The date of the early dinosaur genus Eoraptor at 230 million years ago is important . Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period , roughly 20 million years after the Permianâ `` Triassic extinction event wiped out an estimated 95 % of all life on Earth . Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time .