Eocene Epoch

Eocene Epoch, second division of the Tertiary Sub-Era of the Cenozoic Era of the geological timescale, spanning an interval from about 55 to 34 million years ago. It was preceded by the Palaeocene Epoch and followed by the Oligocene Epoch. Like the later Miocene and Pliocene Epochs, the Eocene (Greek, eos, “dawn”; kainos, “recent”) was originally defined in the 19th century by the British geologist Sir Charles Lyell on the basis of the percentage of modern species of mollusc 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 part of the Americas (see Cordilleras). At the same time, as the former supercontinent of Laurasia continued to be torn apart, sea-floor spreading began in earnest along the northern section of the Mid-Atlantic ridge, thus separating Greenland from northern Europe, and triggering the eruption of great basalt lava flows (see Volcanism), remnants of which can be seen in Ireland, Scotland, Iceland, and Greenland. Europe became an archipelago, separated from Asia by the West Siberian Seaway, where the Ural Mountains now rise. India, an island since it broke free from Africa during the Mesozoic Era, completed its northward drift and collided with Asia. In the southern hemisphere, Australia separated from Antarctica and began drifting northwards.

The climate of Eocene times was warm. It began with a sharp 200,000-year-long warming event, known as the Palaeocene-Eocene Thermal Maximum, the warmest interval of the entire Cenozoic. This caused a major extinction in marine benthic (sea-floor dwelling) micro-organisms. On land, tropical-type evergreen forests initially extended polewards to about 60° latitude in the northern hemisphere, while deciduous forests grew in the Arctic and Antarctic. Crocodiles lived as far north as present-day Arctic Canada. This largely ice-free world began to cool slowly from about 50 million years ago. By the end of the epoch, glaciation had begun on Antarctica, the tropical forest belt had shrunk dramatically, and more open deciduous vegetation grew in mid latitudes.

Most modern orders of mammals are first known from the Eocene. Primates, artiodactyls (cloven-hoofed mammals), perissodactyls (horses, rhinoceroses, and tapirs) and bats appear relatively suddenly at the beginning of the epoch in all three northern hemisphere continents in a major dispersal event. Ecologically these artiodactyls and perissodactyls were very different from their modern descendants. For instance, the earliest horses were barely 30 cm (12 in) tall at the shoulder, with three toes on their hind feet and four on their forefeet. From exceptionally preserved fossils preserving soft tissues and stomach contents at Messel, Germany, we also know that they had manes, fed on leaves and fruit, and bore a single foal. The first known whales and sea cows are from early in the epoch, fossils being discovered in Pakistan and Jamaica respectively. These early aquatic mammals had four walking limbs instead of one pair of flippers and were amphibious. Large size and a leaf-eating diet became more common for mammals as the Eocene progressed and forest productivity dwindled with the cooling climate.