dinosaur, (clade Dinosauria), the common name given to a group of reptiles, often very large, that first appeared roughly 245 million years ago (near the beginning of the Middle Triassic Epoch) and thrived worldwide for nearly 180 million years. Most died out by the end of the Cretaceous Period, about 66 million years ago, but many lines of evidence now show that one lineage evolved into birds about 155 million years ago.
The name dinosaur comes from the Greek words deinos (“terrible” or “fearfully great”) and sauros (“reptile” or “lizard”). The English anatomist Richard Owen proposed the formal term Dinosauria in 1842 to include three giant extinct animals (Megalosaurus, Iguanodon, and Hylaeosaurus) represented by large fossilized bones that had been unearthed at several locations in southern England during the early part of the 19th century. Owen recognized that these reptiles were far different from other known reptiles of the present and the past for three reasons:
1.they were large yet obviously terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known
- they had five vertebrae in their hips, whereas most known reptiles have only two
- rather than holding their limbs sprawled out to the side in the manner of lizards, dinosaurs held their limbs under the body in columnar fashion, like elephants and other large mammals.
Originally applied to just a handful of incomplete specimens, the clade Dinosauria now encompasses more than 800 generic names and at least 1,000 species, with new names being added to the roster every year as the result of scientific explorations around the world.
Not all of these names are valid taxa, however. A great many of them have been based on fragmentary or incomplete material that may actually have come from two or more different dinosaurs. In addition, bones have sometimes been misidentified as dinosaurian when they are not from dinosaurs at all.
Nevertheless, dinosaurs are well documented by abundant fossil remains recovered from every continent on Earth, and the number of known dinosaurian taxa is estimated to be 10–25 percent of actual past diversity.
Dinosaur ancestors
During the early decades of dinosaur discoveries, little thought was given to their evolutionary ancestry. Not only were the few specimens known unlike any living animal, but they were so different from any other reptiles that it was difficult to discern much about their relationships.
Early on it was recognized that, as a group, dinosaurs appear to be most closely allied to crocodilians, though T.H. Huxley had proposed in the 1860s that dinosaurs and birds must have had a very close common ancestor in the distant past. Three anatomic features—socketed teeth, a skull with two large holes (diapsid), and another hole in the lower jaw—are present in both crocodiles and dinosaurs.
The earliest crocodilians occurred nearly simultaneously with the first known dinosaurs, so neither could have given rise to the other. It was long thought that the most likely ancestry of dinosaurs could be found within a poorly understood group of Triassic reptiles termed thecodontians (“socket-toothed reptiles”). Today it is recognized that “thecodontian” is simply a name for the basal, or most primitive, members of the archosaurs (“ruling reptiles”), a group that is distinguished by the three anatomic features mentioned above and that includes dinosaurs, pterosaurs (flying reptiles), crocodiles, and their extinct rel atives.
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