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Dinosaur
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Dinosaur

See Dinosaurs (television) for the sitcom television series.
Dinosaurs
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Archosauria
Superorder: Dinosauria
Orderss
Saurischia
Ornithischia
Dinosaurs are an extinct 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.

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").

Table of contents
1 Overview
2 Saurischians
3 Ornithischians
4 Warm-blooded dinosaurs
5 Feathered dinosaurs and the bird connection
6 Speculation on dinosaur behavior
7 Extinction
8 Classification of dinosaurs
9 Dinosaurs in culture
10 Trivia
11 External links

Overview

Dinosaurs varied greatly in size. The smallest known species were about the size of a chicken, but most were much larger. The biggest dinosaurs were the Sauropoda; the species Argentinosaurus currently holds the record for the largest land animals ever to live, and were second in size among all creatures only to certain species of whale. However, the extraordinarily massive femur of a brachiosaur, titled Ultrasaurus, suggests an animal that could have weighed as much as 130 tons, dwarfing the competition.

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.

Dinosaurs are archosaurs, like modern crocodylians. These are set apart by having diapsid skulls with 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.

Saurischians

Saurischians (from the Greek Saurischia meaning "lizard hip") include all the Theropods, bipedal carnivores such as the tyrannosaurs, and sauropods in the dinosaur classification. (brief further characterization is needed here). For more detail, see Saurischia.

Ornithischians

The other great dinosaurian clade is the Ornithischia (or Predentata). For more details on these dinosaurs, please refer to Ornithischia.

Warm-blooded dinosaurs

There has been a constant and vigorous debate over the method of regulation of temperature of dinosaurian blood, a debate first popularized by Robert T. Bakker. Since the first discovery of dinosaurs, the idea that they were ectothermic creatures was considered most likely. This would have meant that the animals were mostly slow, dormant organisms, comparable to modern reptiles that need the sun to heat their bodies. However, new evidence of dinosaurs in chilly temperate climates and scientific analysis of the blood-vessel structures within dinosaur bone have opened the possibility that some dinosaurs were endothermic, regulating their body temperature by internal biological methods. Skeletal structures suggest active lifestyles for theropods and other creatures. This means that an endothermic cardiovascular system would have been more suitable for dinosaurs. This doesn't mean all dinosaurs were endothermic, though. The debate still continues to this day (although many paleontologists would agree that endothermic systems are more likely now).

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 China, which was 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 dimosaurs 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.

Speculation on dinosaur behavior

The behavior of non-avian dinosaurs will always be a mystery simply because none exist today. The only evidence paleontologists have to go on are fossil tracks, skeletons locked in battle (Velociraptor and Protoceratops), and fossilized nests. All the evidence varies, depicting several different behaviors. Herbivores may have been much more social, migrating in huge herds much like modern day mammals (i.e. African species). This could have been a successful predator warning system, depending on the predator. The carnivorous dinosaurs possibly exhibited social characteristics as well, like wolves and large cats. Families may have traveled together for a very long time in order to maximize survivability. But all of this is speculation, and a more accurate description is likely distant in the future. Considering the success of dinosaurs, as the dominant creatures for millions of years, some type of social order seems likely to have been present.

Extinction

The extinction of the 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.

The survivors of this mass extinction appear to have been two things: small animals hiding underground and/or aquatic creatures relatively safe underwater. Neither of this applies to dinosaurs, most of whom were probably incinerated in the heat blast. Other groups as well as the dinosaurs, including ammonites, mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most kinds of bird, and many groups of mammals, became extinct.

Evidence for Cenozoic dinosaurs

It has been claimed that fossils from El Ojo, South America, represent remains of 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, being then killed by the climatic change.

Classification of dinosaurs

Dinosaurs are divided into two major orders, the Saurischia and the Ornithischia, on the basis of hip structure.

SUPERORDER DINOSAURIA

Dinosaurs in culture

Even though dinosaurs were highly successful life forms for some 150 million years, dinosaur is sometimes used as a derogatory term to describe 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 often describe centralized governments or corporations as dinosaurs.

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 the Arthur Conan Doyle book The Lost World and the Michael Crichton book Jurassic Park. Dinosaurs are also a frequent topic of television documentaries and popular, nonfiction books. Dinosaurs are a common theme in the popular Bill Watterson comic strip Calvin and Hobbes. See also List of fictional dinosaurs.

Trivia

There has been much speculation about the availibility of technology to bring dinosaurs back to life. The idea proposed in Michael Crichton's book Jurassic Park, using blood in fossilized mosquitos that have been suspended in sap since the dinosaur times, and then filling in the gap with aspects of frog genes to create the DNA of a dinosaur is probably impossible. Scientists however, are still unsure about whether or not we could bring these extinct animals back to life.

External links