Dinosaur

There is only incomplete knowledge of the evolutionary relationships of dinosaurs with each other and with other major groups of reptiles. This results from the fact that dinosaurs, like any other extinct organism, can only be studied through their fossilized remains, which are often rare and fragmentary. Nevertheless, some dinosaur species bear clear resemblances to each other but are also obviously distinct from certain other dinosaurs.

The dinosaurs evolved from a group of early reptiles known as thecodonts, which arose during the Permian period (290-250 million years ago) and were dominant throughout the Triassic (250-208 million years ago). It appears that two major groups of dinosaurs evolved from the thecodonts, the ornithischian (bird hips) dinosaurs and the saurischian (lizard hips) dinosaurs. These two groups are distinguished largely on the basis of the anatomical structure of their pelvic or hip bones.

Both of these dinosaur lineages originated at about the same time. Both evolved into many species that were ecologically important and persisted until about 66 million years ago. Both groups included quadrupeds that walked A dinosaur foot print in Tuba City, Arizona. JLM Visuals. Reproduced by permission. on all four legs, as well as bipeds that walked erect on their much-larger hind legs. All of the ornithischians had bird-like beaks on their lower jaws and all were herbivores. Most of the carnivorous, or predatory, dinosaurs were saurischians, as were some of the herbivorous species. Interestingly, despite some resemblance between ornithischian dinosaur and bird physiology, it appears that the first birds actually evolved from saurischians.