Early in the Eocene, the global climate remains warm. As the continents move ever closer to their present-day positions, this plate activity alters ocean and air circulation patterns. By the end of the Eocene, temperatures cool considerably and a drying period commences. In subtropical latitudes, open woodlands covered with ferns and shrubby plants replace forests. Land mammals, which move out of the forests and into the open spaces, become noticeably larger. Rhinoceroses, three-toed horses, and early relatives of pigs, camels, and hippopotamuses first appear. As the temperature cools, some groups go extinct, especially those in higher latitudes. Features apparent in early whale fossils -- specialized teeth, nostrils near the tip the nose, hind limbs, and a pelvis attached to the skeleton -- offer irrefutable evidence that whales evolve from land mammals. While they shed many mammalian characteristics, like hair, whales still bear live young, breathe air, and have large and complex brains as they adapt to aquatic life. (Source: PBS)
The oldest known fossils of earliest modern orders of mammals appear during the Early Eocene. Small, below 10 kg. Ungulates (Artiodactyla and Perissodactyla) became prevalent mammals at this time.