More descendants than true dinosaurs but for most the distinction is unimportant . For the sake of convenience , the dinosaur family is divided into two main groups . To vastly simplify the story , starting about 230 million years ago a subgroup of archosaurs split off into two types of dinosaurs , distinguished by the structure of their hip bones . Saurischian lizardhipped dinosaurs went on to include predators like Tyrannosaurus Rex and huge sauropods like Apatosaurus , while ornithischian birdhipped dinosaurs consisted of a diverse assortment of other herbivores , such as hadrosaurs , ornithopods and stegosaurs . Confusingly , we now know that birds descended from lizardhipped , rather than birdhipped , dinosaurs !