Part 9

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.