Eocene Epoch

Eocene Epoch, second division of the Paleogene Period of the Cenozoic Era of the geologic time scale, spanning an interval from about 56 to 34 million years ago. Like the Paleocene Epoch, which preceded it, and the Oligocene, which followed it, the Eocene (Greek eos, ”dawn”; kainos, ”life”) was originally defined in the 19th century by the British geologist Sir Charles Lyell on the basis of the percentage of modern species of shellfish found in Cenozoic rock strata.

In the western hemisphere, the Eocene marked the last phase of the cordilleran orogeny—the mountain-building episode responsible for uplifting the great chain of ranges that extends north and south along the western portion of the Americas. In the western United States, claylike sediments deposited in broad Eocene lakes were later compacted to form the valuable oil-shale deposits of Wyoming, Colorado, and Utah. At the same time, as the former supercontinent of Laurasia continued to tear apart (see Plate Tectonics), seafloor spreading began in earnest along the northern section of the mid-Atlantic ridge, thus driving Greenland westward, away from northern Europe, and triggering the eruption of great basalt flows; remnants can be seen in Ireland, Scotland, Iceland, and Greenland.

Mountain-building forces generated by continental collisions that had begun at the close of the preceding Mesozoic Era culminated in the upheaval of the Alpine-Himalayan mountain system. Meanwhile huge volumes of basalt poured out over the plains of northeastern India as that newly formed subcontinent, torn loose from Africa during the Cretaceous Period, became welded to Asia. In the southern hemisphere, Antarctica and Australia, joined since breaking free from Gondwanaland in the Mesozoic, finally separated and drifted apart.

The climate of Eocene times was subtropical and moist throughout North America and Europe. In the U.S., palm trees and alligators lived as far north as the Dakotas, while at high northern latitudes in Greenland and Siberia, moist Temperate Zone forests were dominated by giant redwoods and deciduous trees such as beech, chestnut, and elm. In Alaska’s warm Eocene climate, cycads, magnolias, and fig trees flourished.

The rapid evolution of new orders of mammal life continued from the preceding Paleocene. Ancestral forms of the horse, rhinoceros, camel, and other modern groups such as bats, primates, and squirrel-like rodents appeared simultaneously in Europe and North America. Many were strikingly small compared with today’s forms. The earliest horse was barely 30 cm (12 in) high, with three toes on its hind feet and four on its front feet. The dominant carnivores of the time, called creodonts, were not closely related to modern forms such as dogs and cats. The latter part of the epoch witnessed the first adaptation of mammals to life in the sea. Fossil bones of an Eocene whalelike animal are found in the southern U.S., Egypt, and Europe. The largest of these animals was more than 15 m (more than 50 ft) long, but not so large that it could not be preyed upon by the sharks of the time, some species of which had jaws as wide as 1.8 m (6 ft) across.