Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. At the end of the Cretaceous period 65 million years ago, dinosaurs suffered a catastrophic extinction, which ended their dominance on land. Modern birds are the direct descendants of theropod dinosaurs.

Since the first dinosaur was recognized in the 19th century, mounted, fossilized dinosaur skeletons have become major attractions at museums around the world. Dinosaurs have become a part of world culture and remain consistently popular, especially among children. They have been featured in best-selling books and blockbuster films such as Jurassic Park, and new discoveries are regularly covered by the media.

The term dinosaur 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 technically none of these were dinosaurs.

What is a dinosaur?

Definition

The taxon "Dinosauria" was formally named by the English palaeontologist Richard Owen in 1842 as "a distinct tribe or suborder of Saurian reptiles".^[1] The term is derived from the Greek words δεινός (deinos meaning "terrible", "fearsome" or "formidable") and σαύρα (saura meaning "lizard" or "reptile"). Owen chose it to express his awe at the size and majesty of the extinct animals, not out of fear or trepidation at their size and often-formidable arsenal of teeth and claws.

Dinosaurs were extremely varied. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk easily on two or four legs. Regardless of body type, nearly all known dinosaurs were well-adapted for a predominantly terrestrial, rather than aquatic or aerial, habitat.

Dinosaur synapomorphies

All dinosaurs so far discovered share certain modifications to the ancestral archosaurian skeleton. Though 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 structures across a taxonomic group are called synapomorphies.

Dinosaur synapomorphies include reduced fourth and fifth digits on the manus (hand); reduced number of digits on the pes (foot) to three main toes; a sacrum — the region of the vertebral column to which the pelvis attaches — composed of three or more vertebrae; and an open or perforate acetabulum (hip socket), having a hole in the centre, in which the head of the femur (thigh bone) articulates. Dinosaurs are unique among all tetrapods in having this perforate acetabulum.

Other shared anatomical features

Scientists generally agree that a variety of other anatomical features were shared by most dinosaurs. These include front limbs shorter and lighter than hind limbs; an unusual secondary palate that permitted dinosaurs to eat and breathe simultaneously; a relatively straight femur with medially-directed femoral head; two pairs of holes in the temporal region of the skull (i.e. a diapsid skull); rearward-pointing elbows in the front limbs; and forward-pointing knees in the hind limbs.

The hip joint arrangement described above allowed an erect stance, in which hind limbs were situated directly beneath the body or 'underslung'. This stance is like that of most mammals today but unlike that of other reptiles, which have a less erect posture and limbs splayed out to either side. The vertical action of the limbs in dinosaurs allowed for more efficient and faster locomotion, compared to the clumsier and slower movement of other 'sprawled' reptiles. It also allowed many types of dinosaurs to become bipedal.

Taxonomic definition

Under phylogenetic taxonomy, dinosaurs are defined as all descendants of the most recent common ancestor of Triceratops and modern birds. They are divided into Ornithischia (bird-hipped) and Saurischia (lizard-hipped), depending upon pelvic structure. Ornithischian dinosaurs had a four-pronged pelvic configuration, incorporating a caudally-directed (rear-pointing) pubis bone with — most commonly — a forward-pointing process. By contrast, the pelvic structure of saurischian dinosaurs was three-pronged, and featured a pubis bone directed cranially, or forwards, only. Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia, while Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia. It has also been suggested that Dinosauria be defined as all the descendants of the most recent common ancestor of Megalosaurus and Iguanodon.

There is an almost universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor are related, modern birds are dinosaurs and dinosaurs are, therefore, not extinct. Modern birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.

However, referring to birds as "avian dinosaurs" and to all other dinosaurs as "non-avian dinosaurs" is cumbersome. Birds are still referred to as birds, at least in popular usage and among ornithologists. It is also technically correct to refer to birds as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor. Paleontologists mostly use cladistics, which classifies birds as dinosaurs, but some biologists of the older generation do not.

For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur", and "bird" as a synonym for "avian dinosaur" (meaning any animal that evolved from the common ancestor of Archaeopteryx and modern birds). "non-avian dinosaur" will be used for emphasis as needed. It should be noted that this article's definition of "bird" differs from the definition common in everyday language; to most non-scientists, a "bird" is simply a two-legged animal with wings and feathers.

