Dinosaurs were a diverse group of reptiles that lived during the Mesozoic Era, a period beginning roughly 252 million years ago and ending about 66 million years ago. They were the dominant terrestrial animals for most of this time, and included such iconic species as Tyrannosaurus rex, Stegosaurus, Triceratops, and Brachiosaurus. Fossils have provided us with much information about the appearance, behavior, and ecology of these extinct animals, and ongoing research continues to deepen our understanding of their place in the history of life on Earth.