DINOSAURS
There were many reptiles that lived at the same time as the dinosaurs, including some that are alive today, such as alligators and turtles. So what distinguishes dinosaurs from these other reptiles? Dinosaurs were land-dwelling reptiles. They probably arose from a group of archosaurs called thecodonts in the middle to late Triassic Period. Many scientists believe archosaurs evolved a number of improvements to crocodilian leg and muscle arrangement, enabling them to move much faster and hunt more effectively. With dinosaurs the process continued, and the main feature that marks the dinosaurs appeared: an upright stance on straight legs. Other reptiles, such as lizards, have legs that spread to the sides of their bodies, and knee joints that are at an angle. Dinosaurs, with their legs beneath their bodies, grew quite large because their distinctive legs could hold them up. In fact, scientists can recognize a dinosaur fossil just by observing the arrangement of the hip joint, the angle of the knee and ankle joints, and the ridges and knobs on the bones that mark the places where the muscles attached.
Even for those who are not paleontologists, it is fairly easy to recognize a dinosaur. All dinosaurs lived on land and walk upright on column-like legs, they laid eggs with shells and lived only during the Mesozoic era.
All dinosaurs belong to the class Reptilia, and the superorder Archosauria. Below Archosauria are two principal groups, the orders Saurischia and Ornithischia, which differ in their hip arrangement. Some scientists have argued that dinosaurs, which may have been warm-blooded, are so different from the other members of the class Reptilia that they deserve their own class: Dinosauria. If birds, now in a class to themselves, Aves, did descend from dinosaurs, then they should also be classified as Dinosauria, and that would make them the only members of that group to survive the mass extinction at the end of the Cretaceous Period.