The Triassic.

The Triassic, lasting from 252 to 201 million years (Myr) ago, was crucial in the origin of modern ecosystems. It is the seventh of the 11 geological systems or periods into which the Phanerozoic, the fossiliferous last 540 million years, of Earth history is divided. It might seem strange to select just one of these divisions of time as somehow more significant than the others. However, this was the span of time during which all of the key modern vertebrate groups originated - the neoselachian sharks, neopterygian bony fishes, lissamphibians, turtles, lepidosaurs, crocodilomorphs, and mammals. If birds are theropod dinosaurs, which they are, then this clade also arose in the Triassic.