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Dinosaur

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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 considered to be the direct descendants of theropod dinosaurs.

Since the first dinosaur was recognized in the 19th century, their mounted, fossilized 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 also used informally to describe any prehistoric reptile, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs, and mosasaurs, though none of these are actually dinosaurs.

Definition
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The superorder or clade "Dinosauria" was formally named by the English scientist Richard Owen in 1842. The term is a portmanteau derived from the Greek words deinos ("terrible" or "fearfully great" or "formidable") and sauros ("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 the dinosaur Ammosaurus) could walk easily on two or four legs.

Under phylogenetic taxonomy, dinosaurs are defined as all descendants of the most recent common ancestor of Triceratops and modern birds. Ornithischia is defined as all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Saurischia is defined as all 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 clumsy. Birds are still 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, to construct their taxonomies, but many other scientists 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). 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
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Only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. As a result, scientists will probably never be certain of the smallest and largest dinosaurs. 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.

Largest and smallest dinosaurs

A statue of Diplodocus carnegiei, outside the Carnegie Museum of Natural History.
Size of a human compared to a Tyrannosaurus rex.While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. 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.

The tallest and heaviest dinosaur known from a complete skeleton is the Brachiosaurus, which 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.

Dinosaurs were the largest of all terrestrial animals. The largest elephant on record weighed 12,000 kg (13.2 tons), while the tallest giraffe was 6 m (20 ft) tall. Even giant prehistoric mammals such as the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods. 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).

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.

Average size

The meaning of "dinosaur average size" is debatable. However it is defined, current evidence suggests different values for average size in the Triassic, early Jurassic, late Jurassic and Cretaceous periods.[1] According to Bill Erickson, "Estimates of median dinosaur mass range from 500 kg to 5 metric tons [...] Eighty percent of the biomass from the Late Jurassic Morrison formation of the western United States consisted of stegosaurs and sauropods; the latter averaged 20 tons. [...] The typically large size of the dinosaurs, and the comparatively small size of modern mammals, has been quantified by Nicholas Hotton. Based on 63 dinosaur genera, Hotton's data yield an average generic mass in excess of 850 kg (about the size of an average grizzly bear) and a median generic mass of nearly 2 tons (which is comparable to a giraffe). This contrasts sharply with extant mammals (788 genera) whose average generic mass is 863 grams (a large rodent) and a median mass of 631 grams (a smaller 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."

Behavior
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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,[3] 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.

A nesting ground of Maiasaura was discovered in 1978.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.[4][5] 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.[6] Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in the United Kingdom.[7] 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,[8] proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise,[9] this too was confirmed by tooth marks from Madagascar in 2003.[10]

There seem to have been no burrowing and few climbing dinosaur species. This is somewhat surprising when compared to the later mammalian radiation in the Cenozoic, which included many species of these types. As to how the animals moved, biomechanics has provided significant insight. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have demonstrated how fast dinosaurs could run,[11][12] whether diplodocids could create sonic booms via whip-like tail snapping,[13] whether giant theropods had to slow down when rushing for food to avoid fatal injuries,[14] and if sauropods could float.[15]

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Study of dinosaurs
Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.[16][17] Many fields of study contribute to our understanding of dinosaurs, including physics, chemistry, biology, and the earth sciences (of which paleontology is a sub-discipline).

Dinosaur remains have been found on every continent on Earth, including Antarctica. Numerous fossils of the same dinosaur species have been found on completely different continents, corroborating the generally-accepted theory that all land masses were at one time connected in a super-continent called Pangaea. Pangaea began to break apart during the Triassic period roughly 230 million years ago.[18]

The current "dinosaur renaissance"
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The field of dinosaur research has enjoyed a surge in activity that began in the 1970s and is ongoing. This was triggered, in part, by John Ostrom's discovery of Deinonychus, an active, vicious predator that may have been warm-blooded (homeothermic), in marked contrast to the prevailing image of dinosaurs as sluggish and cold-blooded. Vertebrate paleontology, arguably the primary scientific discipline involved in dinosaur research, has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, Madagascar, Antarctica, and most significantly in China (the amazingly well-preserved feathered dinosaurs in China have further solidified the link between dinosaurs and their living descendants, modern birds). The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs. Cladistic analysis, among other modern techniques, helps to compensate for an often incomplete and fragmentary fossil record.

