Dinosaur
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For other uses, see Dinosaur (disambiguation).
Dinosaurs
Temporal range: Late Triassic–Holocene, 231.4 - 0 Mya
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A collection of fossil dinosaur skeletons. Clockwise from top left: Microraptor gui (a winged theropod),Apatosaurus louisae (a giant sauropod),Edmontosaurus regalis (a duck-billed ornithopod),Triceratops horridus (a horned ceratopsian),Stegosaurus stenops (a plated stegosaur),Pinacosaurus grangeri (an armored ankylosaur).
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauriformes
Clade: Dinosauria
Owen, 1842
Major groups
Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, 231.4 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 201 million years ago) until the end of the Cretaceous (66 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic Era. The fossilrecord indicates that birds evolved fromtheropod dinosaurs during the Jurassic Period and, consequently, they are considered a subgroup of dinosaurs.[1]Some birds survived the extinction event that occurred 66 million years ago, and their descendants continue the dinosaur lineage to the present day.[2]
Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10,000 living species,[3] are the most diverse group of vertebrates besidesperciform fish.[4] Using fossil evidence,paleontologists have identified over 500 distinct genera[5] and more than 1,000 different species of non-avian dinosaurs.[6] Dinosaurs are represented on every continent by both extant species and fossil remains.[7] Some are herbivorous, others carnivorous. While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedalspecies, and some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor andspines. Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs. While modern dinosaurs (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs may have achieved lengths of 58 meters (190 feet) and heights of 9.25 meters (30 feet 4 inches).[8] Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long.
Although the word dinosaur means "terrible lizard", the name is somewhat misleading, as dinosaurs are not lizards. Instead, they represent a separate group of reptiles that, like many extinct forms, did not exhibit characteristics traditionally seen as reptilian, such as asprawling limb posture or ectothermy. Additionally, many prehistoric animals, includingmosasaurs, ichthyosaurs, pterosaurs, plesiosaurs, and Dimetrodon, are popularly conceived of as dinosaurs, but are not taxonomically classified as dinosaurs. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Mostresearch conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction.
Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of world culture. The large sizes of some groups, as well as their seemingly monstrous and fantastic nature, have ensured dinosaurs' regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.
Contents
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- 1 Etymology
- 2 Definition
- 3 Evolutionary history
- 4 Classification
- 5 Biology
- 6 Origin of birds
- 7 Extinction of major groups
- 8 History of study
- 9 Cultural depictions
- 10 See also
- 11 Notes and references
- 12 Further reading
- 13 External links
Etymology
The taxon Dinosauria was formally named in 1842 by paleontologist Sir 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.[9] The term is derived from the Greekwords δεινός (deinos, meaning "terrible," "potent," or "fearfully great") and σαῦρος (sauros, meaning "lizard" or "reptile").[9][10] 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.[11]
Definition
Triceratops horridus skeleton,Natural History Museum of Los Angeles County
Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting ofTriceratops, Neornithes [modern birds], their most recent common ancestor (MRCA), and all descendants.[12] It has also been suggested that Dinosauria be defined with respect to the MRCA ofMegalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.[13] Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia +Saurischia", encompassing theropods (mostlybipedal carnivores and birds), ankylosaurians (armored herbivorous quadrupeds),stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), and sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).[14]
The common House sparrow(Passer domesticus) is often used to represent modern birds in definitions of the group Dinosauria.
Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic nomenclature; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well. Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[15]
General description
Using one of the above definitions, dinosaurs can be generally described as archosaurswith hind limbs held erect beneath the body.[16] Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs,pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.[17] Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey.[18]
Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avialan dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.[5] An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record.[19] By September 17, 2008, 1047 different species of dinosaurs had been named.[6] Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some prehistoric species were quadrupeds, and others, such asAmmosaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size. Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the early Jurassic period.[7] Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avialan dinosaurs (such asMicroraptor) could fly or at least glide, and others, such as spinosaurids, had semi-aquatic habits.[20]
