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).