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.
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]).