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Dinosaur Temporal range: Triassic – Cretaceous
Replica of Tyrannosaurus rex at the Senckenberg Museum.
Conservation status
Template:StatusFossil
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Dinosauria Owen, 1842
Orders & Suborders
Saurischia    Sauropodomorpha    Theropoda Ornithischia

2. Part 2

Dinosaurs are vertebrate animals that range from reptile-like to bird-like. Dinosaurs dominated the terrestrial ecosystem for over 160 million years, first appearing around 230 million years ago. 65 million years ago, at the end of the Cretaceous period, all non-avian dinosaurs became extinct. Dinosaurs still exist today in the line of birds (avian dinosaurs). Knowledge about dinosaurs is derived from both fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues. 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 up during the Triassic period around 230 million years ago.

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 dinosaurs.

The ongoing renaissance in the scientific understanding of dinosaurs began in the 1970s and 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.

What is a dinosaur?

Definition

The superorder or clade "Dinosauria" was formally named 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"). The name was chosen to express Owen's 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 are extremely varied. Some were herbivorous, others carnivorous. Some dinosaurs were bipedal, others quadrupedal, while others could walk easily on two or four legs, such as the dinosaur Ammosaurus.

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, birds are morphologically distinct from their reptilian ancestors, and 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 taxa that exclude some descendants of a single common ancestor (paraphyletic taxa). Paleontologists mostly use cladistics, which classifies birds as dinosaurs, to construct their taxonomies, but many other scientists do not. As a result, this article will use "dinosaur" as a synonym for "non-avian dinosaur", and "bird" as a synonym for "avian dinosaur".

Size

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

Largest and smallest dinosaurs

While the largest and smallest dinosaurs will probably remain unknown, and comparisons involving existing specimens are imprecise, it is clear that, as a group, dinosaurs were large. 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 the Brachiosaurus, which was discovered in Tanzania between 1907–12. It is now mounted in 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 dinosaur 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 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 not only debatable but it varies in time (Triassic versus early Jurassic versus late Jurassic versus Cretaceous). "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 average 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 a grizzly bear) and a median generic mass of nearly 2 tons (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 living mammals; the majority of dinosaurs were bigger than all but 2% of living mammals."

Behavior

Interpretations of behavior based on the pose of body fossils and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches rely on speculation, and promise to generate controversy for the foreseeable future. However, it is likely that some behaviors common in both of their closest living relatives, crocodiles and birds, were also common among dinosaurs. It should be noted that nearly all interpretations of evidence are subject to change, as theories surrounding dinosaurs evolve continuously.

The first evidence of herding behavior was the 1878 discovery of 31 Iguanodon dinosaurs that 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 groups. 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, 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, migration, or to care for their young.

Jack Horner's 1978 discovery of a Maiasaura ("good mother dinosaur") nesting ground in Montana demonstrated parental care long after birth among the ornithopods. 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. Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in the United Kingdom. Nests and eggs are known from most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, like modern birds and crocodiles.

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile for active defense, so they were probably used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Communication is also an enigma, but the hollow crests of the lambeosaurines may have been 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, proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise, this too was confirmed by tooth marks from Madagascar in 2003.

There seem to have been no burrowing and few climbing dinosaurs. This is 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, whether diplodocids could create sonic booms via whip-like tail snapping, whether giant theropods had to slow down when rushing for food to avoid fatal injuries, and if sauropods could float.

Study of dinosaurs

Information on dinosaurs is obtained from a variety of fields of study including Physics, Chemistry, Biology, and the Earth Sciences (which includes Paleontology). Activities include the discovery, reconstruction and conservation of dinosaur fossils and the interpretation of those fossils to better understand the evolution, classification and behavior of dinosaurs.

Classification

Main article: Dinosaur classification

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes where 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. Dinosaurs have teeth that grow from sockets (an archosaur characteristic) rather than as direct extensions of the jaw bones, as well as various other archosaur characteristics. Within the archosaur group, the dinosaurs are differentiated 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. 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.

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

The following is a simplified classification of dinosaurs. A more detailed version can be found at List of dinosaur classifications.

The dagger (†) is used to indicate taxa with no living members.

