The Paleogene is the first of three periods of the current geologic era, the Cenozoic. It lasted from about 66 to 23 million years ago, and it included one of the warmest climates of the past 100 million years.
The Paleogene's name derives from the Greek paleo, "old," and genos, "kind" or "lineage." The Paleogene saw a great diversification of birds, fish, and mammals following the mass extinction event that wiped out non-avian dinosaurs. Flightless, carnivorous "terror birds" weighing up to 770 pounds became apex predators, and the first whales evolved from land-dwelling ancestors. Among these was Ambulocetus, a "walking whale" capable of moving on land and in water.