Chapter II: Part 2

The fossil fish from the Green River Formation have a history of discovery stretching back to 1856. At that time Leidy (1856:256) described a fish given to him by a Dr. John Evans. Leidy called the fish _Clupea humilis_, a type of herring. Exactly where Dr. Evans obtained the specimen is not known. He passed through the area of the Green River Formation several times and could have collected it during any one of those trips (Knight 1955:12).

During the construction of the Union Pacific Railroad in the 1860s, workmen blasting a cut through Green River shales, about 2 miles west of Green River, Wyoming, came upon remains of well-preserved fish. Hayden (1871:742) first mentions this cut as "Petrified Fish Cut." Many of the fish from this cut were given to Cope who described them in Hayden’s (1871) report. Insects, plants, and a bird feather were also obtained from "Petrified Fish Cut" at this time.

Sometime in the 1870s the first fish were obtained from Fossil Butte itself. Cope (1877, 1878) described fish from the Green River Formation which may be from Fossil Butte. His locality is "nearer the mainline of Wasatch Mountains" (Cope 1877:807). This may be Fossil Butte. His 1878 locality information is just as poor. He does mention a specimen of _Priscacara peali_ given to him by A. C. Peale. Since it was Peale who first wrote about Fossil Butte, the specimen may have come from the butte.

Peale (1879) first mentioned the quarrying of fish at Fossil Butte, but he had no specific date as to the discovery of the butte or the beginning of quarrying of fish.

Cope (1884) published two large volumes about the Tertiary vertebrates of the West. Here he mentioned three fossil localities for his Green River Fish: "Petrified Fish Cut," "The Mouth of Labarge Creek," and "Twin Creek." Twin Creek is now known as the Fossil Butte site.

Since that time, the quarries at Fossil Butte have been extensively worked, mostly by commercial collectors.

The fish from Fossil Butte form an unusual array of genera (Table 1) unlike any now found living together. Included in the assemblage are forms usually found in marine waters and tropical, fresh-water fish (Schaeffer and Mangus 1965).

TABLE 1. _Simplified classification of fish from Fossil Basin._

Elasmobranchii
Trygonidae
_Xiphotrygon_ (sting ray)
Chondrostei
Polyodontidae
_Polyodon_ (paddle fish)
Holostei
Lepisosteidae
_Lepisosteus_ (gar)
Amiidae
_Amia_ (bowfin)
Isospondyli
Osteoglossidae
_Phareodus_
Gonorhynchidae
_Notogoneus_ (sand fish)
Clupeidae
_Diplomystus_ (herrings, shad)
_Knightia_
Ostariophysi
Siluridae
_Ameiurus_ (catfish)
Xenarchi
Aphredoderidae (pirate perches)
_Erismatopterus_
Asineopidae
_Asineops_
Acanthoptergii
Percidae
_Mioplosus_ (perches)
Serronidae
_Priscacara_

More than 40 species of fish have been described from the Green River shales, but authorities do not agree on how many of these are valid. Because of this, only super specific groups will be considered in the following discussion.

One of the strangest forms to come from Fossil Butte is that of _Xiphotrygon_ (Fig. 14). This is not a true fish, but is a sting ray, related to sharks. Closely related forms still survive, with little change, in coastal waters in many parts of the world. Like sharks, sting rays have a skeleton composed of cartilage. Normally, cartilage is not preserved as a fossil as it disintegrates readily. The excellent skeletons of _Xiphotrygon_ are good evidence of how well the shales of the Green River Formation preserve the fossils. _Xiphotrygon_ is characterized by being shortened from back to front and flattened from top to bottom. It possesses a long, whip-like tail and bears enormous, flat pectoral (anterior) fins. The mouth is on the underside of the body indicating that _Xiphotrygon_, like its modern relatives, fed along the bottom of the lake. The strong flattened teeth form something like a pavement in the mouth, suggesting that it fed on clams and other hard-shelled invertebrates. Close relatives of _Xiphotrygon_ live mostly in marine waters, but occasionally enter fresh water.

