Though mammals first appeared in the Triassic, some 200 million years ago, they lived in the shadow of the dinosaurs and remained rather small and inconspicuous creatures. These therians split into eutherian (placental) and marsupial lines in the Cretaceous (100 million years ago) but did not diversify significantly until the demise of the dinosaurs, 65 million years ago.
Then, over the first two Epochs of the Tertiary (the Paleocene and Eocene), mammals exploded in number and variety. During this period (65 to 38 million years ago), primates, bats, the first ruminants and ancestral elephants appeared. Primates diverged into prosimians (future lemurs, lorises, galagos) and ancestral anthropods (future monkeys, apes, hominids) and, in North America, ancestral horses, rhinos and camels evolved. Canids (wolves, fox, dogs) and felines split from a common ancestor and the earliest cetaceans (whales, dolphins) returned to the sea.
While many of these ancestral species bore little resemblance to modern mammals, the stage was set for mammals to dominate earth's ecosystems. The Cenozoic Era (the Age of Mammals) began with this explosion and continues today. Most assume that humans, which finally graced the scene 125,000 years ago, are the culmination of this process; more likely, we are just another species in the web of mammal evolution.