The Torrejonian assemblage (middle Paleocene) (Gazin 1969) is small and poorly preserved. Its main significance is that the fauna records a definite time interval for the Evanston Formation. Gazin’s work has demonstrated this fauna to be intermediate in composition between similarly aged faunas recovered from sites to the north and south.
The major faunal elements are insectivores, primates, condylarths (primitive ungulates), and multituberculates (a type of extinct rodent-like mammal, with no known descendants).
The poorly preserved condition of these fossils makes discussion impossible except to note that they extend the known time range of mammalian habitation in the Fossil Basin.
The younger Tiffanian fauna (late Paleocene) (Gazin 1956) is also known from largely fragmentary remains. The multituberculate mammal Ptilodus, also present in Torrejonian deposits, is the most “primitive” mammal in the fauna. The multituberculates first appeared in the Jurassic ([Fig. 4]), and are therefore the oldest lineage of mammals that survived into Tertiary times. The molar teeth of these animals are characterized by multiple cusps arranged in parallel rows. Ptilodus, like most other multituberculates, was specialized in that its fourth lower premolars were expanded into shearing blades, the function of which is not known. Ptilodus was in many respects similar to rodents in the development of procumbant incisors and in general build and appearance. Multituberculates are, however, not related to rodents except that both are mammals. The differentiation of rodents in the late Paleocene and their diversification during the Eocene probably presented strong competition for the multituberculates who were unable to compete successfully and so became extinct.
The primates were represented by Plesiadapsis. This animal was about the size of a squirrel and had chisel-like incisors. Flesh reconstructions of Plesiadapsis, based on skeletal remains, show this primate to look much like a rodent with a long-snouted skull, clawed feet, long body, and tail. There is conceivable relationship between the rodents and Plesiadapsis-like primates.
The dominant Tiffanian herbivorous mammals were members of the Order Condylarthra. These primitive ungulates were diverse in both size and appearance. It is probable that later modern ungulates (Artiodactyla, Perissodactyla) evolved from the Condylarthra; however, there are no surviving members of this group. The Paleocene condylarths were mostly small animals and include such forms as Haplaletes, Litomylus, and Gidleyina. These mammals had somewhat insectivore-like teeth, and may have bridged the gap between insectivores and archaic hoofed mammals. One of the larger Tiffanian condylarths was Phenacodus. This mammal had a long, massive skull and a long, probably flexible body. The limbs were stout and short. The body ended in a long tail. Small hooves were present on all digits, which numbered five per foot. The length of the largest Phenacodus was about 6 ft.
Carnivorous and omnivorous condylarthra were also common. Thryptacodon and Claenodon are two types known from the Evanston Formation in the Fossil Basin.
The true carnivores (Order Carnivora) were also present. The Paleocene carnivores were small and possibly arboreal in habit. The stem carnivore stock was represented by the family Miacidae. In the Fossil Basin the miacid Didymictis was common. In Didymictis, the mouth was armed with small, sharp teeth. Most important, the typical carnivoran carnassial (shearing) teeth had developed. Among carnivores, from Paleocene to Recent, the shearing blades developed on the last upper premolar and the first lower molars. Various kinds of shearing teeth have evolved in other mammal lineages.
Mammals from the Wasatch Formation in Fossil Basin represent the Greybullian and Lysitean provincial ages (see [Fig. 6]). These fossils are of earliest Eocene and mid to early Eocene age. Fossil localities in the Wasatch Formation of southwestern Wyoming are Knight Station (partly Lysitean), at Elk Mountain (Greybullian), and at Fossil Butte itself. The age of the Wasatch Formation at Fossil Butte is Lysitean.
The insectivores are represented by Diacodon, a small mammal known only from fragmentary material.