The paddlefish, _Polyodon_, is rare but present at Fossil Butte. Similar forms still survive in China and the United States. The prominent features of _Polyodon_ include a virtually scaleless body, a long, depressed snout, and long gill rakers. The skeleton, unlike that of most modern fish, is composed of cartilage, hence skeletons of _Polyodon_ are rare. The difference between the modern and Eocene paddlefish are so slight that the two probably filled similar or the same ecologic niches. This consists of feeding on plankton and other small organisms which can be obtained from the water. The long gill rakers would provide a large surface area on which food could be trapped as the water passed out through the gill slits. The large mouth would also provide a large surface area for catching and trapping food.

Quite common to streams and rivers of North America from the Cretaceous to the Recent is the garfish, _Lepisosteus_ (Fig. 15). The most striking feature of the gar is its diamond-shaped scales, which are extremely hard and shiny. The body is long, essentially of the same depth throughout, and ending in a broad-based tail that is slightly asymmetrical. The mouth is armed with many small, sharp teeth for catching prey. One specimen over 5 ft long has been found in the Fossil Butte area.

Fossils of the bowfin _Amia_ (Fig. 16) are recorded from the Green River shales at Fossil Butte and in stream deposits from Cretaceous to Recent. This fish is still another “living fossil.” _Amia_ is a nocturnal predator. Presumably, the Eocene _Amia_ had similar habits. The body in _Amia_ has become elongate, as has the dorsal fin. The thickness of the scales has been reduced, but they are still rather heavy and cover the body. Specimens are rather large and deep-bodied. The tail fin is nearly symmetrical above and below. _Amia_ is rare in the Fossil Butte fish quarries.

By far the most abundant fish from Fossil Butte is the genus _Knightia_ (Fig. 17, 18). This fish is a member of the Clupeidae, a family that includes modern herrings, shad, and sardines. The tail fin is small relative to the rest of the body. A row of modified scales extends on the back from the skull to the dorsal fin. Possibly they aided in streamlining the fish. _Knightia_ apparently fed on the large amounts of plankton, especially algae, which lived in the waters of Fossil Lake. Fossils of _Knightia_ are also known from Eocene lake sediments in South America.

_Knightia_ appears to have been susceptible to mass mortalities. Some layers of shale at Fossil Butte contain literally hundreds of thousands of these fish that must have suffered catastrophic mass mortality. Possible reasons for the obvious sudden death of so many fish will be discussed in the section on paleoenvironment and taphonomy.

Another Eocene relative of the herring found at Fossil Butte is _Diplomystus_ (Fig. 19, 20). Like _Knightia_, _Diplomystus_ bears a row of modified scales on its back. _Diplomystus_ is the second most abundant fish at Fossil Butte. The jaw in _Diplomystus_ has a rather pronounced oblique angle to it. The deepest portion of the body is directly behind the gill region, with a continuous narrowing of the body toward the tail region. _Diplomystus_ is known from Cretaceous rocks of Brazil and Syria and from Tertiary sediments in Brazil and West Africa (Schaeffer and Mangus 1965). Close relatives of _Diplomystus_ now inhabit the waters off the coasts of Peru and eastern Australia.

_Phareodus_ (Fig. 21) belongs to a family of fish, the Osteoglossidae, which began as marine forms and has since become a fresh-water family. At present, they are restricted to tropical rivers and lakes of South America, Africa, and Australia. In the past they had a much greater distribution, as its occurrence at Fossil Butte indicates, including North America. It is a deep-bodied fish with a large head. The anal and dorsal fins are close to the tail fins. The many sharp teeth in the mouth attest to the carnivorous habits of _Phareodus_.

