Only a few dinosaurs are known from complete or nearly complete skeletons; almost half of the known species are based only on teeth or bone fragments. The shapes of bones are used for dinosaur classification. Only the hundred or so dinosaurs for which good remains are known can be studied for relationships.
Bones are rarely fossilized. Living things usually decay and vanish after death. It is difficult for paleontologists to describe an incomplete fossil skeleton and to decide what the animal looked and acted like from just a few fossilized remains. The discovery of a new dinosaur-or new fossils of a poorly known dinosaur-may change the family tree. We will never know all the different dinosaur groups that lived, so their family tree will always be incomplete.
Dinosaurs are classified as reptiles, but all reptiles do not form a single clade (a group that includes a common ancestral species and all the species that descended from it). There are two reptilian clades. One clade includes all living reptiles, dinosaurs, ichthyosaurs, plesiosaurs, and birds (the Sauropsida). The other clade is the mammals and the extinct mammallike reptiles (the Theropsida). Crocodilians and birds are more closely related to each other than either is to lizards and snakes. They are part of a smaller sauropsid clade, the Archosauria. Lizards and snakes are in the clade Lepidosauria. Archosaurs had a large opening in the front of each eye. As the many groups of archosaurs evolved, this antorbital fenestra ("window in front of the eye") sometimes closed (in crocodilians and the later plant-eating dinosaurs) or merged with the nostril (in pterosaurs). It was the largest opening in the skull of the large predatory dinosaurs, such as Allosaurus and Tyrannosaurus,
The earliest archosaurs are found in Permian rocks, formed before the Mesozoic Era began. In the beginning of the Mesozoic, when animal life was recovering from the worst mass extinction in the world's history, the archosaurs expanded and quickly spread. Most of those first archosaurs were extinct by the end of the Triassic Period, but the Pterosauria, Saurischia, and Ornithischia survived to the end of the Mesozoic, and the Crocodilia survived to the present. Birds have not been found in the Triassic, although some puzzling Triassic birdlike animals have recently been discovered in Asia, Europe, and Texas.