II.

Ancestry

Dinosaurs belong to a group of advanced reptiles called archosaurs, which appeared late in the Permian Period. Archosaurs survive today in the form of crocodiles and birds. In addition to dinosaurs, extinct archosaurs included pterosaurs (flying reptiles) and a number of other types of reptiles formerly grouped together under the name thecodonts. During the Triassic, two distinct groups of archosaurs evolved, one related to crocodiles and the other related to dinosaurs, birds, and pterosaurs.

Scientists were once confused about the early ancestors of dinosaurs because some of the archosaurs related to crocodiles developed a number of dinosaur-like features, including walking upright on two hind legs. This phenomenon is known as parallel evolution, in which animals that are not directly related come to resemble each other because they have similar life styles or diets. New fossils have allowed paleontologists to better recognize the true ancestors of dinosaurs. The ancestors of dinosaurs also walked on two hind legs, but had S-shaped necks; simple, hingelike ankle bones; and other distinctive features.

Fossil evidence of the earliest dinosaurs dates from about 230 million years ago. This evidence, found in Madagascar in 1999, consists of bones of an animal about the size of a kangaroo. This dinosaur was a type of saurischian and was a member of the plant-eating prosauropods, which were related to ancestors of the giant, long-necked sauropods that included the Apatosaurus. Prior to this discovery, the earliest known dinosaur on record was the Eoraptor, which lived 227 million years ago. Discovered in Argentina in 1992, the Eoraptor was an early saurischian, 1 m (3 ft) long, with a primitive skull.

Scientists have identified remains of a few small dinosaurs representing ornithischians dating from the end of the Triassic Period before about 200 million years ago. By the middle of the Jurassic Period, around 180 million years ago, most of the basic varieties of saurischian and ornithischian dinosaurs had appeared, including some that far surpassed modern elephants in size. Dinosaurs had evolved into the most abundant large animals on land, and the dinosaurian age had begun.