacodus primaevus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: ?Perissodactyla
Family: Phenacodontidae
Genus: Phenacodus
Cope, 1873
Referred species
  • Phenacodus bisonensis
  • Phenacodus condali
  • Phenacodus grangeri
  • Phenacodus intermedius
  • Phenacodus lemoinei
  • Phenacodus magnus
  • Phenacodus matthewi
  • Phenacodus primaevus
  • Phenacodus teilhardi
  • Phenacodus trilobatus
  • Phenacodus vortmani
  • Trispondylus Cope, 1884

Phenacodus was one of the "plain vanilla" mammals of the early Eocene epoch, a medium-sized, vaguely deer- or horse-like herbivore that evolved a mere 10 million years after the dinosaurs had gone extinct. Its importance lies in the fact that it seems to have occupied the root of the ungulate family tree; Phenaocodus (or a close relative) may have been the hoofed mammal from which later perissodactyls (odd-toed ungulates) and artiodactyls (even-toed ungulates) both evolved.

This creature's name, Greek for "obvious teeth," derives from its, well, obvious teeth, which were well-suited to grinding up the tough vegetation of its North American habitat. Phenacodus lived in plains of North America and lived about 55-45 million years ago. It was up to 5 feet (1.5 metres) long and weighed 50-75 lbs. It most likely eats grass and its special characteristics are its long, straight legs, long tail and narrow snout.

The typical Phenacodus primaevus 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, foreshadowing the tridactyl type common in perrisodactyls 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 carnivores Creodonta. 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.

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 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. 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.

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.

Knight Phenacodus

Phenacodus illustartion by Charles R. Knight

Phenacodus originated in the late Paleocene in western North America, where a relatively short time was quite widespread up to the north of Mexico and Canada. In the early Eocene, it spread out well to the south of Western Europe (France, Spain), but in contrast to North America, here they are rare.

These early ungulates were the size of an adult sheep, but apparently more like predators. Their growth rate of 60 cm length reaches 1.7 m, and there was quite a long powerful tail. The front part of the body was squat, and the highest point of the waist was made ​​up of strong bones with the developed neural spines. On each limb, they had five fingers with small hooves, but they moved on three fingers - the middle finger is longer than others, and the first and fifth considerably shortened.

They were probably omnivores, but the structure of their teeth clearly indicates the development of the ability to use relatively rigid plant forage. It lived in forest edges and woods, where plants were their main food - soft leaves and juicy fruits. In addition, they probably will not give up the foods of animal origin, if possible, eating carrion or preying on small vertebrates. The relative speed of movement helped this animal to run away from predators, and during the rescue of more fearsome predators, which at that time were members of the order creodonts. It is not excluded that Phenacodus, like some archaic ungulates living in the woods, had mottled pigmentation. In the first half of the Eocene, Phenacodus disappeared on both continents, probably lost in the struggle for survival, and were more mobile and intelligent competitors of these ungulates.