Development of mammals
The Cenozoic Era is known as the “Age of Mammals.” Small mammals had already existed, though quite inconspicuously, in Wyoming for about 90 million years before Paleocene time. Then about 65 million years ago their proliferation began as a result of the extinction of dinosaurs, obliteration of seaways that were barriers to distribution, and the development of new and varied types of environment. These new environments included savannah plains, low hills and high mountains, freshwater lakes and swamps, and extensive river systems. The mammals increased in size and, for the first time, became abundant in numbers of both species and individuals. The development and widespread distribution of grasses and other forage on which many of the animals depended were highly significant. Successful adaptation of herbivores (vegetation-eating animals) led, in turn, to increased varieties and numbers of predatory carnivores (meat-eating animals).
During early Eocene time, coal swamps formed in eastern Jackson Hole and persisted for thousands of years, as is shown by 60 feet of coal in a single bed at one locality. Continuing on into middle Eocene time, the climate was subtropical and humid, and the terrain was near sea level. Tropical breadfruits, figs, and magnolias flourished along with a more temperate flora of redwood, hickory, maple, and oak. Horses the size of a dog and many other small mammals were abundant. Primates, thriving in an ideal forest habitat, were numerous. Streams contained gar fish and crocodiles ([fig. 50]).
Figure 51. A typical Oligocene landscape showing some of the more abundant types of mammals. Mural painting by Jay H. Matterness; photo courtesy of Smithsonian Institution.
| Trigonias | early rhinoceros |
| Perchoerus | early peccary |
| Mesohippus | 3-toed horse |
| Aepinacodon | remote relative of hippopotamus |
| Archaeotherium | giant piglike mammal |
| Protoceras | bizarre horned ruminant |
| Hesperocyon | ancestral dog |
| Hyracodon | small fleet-footed rhinoceros |
| Poëbrotherium | ancestral camel |
| Hypisodus | very small chevrotainlike ruminant |
| Ictops | small insect-eating mammal |
| Brontotherium | titanothere |
| Protapirus | ancestral tapir |
| Glyptosaurus | extinct lizard |
| Hoplophoneus | saber-toothed cat |
| Subhyracodon | early rhinoceros |
| Merycoidodon | sheeplike grazing mammal |
| Hyaenodon | archaic hyenalike mammal |
| Hypertragulus | chevrotainlike ruminant |
Early in the Oligocene Epoch, between 30 and 35 million years ago, the climate in Jackson Hole became cooler and drier, and the subtropical plants gave way to the warm temperate flora of oak, beech, maple, alder, and ash. The general land surface rose higher above sea level, perhaps by accumulation of several thousand feet of Oligocene volcanic rocks ([fig. 52]) rather than by continental uplift. Titanotheres (large four-legged mammals with the general size and shape of a rhinoceros) flourished in great numbers for a few million years and then abruptly vanished. Horses by now were about the size of a very small modern colt. Rabbits, rodents, carnivores, tiny camels, and other mammals were abundant in Jackson Hole, and the fauna, surprisingly, was essentially the same as that 500 miles to the east, at a much lower elevation, on the plains of Nebraska and South Dakota ([fig. 51]).
The Miocene Epoch (15 to 25 million years ago) was the time of such intense volcanic activity in the Teton region that animals must have found survival very difficult. A few skeletons and fragmentary parts of camels about the size of a small horse and other piglike animals called oreodonts comprise our only record of mammals; nothing is known of the plants. Farther east the climate fluctuated from subtropical to warm temperate, gradually becoming cooler toward the end of the epoch.
Fossils in the Pliocene lake deposits (8 to 10 million years old; see description of Teewinot Formation) include shallow-water types of snails, clams, diatoms, and ostracodes, as well as beavers, mice, suckers, and frogs. Pollen in these beds show that adjacent upland areas supported fir, spruce, pine, juniper, sage, and other trees and shrubs common to the area today. Therefore, the climate must have been much cooler than in Miocene time. No large mammals of Pliocene age have been found in Jackson Hole. The record of life during Quaternary time is discussed later.
Figure 52. Layers of volcanic conglomerate separated by thin white tuff beds in Wiggins Formation. These cliffs, on the north side of Togwotee Pass, are about 1,100 feet high and represent a cross section of part of the enormous blanket of waterlaid debris that spread south and east from the Yellowstone-Absaroka volcanic area. These and younger deposits from the same general source filled the basins and almost completely buried the mountains in this part of Wyoming.