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).
[Illustration: 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.
[Illustration: 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._]