|| Kids Zone || What is a Dinosaur?
    With so many extreme differences between dinosaurs, it is difficult to arrive at one single definition. Even scientists had trouble deciding just what a dinosaur was. The term dinosaur means "terrible lizard", and was applied by the English palaeontologist Richard Owen in 1841. The name is somewhat of a misconception because not all dinosaurs were "terrible" and none were "lizards". The ancestors of the dinosaurs were the thecodonts, a group of reptiles that lived in the Triassic Period, about 230 million years ago. Many thecodonts were small creatures and some adopted a two-legged stance. The thecodonts evolved rapidly, giving rise to not only the dinosaurs, but crocodiles and flying reptiles.

Present day palaeontologists, through discoveries and research, know a lot more about dinosaurs. Some common characteristics that unite all dinosaurs have been identified.

  1. Dinosaurs were reptiles, belonging to a larger group known as the archosaurs (ruling reptiles) which also include the crocodiles, the extinct pterosaurs (flying reptiles), and the extinct marine reptiles (ichthyosaurs and plesiosaurs), and birds.
  2. Dinosaurs lived on land. They did not fly nor did they live in water, although some may have been semi-aquatic as hadrosaurs were once thought to be. Plesiosaurs, ichthyosaurs and mosasaurs were reptiles that lived in the seas, but were not dinosaurs. Similarly, the extraordinary flying reptiles of the Mesozoic era, the pterosaurs, were not dinosaurs.
  3. Dinosaurs lived during the Mesozoic Era of Earth's history, which lasted from about 225 million to 64 million years ago. Therefore, animals that lived either before or after the Mesozoic are not dinosaurs. For example, the large scale sail-backed reptiles such as Dimetrodon lived in the Permian period, before the dinosaurs.
  4. Dinosaurs developed a fully upright posture with their legs positioned directly beneath their body instead of out to the sides. The only other animals to do this were the birds and mammals. An upright gait was a big advantage for dinosaurs as it helped to support increasing body size and allowed for greater variety of movement. This advantage assisted the dinosaurs in their rapid diversification.

Dinosaurs can be divided into two groups on the basis of the shape of their pelvis bones. The first dinosaur order to evolve was the SAURISCHIAN or "lizard-hipped" group. This consisted of the large sauropods and the meat-eating dinosaurs.

Late in the Cretaceous period, though, the Saurischians were outnumbered by the second major order of dinosaurs, the ORNITHISCIANS or "bird-hipped" dinosaurs. All dinosaurs in this group were herbivores, and they possessed a small bone at the front of the jaw, the predentary.

The classification of these two groups of dinosaurs was based on the hip structures of modern lizards and birds. It is believed, however, that birds are descendants from the lizard-hipped dinosaurs, particularly the small, carnivorous species. This hypothesis is based on shared characteristics between the birds and the Saurischians.

Tracing the evolutionary adaptations of lizard-hipped theropod dinosaurs provides clues to the mystery of how dinosaurs conquered the air. More than one hundred characteristics shared by birds and theropods strongly indicate their close relationship.

The lizard-hipped Saurischians were closely related to pterosaurs, their first cousins, and to birds, their descendants. Although most theropods had lizard-like hips, Velociraptor had hips indistinguishable from those of primitive birds like Archaeopteryx.

Birds evolved from theropods before the end of the Jurassic Period, 150 million years ago, but most of the bird-like theropod specimens date from the Late Cretaceous Period, 65 to 100 million years ago.