Classification
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Main article: Dinosaur classification

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes located where the jaw muscles attach, called temporal fenestrae. Most reptiles (including birds) are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra, are anapsids. Anatomically, dinosaurs share many other archosaur characteristics, including teeth that grow from sockets rather than as direct extensions of the jawbones. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodylians sprawl out to either side. All dinosaurs were land animals.

Many other types of reptiles lived at the same time as the dinosaurs. Some of these are commonly, but incorrectly, thought of as dinosaurs, including plesiosaurs (which are not closely related to the dinosaurs) and pterosaurs, which developed separately from reptilian ancestors in the late Triassic period.

Dinosaurs are divided into two orders, the Saurischia and the Ornithischia, on the basis of their hip structure. Saurischians (from the Greek meaning "lizard hip") are dinosaurs that originally retained the hip structure of their ancestors. They include all the theropods (bipedal carnivores) and sauropods (long-necked herbivores). Ornithischians (from the Greek meaning "bird-hip") is the other dinosaurian order, most of which were quadrupedal herbivores.

Evolution
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A reconstruction of Eoraptor, an early dinosaur.Dinosaurs split off from their archosaur ancestors approximately 230 million years ago during the early Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95 percent of all life on Earth.[19] [20] Radiometric dating of fossils from the early dinosaur species Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs; [21] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[22]

Also among the earliest dinosaurs was the primitive Lagosuchus; Saltopus, which was barely larger than a human hand, appeared slightly later. The first few lines of primitive dinosaurs diversified rapidly through the rest of the Triassic period; dinosaur species quickly evolved the specialized features and range of sizes needed to exploit nearly every terrestrial ecological niche. During the period of dinosaur predominance, which encompassed the ensuing Jurassic and Cretaceous periods, nearly every known land animal larger than 1 meter in length was a dinosaur.

The Cretaceous-Tertiary extinction event, which occured approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the line that had already given rise to the first birds. Other diapsid species related to the dinosaurs also survived the event.

Warm-blooded?

Dinosaur models at the Royal Ontario Museum.A vigorous debate on the subject of temperature regulation in dinosaurs has been ongoing since the 1960s. Originally, scientists broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, dinosaur endothermy has become the consensus view, and debate has focused on the mechanisms of temperature regulation.

After dinosaurs were discovered, paleontologists first posited that they were ectothermic creatures: "terrible lizards" as their name suggests. This supposed cold-bloodedness implied that dinosaurs were relatively slow, sluggish organisms, comparable to modern reptiles, which need external sources of heat in order to regulate their body temperature. Dinosaur ectothermy remained a prevalent view until Robert T. "Bob" Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.

Modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaur species must have regulated their body temperature by internal biological means (perhaps aided by the animals' bulk). Evidence of endothermism in dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica (where they would have experienced a cold, dark six-month winter), the discovery of dinosaurs whose feathers may have provided regulatory insulation, and analysis of blood-vessel structures that are typical of endotherms within dinosaur bone. Skeletal structures suggest that theropods and other dinosaurs had active lifestyles better suited to an endothermic cardiovascular system, while sauropods exhibit fewer endothermic characteristics. It is certainly possible that some dinosaurs were endothermic while others were not. Scientific debate over the specifics continues.[23]

Complicating the debate is the fact that warm-bloodedness can emerge based on more than one mechanism. Most discussions of dinosaur endothermy tend to compare them to average birds or mammals, which expend energy to elevate body temperature above that of the environment. Small birds and mammals also possess insulation, such as fat, fur, or feathers, which slows down heat loss. However, large mammals, such as elephants, face a different problem due to their relatively small ratio of surface area to volume (Haldane's principle). This ratio compares the volume of an animal with the area of its skin: as an animal gets bigger, its surface area increases more slowly than its volume. At a certain point, the amount of heat radiated away through the skin drops below the amount of heat produced inside the body, forcing animals to use additional methods to avoid overheating. In the case of elephants, they are hairless, and have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud wallowing). These behaviors increase cooling through evaporation.

Large dinosaurs would presumably have had to deal with similar issues; their body size would dictate that they lost heat relatively slowly to the surrounding air, and so could have been what are called bulk endotherms, animals that are warmer than their environments through sheer size rather than through special adaptations