Dinosauria

Dinosaurs are a superorder of reptiles that first appeared approximately 230 million years ago. A few lines of primitive dinosaurs diversified rapidly after the Triassic; the reign of dinosaurs encompassed the ensuing Jurassic and Cretaceous periods. At the end of the Cretaceous, 65 million years ago, nearly all species of dinosaur became extinct (the Cretaceous-Tertiary extinction event), except for the line that had already led to the first birds. There is now sufficient evidence to demonstrate that the birds are the descendants of theropod dinosaurs. Warm-blooded vertebrates without fur or feathers did not survive what appears to be a very cold period following a meteor strike on the Earth, thus ending the non-avian dinosaurs species which formerly were thought to be all the dinosaur species.

"Ask your average paleontologist who is familiar with the phylogeny of vertebrates and they will probably tell you that yes, birds (avians) are dinosaurs. Using proper terminology, birds are avian dinosaurs; other dinosaurs are non-avian dinosaurs, and (strange as it may sound) birds are technically considered reptiles."

The formal name Dinosauria was first proposed by the English scientist Richard Owen in 1842. The term is a combination of the Greek words deinos ("terrible" or "fearfully great" or "formidable") and sauros ("lizard" or "reptile").

Dinosaurs are archosaurs, like modern crocodylians. These are set apart by having diapsid skulls, having two holes where jaw muscles attach, called temporal fenestrae. Birds and most retiles are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra are anapsids). Dinosaurs also have teeth that grow from sockets, rather than as direct extensions of the jaw bones, as well as various other characteristics. Within this group, the dinosaurs are set apart most noticeably by their gait. Instead of legs that sprawl out to the side, as found in lizards and crocodylians, they have legs held directly under their body.

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

Size
Only a tiny percentage of animals are ever fossilized, and most of those are still buried in the earth. As a result, the smallest and largest non-avian dinosaurs will probably never be discovered. Even among those that are recovered, very few are known from complete skeletons and even impressions of soft tissue like skin is very rare. So reconstructing a skeleton by comparing the size and morphology of the bones to the bones of similar, better-known species is inexact; and restoring the muscles and other organs is at best educated guesswork.

While the largest and smallest will probably remain unknown, and comparisons between existing specimens is imprecise, it is clear that as a group they were very large. But even by dinosaur standards the sauropods were gigantic. 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 ever walked the Earth.

The tallest and heaviest dinosaur known from a complete skeleton is still the Brachiosaurus (now Giraffatitan) which was discovered in Tanzania between 1907–1912, and is now mounted in the Humboldt Museum of Berlin. It is 12 m (38 ft) tall, and probably weighed between 30,000–60,000 kg (30–65 tons). The longest is the 27 m (89 ft) long Diplodocus which was discovered in Wyoming, and mounted in Pittsburgh's Carnegie Natural History Museum in 1907.

There are bigger dinosaurs, but they are known from only a small handful of bones. The current record holders all date from the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000–100,000 kg (90–110 tons); the longest, the 40 m (130 ft) long Supersaurus; and the tallest, the 18 m (60 ft) Sauroposeidon, which could have reached into a 6th-floor window.

No other group of terrestrial animals even comes close. The largest elephant on record weighed a mere 12,000 kg (13.5 tons), and the tallest giraffe was just 6 m (20 ft) tall. Even giant prehistoric mammals like the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods. Only a small handful of aquatic animals approach it in size, of which the blue whale is largest, reaching up to 190,000 kg (210 tons) and 33.5 m (110 ft) in length.

Discounting 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.

Dinosaur behavior
The behavior of non-avian dinosaurs will always be a mystery simply because none exist today. Paleontologists must rely on evidence gleaned from fossil tracks, skeletons locked in battle (Velociraptor and Protoceratops), and fossilized nests. Much evidence varies, depicting several different behaviors. Herbivores may have exhibited significant social behavior, migrating in huge herds much like modern day mammals (i.e. African species). One hypothesis holds that this behavior could have provided a warning system against certain predators. Carnivorous dinosaurs possibly exhibited social characteristics as well, as do present-day wolves and large cats. Family units may have traveled together over long periods to help each other survive. All interpretations of dinosaur behavior rely on speculation and promise to generate controversy for the forseeable future.

Fields of study
Dinosaurs are studied by palaeontologists. Fields of expertise include the discovery, reconstruction and conservation of dinosaur fossils and the interpretation of those fossils to understand better the evolution, classification and behaviour of dinosaurs.

Evolution of dinosaurs

Dinosaurs split off from their archosaur ancestors during the Triassic period.

Classification of dinosaurs
Dinosaurs are divided into two major orders, the Saurischia and the Ornithischia, on the basis of hip structure. See List of dinosaur classifications for more information.

Saurischians
Saurischians (from the Greek, meaning "lizard hip") are dinosaurs that retained the hip structure of their ancestors. They include all the Theropods (bipedal carnivores) and sauropods (long-necked herbivores). For more detail, see Saurischia.

