MAMMALS.

The rocks of the Evanston, Wasatch, and Fowkes formations contain

various fossilized remains of mammals which once inhabited the Fossil

Basin.

Mammal fossils are highly significant since they are used extensively

for correlation and dating of the Tertiary rocks of the West. Fossils

which are readily recognizable, limited in time, and widely distributed

geographically are called “index fossils.” These fossils are restricted

to a particular time horizon in the rock. In this way, rocks of unknown

age which are found to contain index fossils can be dated relatively and

hence correlated with other fossil-bearing rocks. The small condylarth

_Haplomylus_ (Fig. 27) is a very early Eocene index fossil. Its presence

in certain sediments in the Fossil Basin shows conclusively that these

sediments were laid down in earliest Eocene times.

[Illustration: Fig. 27. A jaw of the primitive mammal _Haplomylus_.

Length of original, 1.3 cm. _Collection of University of Wyoming._]

The mammal fossils found in Fossil Basin are also important in

documenting the temporal changes in evolution and environment of the

biotic community. The picture of Eocene Wyoming drawn through

interpretation of these fossils is vastly different than that of the

present day.

Mammals have a long evolutionary history that began over 200 million

years ago in the Triassic period. At that time mammals had only just

evolved from their reptilian ancestors. These earliest mammals were

small, furtive creatures. However, with the extinction of the dinosaurs

some 65 million years ago at the end of the Cretaceous, the mammals were

able to diversify rapidly and fill the empty ecologic niches that the

dinosaurs once occupied.

This filling of ecologic niches voided by the dinosaurs coupled with the

spreading of mammals into hitherto unoccupied niches resulted in the

development of a large variety of mammals. Some became extinct, others

were the ancestors of modern mammals. Fossils of mammals found in

sediments in the Fossil Basin are proof of the extent of this Late

Cretaceous-early Tertiary mammal radiation.

Early mammals bore little resemblance to their later descendants.

General trends in mammalian evolution were increase in size and the

tendency to become more “modern”-looking through adaptation to changing

Tertiary environments.

Most mammal fossils are extremely fragmentary. Teeth are most often

preserved as they are the hardest part of the skeleton and therefore

most resistant to wear and breakage. Thus, it often happens that the

knowledge of a particular fossil mammal is derived entirely from its

preserved dentition. This has obvious limitations for the completeness

of our understanding about mammals in question.

The Paleocene portion of the Evanston Formation has yielded mammals of

Torrejonian and Tiffanian age (Fig. 6).

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.

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.