The family Gonorhynchidae, the living sandfish of the Indo-Pacific area, is represented at Fossil Butte by _Notogoneus_ (Fig. 22). _Notogoneus_ is very long-bodied and quite slender. The body width does not vary much from head to tail, although narrowing does occur in the tail region. The tail fin is symmetrical, forming a good rudder. The anal, dorsal, pectoral, and pelvic fins are all small. The body is covered with small, elongate scales. _Notogoneus_ was apparently a bottom feeder, living on small organisms picked off or out of the bottom of Fossil Lake.

The Recent catfish, _Ameuirus_, has also been found at Fossil Butte in the Green River shales. The form of the Eocene catfish was very much like the Recent one. The habits of the Eocene catfish were probably like those of the modern _Ameuirus_, i.e., adaptations to a scavenger-type existence.

Rather common in the sediments of Lake Gosiute, but not yet known from the Fossil Basin, is _Erismatopterus_, a member of the family Aphredederidae or pirate perches. This small, elongate fish has few distinguishing characters, the rounded front edge of the skull being most distinguishable.

_Asineops_ is an Eocene relative of _Erismatopterus_ and also one of the pirate perches. _Asineops_ is, however, placed in a different family, the Asineopidae. _Asineops_, like _Erismatopterus_, is a rather plain-looking fish. The dorsal fin is long relative to the rest of the body. The body is slightly deeper behind the gills than elsewhere, giving _Asineops_ a rather common appearance.

The Percidae, or perches, are represented at Fossil Butte by _Mioplosus_ (Fig. 23). _Mioplosus_, as indicated by its well-developed teeth, was a carnivore that probably preyed on its piscine relatives in Fossil Lake. The large head blends in well with a strongly built, long body. The anal and dorsal fins are subequal in size and positioned opposite each other. The tail fin is large and fan-shaped. It is easily identified by the presence of two dorsal fins.

The one genus for which no close living relatives can be found is _Priscacara_ (Fig. 24). The family Priscacaridae is thought to be related to the bass. The family is known only from the Eocene. The size of _Priscacara_ is extremely variable. The body is deep and nearly round like that of the common sunfish, the head blending in well with the body and its contours. The most distinguishing feature of _Priscacara_ is the series of strong, stout spines supporting the anal and dorsal fins. These spines may have acted as cut-waters for better swimming and/or they may have protected _Priscacara_ from its more voracious relatives.

AMPHIBIANS.

To date, no amphibian fossils are known from Fossil Butte. They must have been present, as this class of animals is recorded from Devonian rocks more than 350 million years old and from rocks younger than those at Fossil Butte. Living amphibians are typified by frogs, toads, and salamanders. The lack of amphibian remains at Fossil Butte is more likely due to nondiscovery than nonexistence.

REPTILES.

Reptile fossils are very abundant in the Green River and Wasatch formations. Also a jaw of the three-horned _Triceratops_ is known from the Cretaceous part of the Evanston Formation.

Small lizards are normally represented only by jaw fragments and vertebrae. Their remains are most often found by a process of washing and screening of the sediments containing the fossils. Snakes are also represented in the Green River Formation. Remains of these animals are also often restricted to jaws and vertebrae. A complete skeleton of a fossil boa, _Boavus idelmani_, has been recovered from the Green River Formation and is the most complete fossil snake from North America (Schaeffer and Mangus 1965).

Turtles are among the most abundant reptile fossils. They are usually found as isolated shell plates, although many complete shells are known. The turtles _Trionyx_ (Fig. 25), _Emys_, _Baena_, and _Notomorpha_ are known from the Fossil Basin.

The largest Eocene reptiles were the crocodiles (Fig. 26). The presence of these predaceous animals is further evidence for a prevailing warm and humid climate during the Eocene times since their tolerance ranges include only comparable conditions at the present time. A few incomplete specimens have been recovered from near the monument.

BIRDS.

