Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, approximately 230 million years ago, and became the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65.5 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 fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic, and consequently they are considered a type of dinosaur in modern classification systems. Some birds survived the extinction event that occurred 65 million years ago, and continue the dinosaur lineage to the present day.
Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 9,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. Most dinosaurs have been bipedal, though many extinct groups included quadrupedal species, and some were able to shift between these body postures. Many species possess elaborate display structures such as horns or crests, and some prehistoric groups developed skeletal modifications such as bony armor and spines. Birds have been the planet's dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that egg laying and nest building is a trait shared by all dinosaurs. Many prehistoric dinosaurs were large animals—the largest sauropods could reach lengths of almost 60 meters (200 feet) and were several stories tall—and while many extinct theropods were quite large, a majority evolved very small sizes, especially among birds and other advanced groups.
Although the word dinosaur means "terrible lizard," the name is somewhat misleading, as dinosaurs are not lizards. Rather, they represent a separate group of reptiles with a distinct upright posture not found in lizards. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs were sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that ancient dinosaurs, particularly the carnivorous groups, 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 or replicas have been major attractions at museums around the world, and dinosaurs have become a part of world culture. Their diversity, the large sizes of some groups, and their seemingly monstrous and fantastic nature have captured the interest and imagination of the general public for over a century. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media.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.:103 The term is derived from the Greek words δεινός (deinos, meaning "terrible," "potent," or "fearfully great") and σαῦρος (sauros, meaning "lizard" or "reptile").:103 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.
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: "Dinosauria = Ornithischia + Saurischia", encompassing theropods (mostly bipedal 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, perhaps, sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).
Many paleontologists note that the point at which sauropodomorphs and theropods diverged may omit sauropodomorphs from the definition for both saurischians and dinosaurs. To avoid instability, Dinosauria can be more conservatively defined with respect to four anchoring nodes: Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants. This "safer" definition can be expressed as "Dinosauria = Ornithischia + Sauropodomorpha + Theropoda".
There is near universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. In traditional taxonomy, birds were considered a separate "class" which had evolved from dinosaurs. However, a majority of modern paleontologists reject the traditional style of classification in favor of phylogenetic nomenclature, which requires that all descendants of a single common ancestor must be included in a group for that group to be natural. Birds are thus considered by most modern scientists to 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.General description
Using one of the above definitions, dinosaurs can be generally described as archosaurs with limbs held erect beneath the body. 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 limb posture characteristic of true dinosaurs. 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. One notable exception is Repenomamus giganticus, a triconodont weighing between 12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus.
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. An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record. By September 17, 2008, 1047 different species of dinosaurs had been named. Some are herbivorous, others carnivorous. While most dinosaurs have been bipeds, some prehistoric species were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common among dinosaurs, 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 very small in size. Dinosaurs today inhabit every continents, and fossils show that they had achieved global distribution by at least the early Jurassic period. Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avialan dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semi-aquatic habits.Distinguishing anatomical features
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.
A detailed assessment of archosaur interrelations by S. Nesbitt confirmed or found the following 12 unambiguous synapomorphies, some previously known:
- in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra
- epipophyses present in anterior neck vertebrae (except atlas and axis)
- apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
- radius shorter than 80% of humerus length
- fourth trochanter (projection where the caudofemoralis muscle attaches) on the femur (thigh bone) is a sharp flange
- fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft
- on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element
- exocciptials (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity
- proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface
- cnemial crest on the tibia (shinbone) arcs anterolaterally
- distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia
Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cenmial crest on the tibia and of an ascending process on the astragalus, and many others.