Order Saurischia

• †Family Herrerasauridae
• Suborder Theropoda
  • †Superfamily Coelophysoidea
  • †Superfamily Ceratosauria
    • †Family Abelisauridae
  • (unranked) Tetanurae
    • †Family Spinosauridae
    • †(unranked) Carnosauria
    • (unranked) Coelurosauria
      • †Family Coeluridae
      • †Superfamily Tyrannosauroidea
      • †Superfamily Ornithomimosauria
        • †Family Alvarezsauridae
      • (unranked) Maniraptora
        • †(unranked) Oviraptorosauria
          • †Superfamily Therizinosauria
        • †Superfamily Deinonychosauria
          • †Family Troodontidae
          • †Family Dromaeosauridae
          • †Family Microraptoria
        • Class Aves (birds)
• †Suborder Sauropodomorpha
  • †Thecodontosaurus
  • †Family Plateosauridae
  • †Riojasaurus
  • †(unranked) Sauropoda
    • †Family Diplodocidae
    • †(unranked) Macronaria
      • †(unranked) Titanosauriformes
        • †Family Brachiosauridae
        • †Family Titanosauridae

†Order Ornithischia

• †Suborder Thyreophora
  • †Superfamily Stegosauria
  • †Superfamily Ankylosauria
• †Suborder Cerapoda
  • †Family Heterodontosauridae
  • †Superfamily Pachycephalosauria
  • †Superfamily Ceratopsia
    • †Family Psittacosauridae
    • †Family Protoceratopsidae
    • †Family Ceratopsidae
  • †Superfamily Ornithopoda
    • †Family Hypsilophodontidae
    • †Family Iguanodontidae
    • †Family Hadrosauridae

Evolution

Dinosaurs split off from their archosaur ancestors around 230 million years ago during the Triassic period, around 20 million years after the Permian-Triassic extinction event wiped out about 70 percent of all biological diversity on the planet. Radiometric dating of fossils from the early dinosaur species Eoraptor places the species in the fossil record at this time. Paleontologists believe the Eoraptor resembles the common ancestor of all dinosaurs. A few lines of primitive dinosaurs diversified rapidly after the Triassic, quickly filling most of the vacant ecological niches. Some of the earliest dinosaurs were the Eoraptor and the primative Lagosuchus. The tiny Saltopus, barely larger than a human hand, was a slightly later early Triassic dinosaur. During the period of dinosaur predominance, which encompassed the ensuing Jurassic and Cretaceous periods, nearly every (known) terrestrial animal larger than 1 m in length was a dinosaur.

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

Areas of debate

Warm-blooded?

Scientists have waged an ongoing and vigorous debate over temperature regulation in dinosaurs, at first over its possibility, and then over its method. This thermoregulation debate was first popularized by Robert T. "Bob" Bakker. After dinosaurs were discovered, paleontologists first posited that they were ectothermic creatures: "terrible lizards" as their name suggested. This supposed cold-bloodedness implied that dinosaurs were mostly slow, sluggish organisms, comparable to modern reptiles, which need the sun to heat their bodies.

However, more modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaurs regulated their body temperature by internal biological means (perhaps even aided by the animals' bulk). Evidence of endothermism in dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica (where they experienced a cold, dark six-month winter), the discovery of dinosaurs whose feathers 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. Perhaps some dinosaurs were endothermic and others were 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 based on more than one mechanism. Most discussions of dinosaur endothermia 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 of some sort, such as fat, fur, or feathers, to slow 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 lack fur, 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 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 or mammals. However, so far this theory fails to account for the vast number of dog- and goat-sized dinosaur species, which made up the bulk of the ecosystem in the Mesozoic.

Feathered dinosaurs and the bird connection

Birds and non-avian dinosaurs share many features. Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are generally accepted to have been their closest ancient relatives.

Feathers

Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the Solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of reptiles and birds. Brought to light just two years after Darwin's seminal The Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, specimens are commonly mistaken for Compsognathus.

Since the 1990s, a number of additional feathered dinosaurs have been found, providing even clearer evidence of the close relationship between dinosaurs and birds. Most of these specimens were unearthed in Liaoning province in northeastern China, which was part of an island continent in the Cretaceous. Though feathers have been found only in the lagerstätte of the Yixian Formation and a few other places, it is possible that dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered dinosaurs may be due to the fact that delicate features like skin and feathers are not often preserved in the fossil record.