Ornithischians
The other dinosaurian order is the Ornithischia (from the Greek, meaning 'bird-hip'; also known as Predentata), most of which were quadrupedal herbivores. For more details, see Ornithischia.

Were dinosaurs warm-blooded?
Scientists have waged a constant and vigorous debate over the temperature regulation of dinosaur blood— at first over its possibility, then over its method— a debate first popularized by Robert T. Bakker. From the first discovery of dinosaurs, paleontologists posited that they were ectothermic creatures: "terrible lizards" as their name suggested. This axiomatic expectation implied that dinosaurs were mostly slow, sluggish organisms, comparable to modern reptiles, which need the sun to heat their bodies. However, new evidence of dinosaurs in chilly temperate climates, of polar dinosaurs in Australia, where they experienced a six-month chilly and dark winter, of feathered dinosaurs whose feathers provided regulatory insulation, and analysis of blood-vessel structures that are typical of endotherms within dinosaur bone, confirmed the possibility that some dinosaurs regulated their body temperature by internal biological methods, some aided partly by their very bulk. Skeletal structures suggest active lifestyles for theropods and other creatures, behavior more suitable for an endothermic cardiovascular system. Sauropods exhibit fewer endothermic characters. Perhaps some dinosaurs were endothermic and others not. Scientific debate over the details continues, although many paleontologists would now agree that endothermic systems are more likely.

Complicating this debate, warm-bloodedness can emerge from more than one mechanism. Most discussions of dinosaur endothermia compare them to average birds or mammals, which expends energy to elevate body temperature above that of the environment. Small birds and mammals also possess insulation of some sort, such as fat, fur, or feathers, to slow down heat loss. However, large mammals, such as elephants, face a different problem because due to their relatively small surface area to volume ratio. 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 avoid overheating. In the case of elephants, they lack fur, and have large ears which increase their surface area, and have behavioural adaptations as well, such as using the trunk to spray water on themselves and mud wallowing. These behaviours increase cooling through evaporation.

Large dinosaurs would presumably have faced the same situation: their 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 any special adaptations like those of birds and mammals.

Feathered dinosaurs and the bird connection
Since the 1990s, a number of feathered dinosaurs have been found, providing clear evidence of the close relationship between dinosaurs and birds. Most of these specimens were local to Liaoning province in northeastern China, which was part of an island continent in the Cretaceous; however, the feathers were only preserved by the remarkable geology of the Chinese sites and their superbly detailed fossils; it is therefore possible that dinosaurs elsewhere in the world may have been feathered too, even though the feathers have not been preserved.

The feathered dinosaurs discovered so far include Sinosauropteryx, Protarchaeopteryx, Caudipteryx and Confuciusornis, all of which come from northern China's Yixian formation. The dromaeosauridae family in particular seems to have been heavily feathered, and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the association of multiple skeletal features also shared by the two groups is the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds, dinosaurs and the evolution of flight is more complex than has been previously realised. For example, while it was once believed that birds simply evolved from dinosaurs and went their separate way, some scientists now believe that some dinosaurs, such as the dromaeosaurs, may have actually evolved from birds, losing the power of flight while keeping the feathers.

see also: Feathered dinosaurs

Theories of extinction
The extinction of the non-avian dinosaurs is one of the most intriguing problems in paleontology. Only since the 1980s has the nature of this extinction become apparent. The theory first proposed by Walter Alvarez linked the extinction event at the end of the Cretaceous period to a meteorite impact about 65.5 million years ago, based on a sudden change in Iridium levels in fossilized layers. The bulk of the evidence now indicates that a 10-kilometer-wide bolide hit the Yucatan Peninsula 65 million years ago, creating the 170-km wide Chicxulub crater, and caused the extinction. Scientists are still disputing whether dinosaurs were in steady decline or still thriving before the meteor struck.

Although the speed of extinction cannot be deduced from the fossil record alone, the latest models suggest the extinction was extremely rapid. It appears to have been caused by heat caused by the meteorite impact and the matter ejected from the crater re-entering the atmosphere around the world. Other theories link the extinction with increased volcanic activity, decreasing oxygen level in the atmosphere and dropping temperatures.

Other groups as well as the dinosaurs went extinct at the same time, including ammonites, mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most kinds of birds, and many groups of mammals, became extinct.

Evidence for Cenozoic non-avian dinosaurs
It has been claimed that fossils from El Ojo, South America, represent remains of non-avian dinosaurs surviving the extinction and still thriving in the Paleocene epoch. There are also other sporadic claims of post-Cretaceous dinosaur fossils (even a very doubtful finding of dinosaur eggs as late as Eocene). While it is certainly not improbable that some scattered population of some (presumably small) dinosaur species could have survived at least some hundred year after the mass extinction, evidence now points to El Ojo (and most other) findings as Cretaceous fossils contaminating Paleocene strata. Nevertheless it is still theorized that some dinosaur population could have survived the main extinction event isolated in Antarctica, then being killed by the climatic change.

