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Dinosaurs are classified into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or bird-hipped dinosaurs. Dinosaur bones occur in sediments that were deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (240 million to 205 million years ago), the Jurassic (205 million to 138 million years ago), and the Cretaceous (138 million to 65 million years ago). Historical references to dinosaur bones may extend as far back as the 5th century BC. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins—legendary beasts that were part eagle and part lion—guarding nests in central Asia. “Dragon bones” mentioned in a 3rd century AD text from China are thought to refer to bones of dinosaurs.
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The
first dinosaurs studied by paleontologists (scientists who study prehistoric
life) were Megalosaurus and Iguanodon, whose partial bones were discovered
early in the 19th century in England. The shape of their bones indicates
that these animals resembled large, land-dwelling reptiles. The teeth
of Megalosaurus, which are pointed and have serrated edges, indicate
that this animal was a flesh eater, while the flattened, grinding
surfaces of Iguanodon teeth indicate that it was a plant eater. Megalosaurus
lived during the Jurassic Period, and Iguanodon lived during the early
part of the Cretaceous Period. Later in the 19th century, paleontologists
collected and studied more complete skeletons of related dinosaurs
found in New Jersey. From these finds they learned that Megalosaurus
and Iguanodon walked on two legs, not four, as had been thought. In the late 19th and early 20th centuries, as the science of paleontology grew and the search for dinosaur remains was extended around the world, new kinds of dinosaurs were discovered. At present, about 350 different varieties of dinosaur have been identified from bones found on all of the continents as well as on the islands of Greenland, Madagascar, and New Zealand. |
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