Birds, which first appeared in Jurassic time, are rare as fossils in rocks of any age because their skeletal remains are exceedingly fragile. Their fossilization requires special conditions because many of their bones are hollow as a weight-reducing adaptation for flight. These hollow bones are especially susceptible to breakage and poor preservation. Scattered remains of birds, mostly limb bones, are found in the Green River sediments. Feather imprints have also been recovered. A complete skeleton of _Gallinuloides_, a bird related to the South American fowl-like chachalaca, is one of the most significant finds from the Green River Formation. Recently, Brodkorb (1970) described the wings of puff birds similar to those now living in South America from near Fossil Butte.

In the adjacent Green River Basin to the east, paleontologists from the University of Wyoming have found nesting grounds of Eocene flamingos in slightly younger sediments of the Green River Formation. These “rookeries” have yielded many hundreds of fossil bones of this curious bird. Since these bird fossils were found proximal to Fossil Butte, they demonstrate the near subtropical nature of the environment in the Fossil Butte area in Green River times.

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.

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.

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_

PALEOECOLOGY AND TAPHONOMY

Paleoecology is the study of ancient biotic communities and their relationship to the abiotic environment. The conclusions of paleoecology are reached by studying in detail the fossils and sediments and whatever relationships exist between them. Its ultimate aim is to build a picture of the climate, flora, fauna, and topographic setting of an area. For this reason it borrows heavily from paleontology, paleobotany, sedimentology, and climatology.

From studying modern environments, it is observed that each has its own type of sediment. When certain sediments are found in the rock record, it is generally assumed that they represent environments similar to modern ones that produce similar deposits. For example, limestone is forming today in warm, shallow, well-lighted, well-aerated water. When limestone is found in a rock sequence, it is usually assumed that the same or similar environmental conditions occurred in that area in the past. Fossils can further refine the interpretation.

As can be interpreted from foregoing discussions, the sediments deposited in the Fossil Basin vary from stream and flood-plain fluvial to lacustrine. At times, material eroded from surrounding uplands was carried by streams throughout the basin. These rocks are exemplified by the Evanston and parts of the Wasatch and Fowkes formations. When the lake appeared, lacustrine sediments, marlstone, and shale were deposited in the lake, while around the periphery of the basin, fluvial sediments continued to accumulate.

Plants found in the lake sediments of Fossil Basin tell us much about the climatic conditions that prevailed during the Eocene. The flora is quite similar to that now existing in the southeastern United States, reflecting a warm and humid climate. One of the striking examples is that of huge palm fronds that occasionally are found in the fish quarries at Fossil Butte. These seem to be fronds that blew from the trees into the lake. Soon they became water-logged and settled to the bottom to be preserved.

The land animals indicate that a rather wide range of ecologic niches existed over the basin before the lake came into being, around the lake during its presence, and again all over the basin after the lake disappeared. Many of the smaller mammals, some of the rodents, and most of the primates were almost certainly arboreal. The large mammals such as uintatheres, pantodonts, and tapiroids may have been stream-side or marsh dwellers. Probably inhabiting the forest floor and feeding on low bushes and undergrowth were such forms as the condylarths, horses, artiodactyls, and some of the rodents. Feeding on the rodents and other smaller mammals were the creodonts and miacids. Tiny shrew-like forms scampered about the undergrowth and fed upon worms and insects.

Bradley (1963) has estimated possible temperature and precipitation levels in southwestern Wyoming during the Eocene. Bradley’s modern analogues were the Gulf Coast and Great Lakes regions. From a series of calculations, an average annual temperature of 65°F is postulated. This could have fluctuated greatly in the inland setting of Fossil Basin. Precipitation amounts were possibly on the order of 30-43 inches annually. The amount of annual evaporation was also possibly in the range of 30-43 inches.

