WHAT IS A DINOSAUR ?

"Dinosaur" is a term freely used and, more often, misused by the public. A dinosaur was not any four-legged creature that lived long ago. Neither was a dinosaur a flying reptile, or pterosaur, frequently referred to incorrectly as a bird. Nor were marine reptiles (Fig. 10) like plesiosaurs, icthyosaurs, and mosasaurs swimming dinosaurs. And of course, the famous finback reptile Dimetrodon of Permian time was not a dinosaur either.

Although it is difficult to get a precise definition of a dinosaur that all paleontologists will accept, a general description is not too hard to provide. The first dinosaur to be recognized as a giant extinct reptile was Iguanodon, found in England in 1822 by Mary Ann Mantell, a fossil hunter and the wife of Dr. Gideon Mantell, who later described it. In 1842, the noted British anatomist Sir Richard Owen, oddly an opponent of Darwin's Theory of Evolution, gave the official name "Dinosauria" to Iguanodon and two other extinct giant reptiles, Megalosaurus and Hylaeosaurus. From that time forth, dinosaurs have been a fixture in the public consciousness. Whether seen as failures or successes, they have often appeared in our cartoons, our advertising, and as toys.

The generally accepted features of a dinosaur today are that it was a reptile belonging to the Subclass Diapsida (two holes in the skull behind the eyes), Superorder Archosauria (ruling reptile), and either the Order Saurischia (reptile-hipped) or Ornithischia (bird-hipped). The major relationships among dinosaurs and related species are depicted in Figure 1.

Dinosaurs generally had but one type of tooth, although the teeth of one dinosaur may be quite different from the next. Mammals, by contrast, have a variegated dentition including cutting, tearing and chewing teeth. A dinosaur usually had longer hind limbs than forelimbs, reflecting its bipedal ancestry. Brachiosaurus, a close relative of our most common local dinosaur, is a noted exception.

Dinosaurs were basically land dwellers; they did not fly nor did they do a lot of swimming. Swamps were not popular with them, contrary to common belief. As land dwellers, one of their most important features was a fully erect gait (see Figs. 5 and 8). They did not sprawl, but walked with their legs straight under their bodies. This fact is reflected in their hip joints, which were tight cylinder joints, and in their trackways, which were narrow___in some cases even pigeon-toed___with little sign of tail dragging.

They had high skulls and generally deep rib cages. Many walked on two legs. Some of the very large ones were four-legged; however, even the biggest may have been able to rear up on their hind legs when trying to reach high vegetation. There were many more plant-eaters than meat-eaters. They generally laid hard-shelled eggs, but some may have borne live young.

Source from : http://terpconnect.umd.edu

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