Evolution and Radiation of Mammals in the Eocene, and the Diversification of Modern Orders

Abstract

The Eocene was an especially interesting and important time in the history of mammals. Whereas the Paleocene witnessed the rise of mammals from their previously subordinate position among terrestrial vertebrates, it was during the Eocene that the second major radiation of eutherian mammals took place. Many of the modern orders diversified and achieved dominance on land, and mammals even began to invade the seas and take to the air. By the late Eocene, this explosive radiation had produced the greatest ordinal diversity of mammals known at any time in their history (Lillegraven, 1972). If we could travel back to the Paleocene, we would see few familiar mammals; most belonged to archaic groups that long ago became extinct. But a visitor to the Eocene (given wide latitude in space and time, of course) might recognize many of the denizens of Eocene forests and floodplains—bats, prosimian primates, squirrel-like rodents, rabbits, moles, armadillos, and miniature hoofed animals resembling mouse deer. Along the shore, our hypothetical Eocene naturalist might spot a whale or a primitive sea cow. A closer look would reveal that these animals differed in many ways from living forms, of course, but the resemblances are real and, in some cases, striking. In fact, at least 10 orders of mammals alive today appeared in the fossil record for the first time, or experienced major radiations of modern groups, in the Eocene. Not all Eocene mammals belonged to modern orders, however. There was still a considerable proportion of archaic mammals, many representing lines that had begun in the Paleocene or, in some instances, before. And some of them were highly successful too.