It is believed that Fossil Lake was thermally stratified—that is, with colder, denser water at depth (hypolimnion) and warmer, less dense water (epilimnion) nearer the surface. The deeper waters probably would have been devoid of oxygen, hence essentially uninhabited other than by anaerobic bacteria that survive without oxygen. If the bottom had been oxygenated, many types of life would have burrowed into the sediment thus destroying the delicate varves (Fig. 28). The lake was probably deep enough that wind and wave action did not roil the bottom sediments.

There have been many attempts to interpret the taphonomy (see glossary) of concentrations of fish at Fossil Butte. Bradley (1948) interpreted the cause of death and the reason for their preservation as follows:

In this basin (Fossil, Wyo.) hundreds of thousands of beautifully
preserved fish are entombed in the varved sediments. Even the delicate
fin and tail rays and other bones originally held in place only by
tissue are virtually undisturbed, and even the scales are in place
almost completely undisturbed. It seems to me that the picture of this
lake as a thermally stratified water body provides nearly all the
necessary information to account for the excellent preservation of
these fish. Only in the stagnant hypolimnion could they have escaped
being torn to pieces by scavengers or distorted by bottom feeders. It
is significant that all the well preserved fish are in varved
sediments. Those in non-varved sediments are a disordered mass of
broken and chewed up bones.

The only part of the story lacking now is how the fish died and got
into the hypolimnion. Limnology offers two possible explanations.
Sometimes when the surface of a lake gets excessively warm, fish will
plunge into deep water and might thus penetrate the hypolimnion, be
overcome by hydrogen sulphide, and also have the gas in their swim
bladders chilled so that they sank at once to the bottom. Once there,
only anaerobic bacteria would attack them. The other hypothesis is
that the thermally stratified lake was suddenly chilled so that it
overturned more rapidly than the hydrogen sulphide could be oxydized
and so killed off large numbers of fish. This seems a little more
probable as the fossil fish are of all ages and sizes.

The fish in these quarries have been collected since the 1870s but mainly by commercial collectors. Most museum collections were purchased from these commercial collectors, hence they consist almost entirely of perfectly preserved fish, as poorly preserved specimens would be discarded by the collectors as of no monetary value. The result has been that most people, including Bradley, were misled into believing all of the fish in the varved sediments of the quarry were perfectly preserved. As nearly as can be determined, the first attempt to systematically collect and study this concentration of fish was that by paleontologists from the University of Wyoming (McGrew 1974). This work threw much new light on the occurrence and made available much new data that permit new interpretations.

Shales of the Green River Formation in general have been described by Bradley (1931) and he specifically mentioned those of the fish layer in the Fossil Basin as follows: “Plate 1 shows a thin section of the varved marlstone in the small, unnamed Green River Lake west of Gosiute Lake, where the varves are better developed. Each varve or annual deposit, consists of a layer of microgranular carbonates and a thinner, dark layer of organic matter” (Bradley 1948:645). The X-ray diffraction analyses performed by John Ward Smith of the Laramie Energy Research Center showed that the shales of the “fish layer” consist predominantly of calcite (roughly 60%), aragonite and dolomite (approximately 20%), and quartz (up to 10%).

Although fish are numerous throughout the thickness of this “fish layer,” there are three laminae that contain so many fish that it is almost certain that they represent catastrophic mass mortalities. Two are made up primarily of _Priscacara_ and one consists almost exclusively of _Knightia_.

X-ray photos show that a rather high percentage of the fish in the shales are not perfectly preserved but have undergone varying amounts of disarticulation. There appears to be an orderly sequence of stages of decomposition—from essentially perfect articulation to total disarticulation. Disarticulation first appears in the most anterior vertebrae. From there it rapidly proceeds anteriorly into the head region and appears to do so posteriorly at a slower rate. In many specimens the head and anterior half of the body are completely disarticulated, while the posterior part of the body shows no disarticulation whatever.

It is assumed that after the dead fish settled to the bottom of the lake, external anaerobic bacteria found access to the “innards” of the fish via the opercular opening. This would account for the first sign of decomposition and disarticulation being just back of the head.

