Primates also continued to expand and diversify in Wasatchian time. Microsyops was a very unusual type that belongs to the extinct family Microsyopidae. The teeth were sharp and adapted for eating fruit and/or insects. Microsyops was about the size of a rat or slightly larger. The microsyopid primates were limited to the Paleocene and Eocene and left no later known descendants.
Another group of primates is represented by Pelycodus. This was a small lemur-like animal that may possibly have been an ancestor of higher primates (monkeys, apes, and men). Pelycodus, like Microsyops, was an arboreal (tree-dwelling) animal which inhabited the forests of the Fossil Basin during the early Eocene.
The Order Taeniodonta is a strange and little known group of mammals. In the sediments at Fossil Butte the taeniodonts are represented by Ectoganus. This was a moderately large animal. The skeletal adaptations were similar to those of moles, but there was no actual relationship between the two groups. The front feet and legs were robust, and bore large claws. Ectoganus probably used these structures to grub for food, possibly roots. The incisor teeth were rootless and persistently growing. The tooth enamel was restricted to two bands on either side of the teeth. The single pair of upper and lower incisors was greatly enlarged. The taeniodonts became extinct in the Eocene and left no descendants.
Equally as strange a group of animals as the taeniodonts are mammals in the Order Tillodonta. These were herbivorous animals, some of which became quite large. They have many rodent-like characters, but were not related to rodents. In many respects, the morphology of rodents seems to have lent itself to convergence. Esthonyx was the most common tillodont from the Fossil Basin. Prominent, rootless incisors were a characteristic of the later members of the group. These incisors were chisel-like, as in rodents. The molars were unusual and bore certain resemblances to primitive carnivoran and insectivoran teeth. The tillodonts, however, bore uncertain relationships to other mammals and left no descendants beyond the middle Eocene.
In comparison with recent faunas, Paleocene rodents were rare faunal elements. Two early rodents were found in the Fossil Basin Wasatch Formation:
Paramys was relatively unspecialized but was diverse in the number of species. One species of Paramys was about 2 ft long and possessed a long tail. The body was long and slender. The skull was small and rather squirrel-like. The cheek teeth were quadrate and had low, blunt cusps. The typical, single pair of chisel incisors was present both on lower and upper jaws. Paramys was probably an arboreal form that did not look too unlike a modern squirrel and may have had a similar mode of life. Another rodent, Reithroparamys, was somewhat similar to Paramys in size and appearance. The differences between the two animals were mainly in the teeth and hind limbs. The latter showed certain very minor structural modifications that suggested a saltatorial (jumping) mode of locomotion for Reithroparamys.
Bats (Order Chiroptera) are extremely rare as fossils because their volant and cave-dwelling habitat as well as a fragile skeleton did not lend their remains to preservation. From the Green River Shales, from near Fossil Butte, a complete articulated skeleton of a bat has been found (Jepsen 1966). This animal, named Caronycteris index, was small and generalized in form. Superficially, it looked much like a typical brown bat. This fossil is important because it demonstrates that the bats had already become good fliers by early Eocene time.
The condylarths were prominent in the early Eocene but diminished in importance following this time. Phenacodus was still extant; however, it was replaced in importance by the smaller Hyopsodus.
Hyopsodus is perhaps the most commonly recovered Eocene fossil mammal. This animal was small and long-bodied and retained a more-or-less generalized structural pattern. In some respects Hyopsodus was similar to both insectivores and primates and was at one time or another regarded as belonging to either of these two orders.
Some of the later condylarths paralleled the more advanced ungulates. Indeed, the Perissodactyla (horses, tapirs, rhinos) and Artiodactyla (bovids, deer, pigs, sheep, etc.) were derived from early Tertiary condylarths. Meniscotherium was one such advanced condylarth. The cusps on its molars, instead of being blunt points, had developed into crescentic patterns, somewhat like those in deer or camels. The relationship of Meniscotherium to modern ungulates is only one of parallel dental development. Meniscotherium was a medium-sized animal, but about the same build and size as that of a cocker spaniel. Meniscotherium, however, had hoofs on its toes and was a forest-living browser.