الوسم: الديناصورات

Dinosaurs ("terrible, powerful, or wondrous lizards") were the dominant terrestrial vertebrate animals for over 160 million years,
from the late Triassic period (about 230 million years ago) until the end of the Cretaceous period (about 65 million years ago), when the Cretaceous–Tertiary extinction event caused the extinction of most dinosaur species. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, and most paleontologists regard them as the only clade of dinosaurs to have survived until the present day.^[1]
Dinosaurs were a varied group of animals. Paleontologists have identified over 500 distinct genera^[2] and more than 1,000 different species of dinosaur,^[3] and remains have been found on every continent on Earth.^[4] Some dinosaurs were herbivorous, others carnivorous. Some were bipedal, others quadrupedal, and others were able to shift between these body postures. Many species developed elaborate skeletal modifications such as bony armor, horns or crests. Although generally known for their large size, many dinosaurs were human-sized or even smaller. Most major groups of dinosaurs are known to have built nests and laid eggs, suggesting an oviparity similar to that of modern birds.
The term "dinosaur" was coined in 1842 by Sir Richard Owen and derives from Greek δεινός (deinos) "terrible, powerful, wondrous" + σαῦρος (sauros) "lizard". Through the first half of the twentieth century, most of the scientific community believed dinosaurs to have been sluggish, unintelligent cold-blooded animals. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated ****bolisms and numerous adaptations for social interaction.
Since the first dinosaur fossils were recognized in the early nineteenth century, mounted dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become a part of world culture. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media. As a result, the word "dinosaur" has entered the vernacular, although its use and meaning in colloquial speech may be inconsistent with modern science. In English, for example, "dinosaur" is commonly used to describe anything that is impractically large, slow-moving, obsolete, or bound for extinction.^[5]
Etymology

The taxon Dinosauria was formally named in 1842 by English paleontologist Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.^[6] The term is derived from the Greek words δεινός (deinos meaning "terrible", "powerful", or "wondrous") and σαῦρος (sauros meaning "lizard" or "reptile").^[7] Though the taxonomic name has often been interpreted as a reference to dinosaurs’ teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.^[8] In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person,^[9] despite the dinosaurs’ 160 million year reign and the global abundance and diversity of their avian descendants: modern-day birds.
Modern definition

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Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of "Triceratops, Neornithes [modern birds], their most recent common ancestor, and all descendants."^[10] It has also been suggested that Dinosauria be defined with respect to the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.^[11] Both definitions result in the same set of animals being defined as dinosaurs, including theropods (mostly bipedal carnivores), sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), and ornithopods (bipedal or quadrupedal herbivores including "duck-bills"). These definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term "dinosaur" means.
There is a wide consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds would thus be dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.^[12]
From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" does not include birds. Additionally, referring to dinosaurs that are not birds as "non-avian dinosaurs" is cumbersome. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur". The term "non-avian dinosaur" will be used for emphasis as needed.

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