In the blocks of shale covered by X-ray there were 385 fish. For convenience, these were classified into six groups, group I showing no discernable disarticulation and group VI (Fig. 29) showing total disarticulation. The number and percentage of fish in each group are as follows:

Group I 223 fish 58%
Group II 38 fish 10%
Group III 14 fish 4%
Group IV 27 fish 7%
Group V 24 fish 6%
Group VI 59 fish 15%

Because of the predominance of completely articulated fish throughout the quarry and the fact that the fish involved in the mass mortalities show no disarticulation, it seems probable that some connection might exist between the death of the fish and conditions of the lake bottom that would cause their perfect preservation. It would seem that rapid burial might be the most obvious reason for excellent preservation. Thus any factor or combination of factors that would cause rapid precipitation of carbonates and also would cause mortality of fish would satisfy our requirements. One obvious factor that could, at least theoretically, fulfill these requirements would be an annual bloom of blue-green algae that are known to be toxic to fish (Prescott 1948). Such blooms usually occur during the warmest part of the summer when CaCO₃ is least soluble. By extracting CO₂ from the water, these algae are known to cause precipitation of CaCO₃. Thus we have a possible cause for some annual fish kill and perhaps an occasional superbloom that would bring about a catastrophic mass mortality. The highest mortality of fish then might have occurred during late summer algal blooms and at this time also would occur the most rapid precipitation of CaCO₃.

It is not known how much deposition of CaCO₃ would be required to protect a fish from disarticulation. It may be that a very thin layer, especially if mixed with organic ooze, might provide an effective seal to slow decomposition and prevent disarticulation. If sufficient CaCO₃ was precipitated and deposited to cover the fish that died during this period, one might expect perfect preservation. Such fish would fit into group I. Those fish that died just after this period might lie exposed on the lake bottom for most of a year and be subject to disarticulation. If little or no deposition took place during the rest of the year, fish that died just after the period of deposition should be the most completely disarticulated and fit into group VI. During the fall, winter, and spring, fish that died of attritional mortality would be disarticulated according to the length of time they lay on the bottom prior to the next period of deposition.

If the foregoing is true, one should be able to determine the approximate time of year fish died by the degree of disarticulation. One might assume that blooms of blue-green algae and hence precipitation of CaCO₃ would take place sometime during August and/or September. Thus we should expect the most fish and those most perfectly preserved to have died during this time period (group I). Those fish that died in October and/or November should be the most completely disarticulated (group VI), and those that died in June or July should show only a slight degree of disarticulation (group II). The distribution of the stages of disarticulation seems to fit almost exactly the pattern that one would expect if this interpretation is correct.

Gunter (1947) has shown that annual periods of excess salinity in Texas lagoons cause an annual increase in the death of fish and occasionally a catastrophic mass mortality. Because Lake Gosiute is known to have been saline, it might be assumed that somewhat similar chemical conditions prevailed in the Fossil Lake. It is not known definitely that the two lakes were ever connected but if they were, it was most probably a narrow connection near the southern end of Fossil Lake and probably a rather temporary connection.

That a rather long period of aridity occurred in the general region is demonstrated by various depositional features, primary structures, and salt deposition in Gosiute Lake in a part of the Green River Formation. These deposits appear to have settled down during Lostcabinian (late to early Eocene) times. Because the Wasatch Formation immediately underlying the Fossil Butte Member of the Green River Formation in the Fossil Syncline Basin is Lysitean (mid to early Eocene), it is probable that the fish deposits are of Lostcabinian age. Thus it may well be that the long period of aridity occurred during the deposition of the fish beds. While the Fossil Lake probably never reached the high degree of salinity present in Lake Gosiute, it was probably sufficiently saline that periods of excessive evaporation could increase the salinity enough to contribute to the mortality of fish and occasionally cause a catastrophic mass mortality such as those described by Gunter. The presence of aragonite and dolomite in the shales suggests that at least some of the carbonate deposition might well have been because of excessive evaporation and high concentrations of carbonates (Smith 1974, pers. comm.).

