Dinosaurs are an extinct superorder of reptiles that first appeared approximately 230 million years ago. A few lines of primitive dinosaurs diversified rapidly after the Triassic; the reign of dinosaurs encompassed the ensuing Jurassic and Cretaceous periods. At the end of the Cretaceous, 65 million years ago, nearly all species of dinosaur became extinct (the Cretaceous-Tertiary extinction event), except for the line that had already led to the first birds. There is now sufficient evidence to demonstrate that the birds are the descendants of theropod dinosaurs.
The formal name Dinosauria was first proposed by the English scientist Richard Owen in 1842. The term is a combination of the Greek words deinos ("terrible" or "fearfully great" or "formidable") and sauros ("lizard" or "reptile").
Dinosaurs varied greatly in size. The smallest known species were about the size of a chicken, but most were much larger. The biggest dinosaurs were the Sauropoda; the species Argentinosaurus currently holds the record for the largest land animals ever to live, and were second in size among all creatures only to certain species of whale. However, the extraordinarily massive femur of a brachiosaur, titled Ultrasaurus, suggests an animal that could have weighed as much as 130 tons, dwarfing the competition.
Many other types of reptiles lived at the same time as the dinosaurs. Some of these are commonly, but incorrectly, thought of as dinosaurs: these include plesiosaurs (which are not closely related to the dinosaurs), and Pterosaurs, which developed separately from reptile ancestors in the late Triassic.
Dinosaurs are archosaurs, like modern crocodylians. These are set apart by having diapsid skulls with teeth that grow from sockets, rather than as direct extensions of the jaw bones, as well as various other characteristics. Within this group, the dinosaurs are set apart most noticeably by their gait. Instead of legs that sprawl out to the side, as found in lizards and crocodylians, they have legs held directly under their body.