The feathered dinosaurs discovered so far include Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, Sinosauropteryx, and potentially Adasaurus. Dinosaur-like birds like Confuciusornis, anatomically closer to modern avians, have also been discovered. All of these specimens come from the same formation in northern China. The dromaeosauridae family in particular seems to have been heavily feathered, and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul, conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich and other ratites.

Comparison of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called maniraptors. Skeletal similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle and breast bone.

Reproduction biology

A discovery of features in a Tyrannosaurus rex skeleton provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone, which forms a layer inside the hard outer bone, is rich in calcium and is used for making eggshells. The presence of endosteally-derived bone tissues lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen's hind limb suggested similar reproductive strategies, and revealed the specimen to be female.

A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur. It is possible that adult dinosaurs regurgitated nutrition into a young dinosaur's mouth, a behavior that is also characteristic of numerous modern-day bird species.

Lungs

Large meat-eating dinosaurs had a complex system of air sacs similar to those in today's birds, according to an investigation led by Patrick O'Connor of Ohio University. The lungs of theropod dinosaurs, carnivores that walked on two legs and had birdlike feet, likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study was funded in part by the National Science Foundation.

Heart and sleeping posture

Modern computerized tomography (CT) scans of dinosaur chest cavities, conducted in 2000, found the apparent remnants of complex four-chambered hearts, much like those of today's mammals and birds. A recently discovered troodont fossil demonstrates that the dinosaurs slept like certain birds today, with their heads tucked under their arms. This behavior, which would allow the head to be kept warm, is also characteristic of modern birds.

Gizzard

Another piece of evidence that birds and dinosaurs are closely related is the use of gizzard stones by both groups. These stones are swallowed by the animal to aid digestion and break down hard fibres and food once it enters the stomach. When found in association with fossils, gizzard stones are called gastroliths. Because a particular stone could have been swallowed at a certain point before being carried to another point during migration, paleontologists sometimes use the stones found in dinosaur stomachs to determine migration routes.

Evidence for Cenozoic dinosaurs

In 2002, paleontologists Zielinski and Budahn reported the discovery of a single hadrosaur leg bone fossil from El Ojo, South America, a formation dated to the early Paleocene epoch 64.5 million years ago. If this bone was not re-deposited by weathering action, it would mean that some dinosaur populations survived at least a half million years into the Cenozoic Era.

Bringing dinosaurs back to life

There has been much speculation about the use of technology to bring dinosaurs back to life. In Michael Crichton's book Jurassic Park, which popularized the idea, scientists use blood from fossilized mosquitos that have been suspended in tree sap since the Mesozoic to reconstruct the DNA of dinosaurs, filling chromosomal gaps with modern frog genes. It is probably impossible to resurrect dinosaurs in this manner. One problem with the amber extraction method is that DNA decays over time by exposure to air, water and radiation, thus making it unlikely that such an approach would recover any useful DNA (DNA decay can be measured by a racemization test).

The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed. However, a functional 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).

Even if dinosaur DNA could be reconstructed, it would be exceedingly difficult to "grow" dinosaurs using current technology since no closely related species exist to provide zygotes or a suitable environment for embryonic development.

Discovery of probable soft tissue from dinosaur fossils

In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of material resembling soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team.

When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this flexible material are not yet clear, although many news reports immediately linked it with the movie "Jurassic Park". Interpretation of the artifact is ongoing, and the relative importance of Dr. Schweitzer's discovery is not yet clear.

Extinction theories

The sudden extinction of the non-avian dinosaurs, which occurred around 65 million years ago, is one of the most intriguing problems in paleontology. Many other animal groups also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most birds, and many groups of mammals. The nature of the event that caused this mass extinction has been researched extensively only since the 1970s. At present, several related theories have broad support among paleontologists.

Asteroid collision

This theory, first proposed by Walter Alvarez in the late 1970s, links the extinction event at the end of the Cretaceous period to a bolide impact about 65.5 million years ago. Alvarez proposed that a sudden increase in iridium levels, recorded in rock strata around the world, was a direct result of the impact. The bulk of the evidence now indicates that a 10 km wide bolide hit the Yucatán Peninsula 65 million years ago, creating the 170 km-wide Chicxulub Crater and causing the extinction. Scientists are still disputing whether dinosaurs were thriving or declining before the meteor struck. Some scientists propose that the meteor would have caused a long and unnatural drop in Earth's atmospheric temperatures, while others claim that it would have created an unusual heat wave.