At the University of Wyoming, an attempt is being made to interpret scales of fish from the quarries. In a number of specimens annuli can be observed and circuli counted. Although removal of scales for study is extremely difficult, a number has been removed and photographed with a scanning electron microscope. In Lake George, Florida, black crappie develop annuli during January, February, March, and April (Huish 1954). Climatic conditions in north central Florida may be similar to those that existed in western Wyoming during the early Eocene. Thus annuli may have developed at the same time of year. By counting the number of circuli between the last annulus and the edge of the scale, it should be possible to determine the approximate time of death of a fossil fish. If this should correlate with the degree of disarticulation, a check on this interpretation should be possible. Sufficient data are not yet available, however, for the results to be conclusive.

It is not intended to suggest that the conditions outlined above could account for all of the fish concentrations in the Green River Shales. In certain shales in the Green River Basin, for example, the concentrations of _Knightia_ in nonvarved shales appear to have been deposited in quite shallow water. These fish are extremely well preserved but were obviously laid down under conditions quite different from those at Fossil Butte. Much study of these occurrences will be necessary before interpretations are possible.

The story, as told here, should make it clear that the earth is ever-changing. What was once a beautiful, deep lake in an area of lush tropical forests is now a dry, sagebrush desert. What was once an area in which sediments were accumulating is now an area of erosion, occasioned by a broad uplift of the region near the end of Tertiary time.

It is hoped that this brief story of the geologic history of Fossil Butte National Monument will give the reader some appreciation of the geologic complexity not only of the monument itself but of the surrounding area as well. Certainly such knowledge will add to the enjoyment of a visit to the monument.

It may seem that geologists have all of the answers. This, however, is not so. Interpretations are made on the basis of available evidence. Each time a geologist studies an area more geologic data and more evidence become available and our interpretations become a little more accurate. It will be many years before all details of this fascinating history will be known.

