Fossil range: Late Triassic - Late Cretaceous
Mounted skeletons of Tyrannosaurus (left) and Apatosaurus (right) at the American Museum of Natural History.
Dinosaurs were the dominant vertebrate animals of terrestrial ecosystems for over 160 million years, from the Late Triassic period (about 230 million years ago) until the end of the Cretaceous period (65 million years ago), when most of them became extinct in the Cretaceous–Tertiary extinction event. The 10,000 living species of birds have been classified as dinosaurs.
The term "dinosaur" was coined in 1842 by Sir Richard Owen. It is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although none of these animals were dinosaurs. Through the first half of the 20th century, most of the scientific community believed dinosaurs to have been slow, unintelligent cold-blooded animals. Most research conducted since the 1970s, however, has supported the view that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. The resulting transformation in the scientific understanding of dinosaurs has gradually filtered into popular consciousness.
The 1861 discovery of the primitive bird Archaeopteryx first suggested a close relationship between dinosaurs and birds. Aside from the presence of fossilized feather impressions, Archaeopteryx was very similar to the contemporary small predatory dinosaur Compsognathus. Research has since identified theropod dinosaurs as the most likely direct ancestors of birds; most paleontologists today regard birds as the only surviving dinosaurs, and some suggest that dinosaurs and birds should be grouped into one biological class. Aside from birds, crocodilians are the only other close relatives of dinosaurs to have survived until the present day. Like dinosaurs and birds, crocodilians are members of Archosauria, a group of reptiles that first appeared in the very Late Permian and came to predominate in the mid-Triassic.
The taxon Dinosauria was formally named in 1842 by English palaeontologist 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. The term is derived from the Greek words δεινός (deinos meaning "terrible", "powerful", or "wondrous") and σαύρα (saura meaning "lizard" or "reptile"). 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. In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person, despite the dinosaurs' 160 million year reign and the global abundance and diversity of their descendants, the birds.
Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of "Triceratops, Neornithes [modern birds], their most recent common ancestor, and all descendants." 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. 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.
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. It is also technically correct to refer to dinosaurs as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor.
Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial archosaurian reptiles with limbs held erect beneath the body, that existed from the Late Triassic (first appearing in the Carnian faunal stage) to the Late Cretaceous (going extinct at the end of the Maastrichtian). Many prehistoric animals are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs. Marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs were neither terrestrial nor archosaurs; pterosaurs were archosaurs but not terrestrial; and Dimetrodon was a Permian animal more closely related to mammals. Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous. 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. One notable exception is Repenomamus giganticus, a triconodont weighing between12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus.
Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 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. An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record. As of September 17, 2008, 1047 different species of dinosaurs have been named. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller. Dinosaur remains have been found on every continent on Earth, including Antarctica. No dinosaurs are known to have lived in marine or aerial habitats, although it is possible some feathered theropods were flyers.
While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common features across a taxonomic group are called synapomorphies.
Dinosaur synapomorphies include an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.
A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsid reptiles, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw. Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus); and an acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example). Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.
Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. Their posture was due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur. Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles. Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs. Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.
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- ^ Benton, Michael J. (2004). "Origin and relationships of Dinosauria". in Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 7–19. ISBN 0-520-24209-2.
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- ^ Morales, Michael (1997). "Nondinosaurian vertebrates of the Mesozoic". in Farlow, James O.; and Brett-Surman, Michael K. (eds.). The Complete Dinosaur. Bloomington: Indiana University Press. pp. 607–624. ISBN 0-253-33349-0.
- ^ Hu Yaoming; Meng Jin; Wang Yuanqing; and Li Chuankui (2005). "Large Mesozoic mammals fed on dinosaurs". Nature 433: 149–152. doi:10.1038/nature03102.
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- ^ Russell, Dale A. (1995). "China and the lost worlds of the dinosaurian era". Historical Biology 10: 3–12.
- ^ Will the real dinosaurs stand up?, BBC, September 17, 2008
- ^ Cite error: Invalid
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- ^ a b Benton, Michael J. (2004). "Origin and relationships of Dinosauria". in Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 7–19. ISBN 0-520-24209-2.
- ^ Holtz, Jr., T.R. (2000). "Classification and evolution of the dinosaur groups". in Paul, G.S. (ed.). The Scientific American Book of Dinosaurs. St. Martin's Press. pp. 140–168. ISBN 0-312-26226-4.
- ^ Langer, M.C., Abdala, F., Richter, M., and Benton, M.J. (1999). "A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil". Comptes Rendus de l'Academie des Sciences, Paris: Sciences de la terre et des planètes 329: 511–517.
- ^ Nesbitt, Sterling J.; Irmis, Randall B.; and Parker, William G. (2007). "A critical re-evaluation of the Late Triassic dinosaur taxa of North America". Journal of Systematic Palaeontology 5 (2): 209–243. doi:10.1017/S1477201907002040.
- ^ This was recognized not later than 1909: "Dr. Holland and the Sprawling Sauropods". http://www.hmnh.org/library/diplodocus/holland1910.html. The arguments and many of the images are also presented in Desmond, A. (1976). Hot Blooded Dinosaurs. DoubleDay. ISBN 0385270631.
- ^ Benton, M.J. (2004). Vertebrate Paleontology. Blackwell Publishers. xii-452. ISBN 0-632-05614-2.
- ^ Cowen, Richard (2004). "Dinosaurs". History of Life (4th edition ed.). Blackwell Publishing. pp. 151–175. ISBN 1405117567. OCLC 53970577.
- ^ a b Kubo, T.; M.J. Benton (2007). "Evolution of hindlimb posture in archosaurs: limb stresses in extinct vertebrates". Palaeontology 50 (6): 1519–1529. doi:10.1111/j.1475-4983.2007.00723.x.