Although the speed of extinction cannot be deduced from the fossil record alone, the latest models suggest that the extinction was extremely rapid. The current consensus view among scientists who support this theory is that the impact caused extinction directly (by heat from the meteorite impact) and also indirectly (via worldwide cooling brought about by matter ejected from the crater).

The Oort cloud

Similar to Alvarez's theory, which involved a single asteroid or comet, this theory suggests that a shower of comets was dislodged from the Oort cloud due to gravitational disruption caused by a passing star. According to this theory, some of these comets hit the Earth at approximately the same time, causing a worldwide extinction. The end result would again have been a sudden drop in global temperatures, followed by a protracted cool period.

Environment changes

At the dinosaur era's peak, sea levels are estimated to have been between 100 metres and 250 metres (330 feet to 820 feet) higher than today, with no polar ice caps. The planet's temperature was much more uniform, with only 25 degrees Celsius separating average polar temperatures from those at the equator. On average, atmospheric temperatures were also much warmer; the poles, for example, were 50 °C warmer than today.

The atmosphere's composition during the dinosaur era was vastly different as well. Carbon dioxide levels were up to 12 times higher than today's levels, and oxygen formed 32 to 35 percent of the atmosphere, as compared with 21 percent today. However, by the late Cretacious, the environment was changing dramatically. Volcanic activity was decreasing, which led to a cooling trend as levels of atmospheric carbon dioxide dropped. Oxygen levels in the atmosphere also started to fluctuate and would ultimately fall considerably. Some scientists hypothesize that climate change, combined with lower oxygen levels, might have led directly to the demise of many species. If the dinosaurs had respiratory systems similar to those commonly found in modern birds, it may have been particularly difficult for them to cope with reduced respiratory efficiency, given the enormous oxygen demands of their very large bodies.

History of discovery

Dinosaur fossils have been known for millennia, though their true nature was not recognized; 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 recognized 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 paleontologist Richard Owen coined the term "dinosaur". He recognized 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 in the small town of Haddonfield, New Jersey (although fossils had been found before, their nature had not been correctly discerned). 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 most scientists had thought that dinosaurs walked on four feet like lizards. Foulke's discoveries sparked a dinosaur mania in the United States.

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, who raced to find new dinosaurs in what came to be known as the Bone Wars. The feud probably originated when Marsh publicly criticized Cope for putting the bones of an Elastomosaurus on back to front. The fight between the two scientists lasted over 30 years, ending only in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh won the contest by virtue of being better funded through the US Geological Survey. Unfortunately, many valuable dinosaur specimens were destroyed or damaged due to the pair's rough approach; for example, their diggers often used dynamite to unearth bones. Despite the pair's unrefined methods, Marsh and Cope's contributions to paleontology were vast; Marsh unearthed 86 new species of dinosaur and Cope discovered 56, for a total of 142 new species. 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 extended to every continent on Earth, including Antarctica. The first Antarctic dinosaur to be discovered, the nodosaurid Ankylosaurus, was found on Ross Island in 1986, though it was 1994 before an Antarctic dinosaur, the Cryolophosaurus ellioti, was formally named and described in a scientific journal.

Current dinosaur "hotspots" include southern South America (especially Argentina) and China. China in particular has produced many exceptional feathered dinosaur specimens due to the unique geology of its dinosaur beds, as well as an ancient arid climate particularly conducive to skeleton preservation.

In popular culture

Dinosaurs were highly successful creatures; they were the dominant land animals on Earth for over 150 million years. However, it is their sudden extinction as much as their success that has made them an enduring part of human popular culture. Hence dinosaur is sometimes used colloquially as a metaphor for people and things that are perceived to be out of date or no longer in touch with the spirit of the times, and therefore ought to be extinct. For example, members of the punk movement derided the "progressive" bands that preceded them as "dinosaur groups".

Perhaps because they are popularly perceived to have been large, aggressive animals, dinosaurs have long both fascinated and terrified the public. This is reflected in many fictional and non-fictional works.