GLOSSARY

ANAEROBIC. Usually in reference to organisms that can live without
oxygen.
ANGULAR UNCONFORMITY. Two rock layers which are not parallel; the
underlying older layer dips at a different angle (usually
steeper) than the younger top strata.
ANNULI. Marks on fish scales produced by periods (usually winter) of
nongrowth.
ANTICLINE. A fold in stratified rock with the strata sloping downward
in opposite directions from the fold crest.
ARAGONITE. A carbonate mineral with specific characteristics.
AUTHIGENIC. A mineral (such as quartz or feldspar) which is formed
after the deposition of a sedimentary layer in which it
occurs.
BENTONITE. A light-colored, soft, porous rock formed from the minute
clay crystals of eroded volcanic ash. It has the
characteristic of swelling when wet (water absorption) and
contracting when dry.
BRACKISH. A condition in a body of water in which the salinity (salt
level) is below that of sea water, but higher than that of
fresh water.
CARBONACEOUS. Rock or sediment which contains carbon or altered
organic material such as coal.
CHERTY. Containing chert: a dull-colored, flint-like quartz often
found in limestone.
CIRCULI. Ridges on fish scales produced during growth of the scales.
CLAST. Rock fragments which are the result of weathering of a larger
rock mass.
CLASTIC. Rocks that consist of particles derived from pre-existing
rocks or minerals.
CLAYSTONE. An indurated clay without the lamination or fissility of
shale.
CONGLOMERATE. A coarse-grained sedimentary rock composed of fragments
larger than 2 mm in diameter in a fine-grained matrix.
CROSS-BEDDING. An internal structure in sedimentary rock in which the
upper sedimentary layer runs across the grain of the main bed;
it is caused by changing currents depositing sediment across
the grain of the original deposits.
DIAMICTITE. A sedimentary rock containing a wide range of particle
sizes.
DIP. The downward inclination of a rock layer; the vertical angle is
determined by its relationship to a horizontal plane.
DOLOMITIC. Containing a measurable amount of the mineral dolomite; a
mineral consisting mainly of magnesium carbonate and calcium
carbonate.
FACIES. Lateral variations in the appearance or composition of a rock
layer. The variations can be lithologic or paleontologic.
FAULT. A fracture in the earth’s crust along which displacement
(movement) has occurred.
FLUVIAL. Pertaining to a river or rivers. Fluvial sediments are those
transported and deposited by stream action.
FORMATION. A rock layer that is mappable; has a distinctive lithology
or series of lithologies. A mappable sequence of uniform or
uniformly varying rocks.
GASTROPODAL. A rock containing an abundance of gastropods.
HOGBACK. A long, narrow, sharp-crested ridge formed by the outcropping
edges of steeply inclined resistant rocks.
IGNEOUS ROCKS. A rock or mineral that has solidified from molten or
partly molten material.
INTRUSIVE. Igneous rock formed by the forcing of molten material into
a pre-existing rock.
IRONSTONE. A rock composed of various iron minerals that accumulated
during or shortly after deposition of the enclosing sediments.
LACUSTRINE. Pertaining to, produced by, or formed in a lake or lakes.
LATERITIC. Containing laterite: a red, porous material usually
developed in a tropical to temperate climate. It is a residual
or end-product of weathering.
LIGNITE. A brownish-black coal that is intermediate in coalification
between peat and subbituminous coal.
LITHOLOGY. The scientific study of rocks: composition, texture, color,
origin, etc.
MAGNETITE. A black, opaque mineral that is strongly magnetic.
MARLSTONE. An impure limestone.
METAMORPHIC. Rocks whose structure has been changed by pressure, heat,
chemical reaction, etc., such as limestone into marble.
MUDSTONE. An indurated mud without the lamination or fissility of
shale.
OPERCULAR OPENING. The gill opening of fish.
OSTRACODAL. A rock, usually a limestone, that contains an abundance of
the small crustacean, ostracods.
OVERTHRUST. A low-angle thrust fault of large scale, usually measured
in miles.
PALEOLIMNOLOGY. The study of ancient lakes.
PAPER SHALE. A form of finely laminated shale that weathers into
extremely thin, curled flakes.
PHOSPHATIC. Containing phosphates.
PHYTOPLANKTON. Floating microscopic plant life that occurs abundantly
in lakes and oceans.
PLATY. Rocks (sandstone or limestone) which separate into small slabs.
PORCELLANITE. A dense cherty rock resembling porcelain.
PUDDINGSTONE. A conglomerate consisting of well-rounded pebbles and
cobbles sparsely packed in a fine-grained matrix.
SILTSTONE. An indurated silt without the lamination or fissility of
shale.
STRATA. Rock layers of distinct composition and origin.
SYNCLINE. A fold in stratified rock in which the strata slope up from
the axis of the fold forming a v opposed to anticline.
TAPHONOMY. The branch of paleoecology which deals with the change from
living animals to fossils.
TECTONIC. The forces which result in structural changes in the earth’s
crust.
THRUST FAULT. A fault in which an upper segment of rock (hanging wall)
moves upward at a low angle (less than 45°) relative to a
lower segment (footwall).
TONGUE. A rock unit that wedges into, but disappears within, another
rock unit.
TUFFACEOUS. Sediment that contains up to 50% volcanic ash or dust.
UNCONFORMITY. A substantial break or gap in the geologic or
stratigraphic record.
UNGULATES. Hooved mammals.
VARVE. A set of rock laminae in which different types of sediment were
deposited in the winter and in the summer. Thus a couplet of
each sediment type would represent the deposition of one year.
WELL SORTED. A rock in which nearly all of the sediment particles are
of one grain size.
ZOOPLANKTON. Floating microscopic animal life that occurs abundantly
in lakes and oceans.

REFERENCES