Part 10

The insectivores are represented by Diacodon, a small mammal known only from fragmentary material.

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.

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).

The true carnivores, represented by Miacis, remained small in size, perhaps to avoid competition with the larger surviving creodonts.

Insectivores from the Fowkes Formation are abundant and varied. Unfortunately, they are known almost exclusively from teeth. Fowkes insectivores include the hedge-hog-like Nyctitherium, Talpavus, and Scenopagus. Apatemys is an insectivore of uncertain affinities. The feeding habits of these early insectivores were probably similar to those of contemporary insectivore species (moles and shrews); however, there is some evidence to suggest many of these small creatures may have been semi-arboreal.

One of the most unusual of middle Eocene creatures is the giant Uintatherium. This was a large browsing animal, about the size of a rhinoceros. What was most unusual about Uintatherium was its skull which was large and strongly built. The large upper canines were apparently formidable defensive weapons. The skulls of the males bore six prominent bony protuberances which grew from the frontal region of the skull. The function of these structures is unknown. They may have been of use for defense.

During Bridgerian times, the rodents underwent an explosive adaptive radiation which ultimately may have led to the near extinction of the less well-adapted multituberculates. The diversity of Bridgerian rodents from Fossil Basin is impressive. Species of Leptotomus, Paramys, Thisbemys, Microparamys, Sciuravus, Mysops, and Pauromys demonstrate that a wide range of adaptations of environments was exploited by the rodents. This diversity in life habits has been a mark of rodent evolution from the middle Eocene to the present day.

The above descriptions of the known mammal fossils from the Fossil Butte area are intended to demonstrate the varied life forms that once existed in southwestern Wyoming. Further collecting will almost certainly expand the number of fossil mammal species from Fossil Basin and increase our knowledge of the succession of ancient environments which prevailed at different times in the Fossil Basin area.

TABLE 2. Mammals known from Fossil Basin

Faunal lists—Middle Paleocene: Torrejonian Class Mammalia Multituberculata Ptilodus Neoplagiaulax Ectypodus Insectivora Leptacodon Aphronorus Primates Torrejonia Pronothodectes Condylarthra Chriacus Tricentes Promioclaenus Litaletes Haplaletes Late Paleocene: Tiffanian Multituberculata Ptilodus? Primates Plesiadapis Condylarthra Thryptacodon Claenodon Litomylus Haplaletes Gidleyina Phenacodus Carnivora Didymictis Pantodonta Genus indeterminant. Early Eocene: Graybullian Primates Pelycodus Tillodontia Esthonyx Rodentia Paramys Creodonta Proviverra Carnivora Didymictis Vassacyon Condylarthra Pachyaena Haplomylus Hyopsodus Phenacodus Meniscotherium? Pantodonta Coryphodon Perissodactyla Hyracotherium Artiodactyla Diacodexis Early Eocene: Lysitean Insectivora Diacodon Primates Pelycodus Microsyops Tillodontia Esthonyx Rodentia Paramys Reithroparamys Creodonta Proviverra Carnivora Didymictis Vulpavus Condylarthra Pachyaena? Hyopsodus Phenacodus Meniscotherium? Pantodonta Coryphodon Perissodactyla Hyracotherium Heptodon Artiodactyla Protodichobune Early Eocene: Wasatchian (general) Chiroptera Icaronycteris Late Middle Eocene: Late Bridgerian Marsupialia Peratherium Insectivora Apatemys Nyctitherium Scenopagus Talpavus Primates Hemiacodon Omomys Notharctus Condylarthra Hyopsodus Rodentia Leptotomus Microparamys Mysops Paramys Pauromys Sciuravus Thisbemys Dinocerata Uintatherium Perissodactyla Orohippus Hyrachyus