The condylarth Haplomylus is found at the Elk Mountain Wasatchian locality. This small animal is typical of the Greybullian level of faunal organization (earliest Eocene). A supposed carnivorous condylarth was Pachyaena. Pachyaena was a fairly large animal for its time. The strong, robust jaws and teeth and heavy build suggest that Pachyaena was a predatory animal suited for preying on larger animals such as the amphibious pantodont Coryphodon. The affinities of Pachyaena to the condylarths are uncertain and are based primarily on foot structure.
The miacid carnivores were abundant and varied. Didymictis persisted and both Vassacyon and Vulpavus continued the arboreal, forest-dwelling habits of their Paleocene precursors.
The main carnivorous animals of the Eocene, however, were the creodonts (Order Creodonta) which were essentially early experimenters in carnivorous habits. As such they were diversely specialized. Proviverra (=Sinopa) was one of the smaller, predaceous creodonts that inhabited Fossil Basin during Eocene times.
One of the most characteristic fossil mammals from the early Eocene of Fossil Basin was Coryphodon. A specimen of Coryphodon was also the first mammal fossil to be found in the Fossil Basin. The order of mammals to which Coryphodon belongs, the Pantodonta, was a group of large and heavily built, herbivorous mammals. Coryphodon was about the size of a small rhinoceros. The skull was large and heavily built. The brain, however, was small. The canine teeth were somewhat enlarged and strong. Stout limbs supported the bulky body. In habits, Coryphodon was possibly semi-aquatic.
The modern ungulates, Order Perissodactyla (odd-toed ungulates) and Order Artiodactyla (even-toed ungulates), first appeared in the early Eocene. The earliest members of both orders were small and relatively unspecialized.
The earliest perissodactyls were the horses and tapirs. The earliest horses were represented by Hyracotherium. This small, slender, and lightly built horse was about the size of a fox terrier. The front feet had four toes, and the hind feet three toes. The teeth suggest that this animal was probably a browser, living in the forests. In the Fossil Basin, tapirs were known from deposits of Lysitean (mid to early Eocene) age of which Heptodon was representative. The tapirs now live in the tropics of Malaysia and Latin America. Their present habitat suggests a similar environment may have been favored by the Eocene tapirs of the Fossil Basin.
The first Artiodactyls were pig-like forms, although they are only distantly related to true suids. Protodichobune was small and may have looked much like some of the early Perissodactyls. There are, however, a complex of structural features which serve to distinguish these mammalian orders from each other.
The mammal fauna from the Fowkes Formation places a late Bridgerian age on the Sillem and Bulldog Hollow members (Nelson 1973). Small mammals were most abundant in the Fowkes, especially primates and rodents. The marsupials were characterized by Peratherium which was a small, probably arboreal, opossum-like animal. Peratherium was but one stage in a slow and relatively conservative evolution of opossums that began in the Cretaceous.
The condylarths decreased in importance during Fowkes time and were gradually replaced by more advanced ungulates, the artiodactyls and perissodactyls. Only the small Hyopsodus survived into the Bridgerian of the Fossil Basin. The horses continued to differentiate as evidenced by the appearance of Orohippus, an animal similar to Hyracotherium, but with minor dental differences. The primitive tapirs were represented by Hyrachyus, a medium-sized herbivore which was nearly as closely related to the rhinoceroses as it was to the tapirs.
The primates were also diverse in Fowkes time; however, this group began to decline in importance following Bridgerian times. As a group, the primates were restricted mainly to warm and forested environments. The lemur-like Notharctus and the tarsier-like Omomys and Hemiacodon were common in the Fowkes. Hemiacodon was typical of the late Bridgerian (an index fossil).