Dinosaurs evolved from small, two-legged (bipedal) ancestors known as thecodonts (thee-co-donts) sometime during the Late Triassic Period (about 225 million years ago). Thecodonts belong to a larger group of reptiles known as archosaurs (ark-o-soars), which in addition to dinosaurs, also includes modern-day crocodiles and the extinct flying reptiles known as pterosaurs (terr-o-soars). Although the name "pterosaur" may seem unfamiliar, the group includes the famous bat-like creatures known as pterodactyls (terr-o-dack-tills).
Most dinosaurs are distinguished from thecodonts and other archosaurs by having a hip socket that is completely pierced through. Rather than having a hollow, cup-like socket into which the head of the thigh bone fits (as in most land creatures) there is a circular hole at this location instead. Thus, one can look through the socket all the way to the other side. Presumably this reflects the upright posture that is so characteristic of the dinosaur. When standing, the weight of the body rested against the upper rim of the hip socket and not in the socket itself. Hence, bone could be removed from there. The leg moves in a fore-and-aft plane, allowing for efficient and relatively rapid forward movement.
There are two basic kinds of dinosaur, distinguished from each other by the overall shape of the hip. One group has a hip similar to that of the dinosaur's thecodont ancestors. These are known as "reptile-hipped" dinosaurs (saurischians - soar-iss-key-ans). They include the long-necked brontosaurs, and the meat-eating (carnivorous) dinosaurs, such as Tyrannosaurus.
The other dinosaur group is the "bird-hipped" dinosaurs (ornithischians - ore-nith-iss-key-ans). In these dinosaurs, the pubis (one of the three hip bones) is rotated backward so that it parallels the ischium (another hip bone - see picture). Also, the front teeth are usually small or absent entirely and replaced by a horny beak. On the lower jaw, this beak is formed by a unique bone, called the predentary, which is not found in any other creature! Bird-hipped dinosaurs include the armored dinosaurs (such as Ankylosaurus), the horned dinosaurs (such as Triceratops), and the duck-billed dinosaurs (such as Edmontosaurus), all of which were plant-eaters (herbivores).
Within each of these groups there were many different species. The navigation bar to your left will take you to pages that discuss some of them.
Most people without formal scientific training tend to describe any extinct reptile as a "dinosaur." However, dinosaurs only include the creatures described on this page. Reptiles such as the flying pterodactyls and the sea serpent-like plesiosaurs also lived during the Age of Dinosaurs, but were not members of the dinosaur family.