Size

While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. Even by dinosaur standards, the sauropods were gigantic. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods, and only a handful of modern aquatic animals approach them in size — most notably the blue whale, which reaches up to 190,000 kg (209 tons) and 33.5 m (110 ft) in length.

Most dinosaurs, however, were much smaller than the giant sauropods. Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods.^[2] According to paleontologist Bill Erickson, estimates of median dinosaur weight range from 500 kg to 5 metric tons; a recent study of 63 dinosaur genera yielded an average weight in excess of 850 kg — comparable to the weight of a grizzly bear — and a median weight of nearly 2 tons, or about as much as a giraffe. This contrasts sharply with the size of modern mammals; on average, mammals weigh only 863 grams, or about as much as a large rodent. The smallest dinosaur was bigger than two-thirds of all current mammals; the majority of dinosaurs were bigger than all but 2% of living mammals. ^[3]

Largest and smallest dinosaurs

Only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork. As a result, scientists will probably never be certain of the largest and smallest dinosaurs.

The tallest and heaviest dinosaur known from a complete skeleton is the Brachiosaurus specimen that was discovered in Tanzania between 1907–12. It is now mounted and on display at the Humboldt Museum of Berlin and is 12 m (38 ft) tall and probably weighed between 30,000–60,000 kg (33–66 short tons). The longest complete dinosaur is the 27 m (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Natural History Museum in 1907.

There were larger dinosaurs, but knowledge of them is based entirely on a small number of incomplete fossil samples. The largest specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000–100,000 kg (88–121 tons); the longest, the 40 m (130 ft) long Supersaurus; and the tallest, the 18 m (60 ft) Sauroposeidon, which could have reached a sixth-floor window. The largest meat-eating dinosaur was the Giganotosaurus, reaching a length of 14-15 meters (45-50 ft), and weighing in at 9 tons. Giganotosaurus is somewhat shorter than its contemporaries Spinosaurus and Mapusaurus, but outweighed them both. Other challenges to Giganotosaurus include T. rex and Carcharodontosaurus, both 40-45 feet long but was longer and heavier than either of them. It also killed larger prey than any of these contenders, except Mapusaurus, which lived in the same place at the same sime.

Not including modern birds like the bee hummingbird, the smallest dinosaurs known were about the size of a crow or a chicken. The Microraptor, Parvicursor, and Saltopus were all under 60 cm (2 ft) in length.

Behavior

Interpretations of dinosaur behavior are generally based on the pose of body fossils and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches. As such, the current understanding of dinosaur behavior relies on speculation, and will likely remain controversial for the foreseeable future. However, there is general agreement that some behaviors which are common in crocodiles and birds, dinosaurs' closest living relatives, were also common among dinosaurs.

The first direct evidence of herding behavior was the 1878 discovery of 31 Iguanodon dinosaurs which perished together in Bernissart, Belgium, after they fell into a deep, flooded ravine and drowned. Similar mass deaths and trackways suggest that herd or pack behavior was common in many dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-bills (hadrosaurids) may have moved in great herds, like the American Bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxford, England,^[4] and others kept their young in the middle of the herd for defense according to trackways at Davenport Ranch, Texas. Dinosaurs may have congregated in herds for defense, for migratory purposes, or to provide protection for their young.

Jack Horner's 1978 discovery of a Maiasaura ("good mother dinosaur") nesting ground in Montana demonstrated that parental care continued long after birth among the ornithopods.^[5][6] There is also evidence that other Cretaceous-era dinosaurs, like the Patagonian sauropod Saltasaurus (1997 discovery), had similar nesting behaviors, and that the animals congregated in huge nesting colonies like those of penguins. The Mongolian maniraptoran Oviraptor was discovered in a chicken-like brooding position in 1993, which may mean it was covered with an insulating layer of feathers that kept the eggs warm.^[7] Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in northwestern Scotland.^[8] Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, in a manner similar to modern birds and crocodiles.

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. The nature of dinosaur communication also remains enigmatic, and is an active area of research. For example, recent evidence suggests that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,^[9] proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise,^[10] this too was confirmed by tooth marks from Madagascar in 2003.^[11]