Notable examples of older fictional works featuring dinosaurs include Arthur Conan Doyle's book The Lost World, the 1933 film King Kong and Godzilla. The (imaginary) depiction of humans and dinosaurs living together has been a recurring theme in fiction. Films which famously portrayed this idea include The Valley of Gwangi (1969) and One Million Years BC (1966). Ray Harryhausen brought the dinosaurs to life in both films using model animation. Other classic films in which dinosaurs are featured prominently include Pterodactyl and Spot from The Munsters. Older films and literature generally depict dinosaurs as sluggish, unintelligent, lizard-like creatures.

More recently, the portrayal of dinosaurs in works intended for popular consumption has tended to better reflect a more nuanced modern scientific understanding of the animals. In particular, the development and refinement of computer-generated imagery has led to a watershed in the depiction of dinosaurs on film. Perhaps the most prominent example of CGI dinosaurs remains the film Jurassic Park, directed by Stephen Spielberg. For the movie, the special effects company ILM used computers to create a believable dinosaur ecosystem on-screen. The success of Jurassic Park and its two sequels, The Lost World: Jurassic Park and Jurassic Park III, demonstrates how the popularity of dinosaurs continues unabated today. The falling cost of computer-generated effects has also recently allowed the production of documentaries for television; the 1999 BBC series Walking with Dinosaurs is a notable example.

Dinosaurs are often anthropomorphized in fiction. In works intended for young children, they are imbued with friendly, even loving personalities. Examples of this trend include the 1970s show Land of the Lost, the 1990s' Dinosaurs and the more recent Barney & Friends.

Cartoons, comic books and comic strips also regularly depict dinosaurs. The cartoon The Flintstones showcased a stone age family living with dinosaurs (though in reality humans appeared millions of years after the extinction of non-avian dinosaurs). Comic strips such as Calvin and Hobbes and The Far Side often featured dinosaur-oriented content. The comics title Dinosaurs for Hire portrayed anthropomorphic dinosaurs in a very unusual way; the comic's trio of gun-toting, trigger-happy dinos were notable for their "Who's Extinct?" t-shirts and their love of "Kojak".

Due to their consumer appeal, many computer and console games have featured dinosaurs as characters. The Jurassic Park films inspired multiple computer games (see Jurassic Park video games). Crash Bandicoot: Warped, Ape Escape, the Turok series, and even Zoo Tycoon have involved dinosaurs in their storylines.

See also

• Fossils
• List of dinosaurs
• List of dinosaur classifications
• Prehistoric life
• Prehistoric reptiles