Bringing dinosaurs back to life
There has been much speculation about the availability of technology to bring dinosaurs back to life. The idea proposed in Michael Crichton's book Jurassic Park, using blood from fossilized mosquitos that have been suspended in sap since the dinosaur times and then filling in the gaps with frog genes to create the DNA of a dinosaur, is probably impossible.

There have been two claims about the successful extraction of ancient DNA from dinosaur fossils, but upon further inspection, neither of these reports could be confirmed (Wang et al., 1997). However, a working visual peptide of a (theoretical) dinosaur has been inferred, using analytical phylogenetic reconstruction methods on gene sequences of still-living related species (reptiles and birds) (Chang et al., 2002).

History of the study of dinosaurs
Dinosaur fossils have been known about for millennia, though their true nature was not recognised; the Chinese considered them to be dragon bones, while Europeans believed them to be the remains of giants and other creatures killed by the Great Flood. The first dinosaur species to be identified and named was Iguanodon, discovered in 1822 by the English geologist Gideon Mantell, who recognised similarities between his fossils and the bones of modern iguanas. Two years later, the Rev William Buckland, professor of geology at Oxford University, became the first person to describe a dinosaur in a scientific journal — in this case Megalosaurus bucklandii, found near Oxford. The study of these "great fossil lizards" became of great interest to European and American scientists, and in 1842 the English palaeontologist Richard Owen coined the term "dinosaur". He recognised that the remains that had been found so far — Iguanodon, Megalosaurus and Hylaeosaurus — had a number of features in common, so decided to present them as a distinct taxonomic group. With the backing of Prince Albert of Saxe-Coburg-Gotha, husband of Queen Victoria, Owen established the Natural History Museum in South Kensington, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.

In 1858, the first known American dinosaur was discovered in marl pits of the small town of Haddonfield, New Jersey (although fossils had been found before, their nature had not been identified). The creature was named Hadrosaurus foulkii, after the town and the discoverer, William Parker Foulke. It was an extremely important find: Hadrosaurus was the first nearly complete dinosaur skeleton ever found and it was clearly a bipedal creature. This was a revolutionary discovery, as it had been thought by most scientists that dinosaurs walked on four feet like lizards. Foulke's discoveries sparked a dinosaur mania in the United States which was exemplified by the fierce rivalry of Edward Drinker Cope and Othniel Charles Marsh, who each competed to outdo the other in finding new dinosaurs in what came to be known as the Bone Wars. Their feud lasted for nearly 30 years and only ended in 1897 when Cope died after spending his entire fortune in the dinosaur hunt. Marsh won the contest by virtue of being better funded through the US Geological Survey. Cope's collection is now at the American Museum of Natural History in New York, while Marsh's is displayed at the Peabody Museum of Natural History at Yale University.

Since then, the search for dinosaurs has been carried to every continent on Earth. This includes Antarctica, where the first dinosaur, a nodosaurid Ankylosaurus, was discovered on Ross Island in 1986, though it was 1994 before an Antartic dinosaur, the Cryolophosaurus ellioti, was formally named and described in a scientific journal. Current "hotspots" include southern South America (especially Chile) and China, which has produced many exceptionally well-preserved feathered dinosaurs.

Dinosaurs in popular culture
Dinosaurs were highly successful life forms for some 150 million years, however, rather than their success, it is their extinction that has become part of human culture. Hence dinosaur is sometimes used as a metaphor for people and things that are perceived as being out of date or no longer in touch with the spirit of the times, and therefore ought to be extinct. An example was the manner in which the punk movement described the "progressive" bands that preceded them as "dinosaur groups." Decentralized social movements have sometimes described centralized governments or corporations as dinosaurs as well.

Dinosaurs have long captured the public mind, and children are especially fascinated with them. This is evidenced by the many dinosaurs in fictional works. Notable examples include Arthur Conan Doyle's book The Lost World and Michael Crichton's book Jurassic Park.

In Jurassic Park dinosaurs are brought into contact with humans when they are genetically resurrected. The film based on this book was a big success in 1993. Even in the early days of cinematography a dinosaur could become a movie star, as was the case in 1914 with Gertie the Dinosaur by animation pioneer Winsor McCay. Further films used artistic license and showed humans as living contemporaries of dinosaurs. For example The Valley of Gwangi (1969) and One Million Years BC (1966) (famously starring Raquel Welch in a fur bikini). Ray Harryhausen brought the dinoasaurs to life in both films using model animation.

Dinosaurs are also a frequent topic of television documentaries and popular, nonfiction books. Dinosaurs are a common theme in the popular comic strip Calvin and Hobbes. See also List of fictional dinosaurs.

Dinosaurs are also the main power sources from Mighty Morphin Power Rangers.