Notes

1. From the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians and turtles, grouped together as the class Reptilia. However, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined here would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Thus, some cladists redefine Reptilia as a monophyletic group, including both the classic reptiles as well as the birds and perhaps the mammals (depending on ideas about their relationships). Others abandon it as a formal taxon altogether, dividing it into several different classes.
2. Dal Sasso, C. and Singnore, M. (1998). Exceptional soft-tissue preservation in a theropod dinosaur from Italy. Nature 292:383-387. See commentary on the article
3. Schweitzer, M.H., Wittmeyer, J.L. and Horner, J.R. (2005). Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science 307:1952 - 1955. See commentary on the article
4. Evans, J. (1998). Ultimate Visual Dictionary - 1998 Edition. Dorling Kindersley Books. 66-69. ISBN 1871854008.
5. Day, J.J. and Upchurch, P. (2002). Sauropod Trackways, Evolution, and Behavior. Science 296:1659. See commentary on the article
6. Lessem, D. and Glut, D.F. (1993). The Dinosaur Society's Dinosaur Encyclopedia. Random House Inc. ISBN 0679417702. See commentary on the article
7. Juvenile Tyrannosaur A juvenile Tyrannosaur skeleton was found.
8. Oviraptor nesting Oviraptor nests or Protoceratops?
9. Dinosaur family tracks Footprints show maternal instinct after leaving the nest.
10. Joined forever in death The discovery of two fossil dinosaurs entangled together proved many theories.
11. Cannibalistic Dinosaur The mystery of a dinosaur cannibal.
12. Rogers, R.R., Krause, D.W. and Rogers, K.C. (2003). Cannibalism in the Madagascan dinosaur Majungatholus atopus. Nature 422:515-518.See commentary on the article.
13. Gait and Dinosaur speed Gait and his formula on estimating a dinosaur's speed.
14. Calculate your own Dinosaur speed More on Gait and his speed calculations.
15. Douglas, K. and Young, S. (1998). The dinosaur detectives. New Scientist 2130:24. See commentary on the article.
16. Hecht, J. (1998). The deadly dinos that took a dive. New Scientist 2130. See commentary on the article.
17. Henderson, D.M. (2003). Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: A computational analysis. Canadian Journal of Zoology 81:1346-1357. See commentary on the article.
18. Hayward, T. (1997). The First Dinosaurs. Dinosaur Cards. Orbis Publishing Ltd. D36040612.
19. Parsons, K.M. (2001). Drawing Out Leviathan. Indiana University Press. 22-48. ISBN 0253339375.
20. Mayr, G., Pohl, B. and Peters, D.S. (2005). A Well-Preserved Archaeopteryx Specimen with Theropod Features. Science 310:1483-1486.See commentary on the article.
21. O'Connor, P.M. and Claessens, L.P.A.M. (2005). Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs. Nature 436:253.
22. Xu, X. and Norell, M.A. (2004). A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431:838-841.See commentary on the article.
23. Fassett, J, R.A. Zielinski, & J.R. Budahn. (2002). Dinosaurs that did not die; evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone, San Juan Basin, New Mexico. In: Catastrophic events and mass extinctions; impacts and beyond. (Eds. Koeberl, C. & K. MacLeod): Special Paper - Geological Society of America 356: 307-336.
24. Wang, H., Yan, Z. and Jin, D. (1997). Reanalysis of published DNA sequence amplified from Cretaceous dinosaur egg fossil. Molecular Biology and Evolution. 14:589-591. See commentary on the article.
25. Chang, B.S.W., Jönsson, K., Kazmi, M.A., Donoghue, M.J. and Sakmar, T.P. (2002). Recreating a Functional Ancestral Archosaur Visual Pigment. Molecular Biology and Evolution 19:1483-1489. See commentary on the article.
26. Schweitzer, M.H., Wittmeyer, J.L. and Horner, J.R. (2005). Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science 307:1952-1955. Also covers the Reproduction Biology paragraph in the Feathered dinosaurs and the bird connection section. See commentary on the article
27. Koeberl, C. and MacLeod, K.G. (2002). Catastrophic Events and Mass Extinctions. Geological Society of America. ISBN 0813723566.
28. (Nov 2000). Earthwatch :6-13.
29. Williams, P. (1997). The Battle of the Bones. Dinosaur Cards. Orbis Publishing Ltd. D36040607.

References

• Kevin Padian, and Philip J. Currie. (1997). Encyclopedia of Dinosaurs. Academic Press. ISBN 0122268105. (Articles are written by experts in the field).
• Paul, Gregory S. (2000). The Scientific American Book of Dinosaurs. St. Martin's Press. ISBN 0312262264.
• Paul, Gregory S. (2002). Dinosaurs of the Air: The Evolution and Loss of flight in Dinosaurs and Birds. Baltimore: The Johns Hopkins University Press. ISBN 0801867630.
• Weishampel, David B. (2004). The Dinosauria. University of California Press; 2nd edition. ISBN 0520242092.

External links

For children

• Dinosaur Time Machine from MantyWeb Educational Software From MantyWeb Educational Software. Kid's site, facts, games.
• Dinopedia From Yahooligans! Science. Glossaries, dino cards and indexes.
• Zoom Dinosaurs From Enchanted Learning. Kid's site, info pages, theories, history.

Popular

• Dinosaurs & other extinct creatures From the Natural History Museum. London popular site, well illustrated dino directory.
• History of Dinosaur discovery Timeline of the discovery of Dinosaurs.
• Dinosaurs: Facts and Fiction From the United States Geological Survey. Popular overview.
• Dinosaurs From the BBC. Popular site, very well illustrated.
• Discussions From DinoData. Summaries of modern debates about dinosaurs.
• Dinosauria From UC Berkeley Museum of Paleontology Detailed information - scroll down for menu.
• The Dinosaur News The Dino-headlines from around the world. Recent news on dinosaurs, including finds and discoveries, lots of links.
• OPUS: Dinosaur by Daniel Bensen A gallery of dino-paintings.

Technical

• Prehistoric Planet From PaleoClones. Current dino news.
• A Fiery Death for Dinosaurs? by Amit Asaravala From Wired. Article on the rapid extinction of dinosaurs.
• The Rex Files From the New Scientist. Articles, latest news but out of date.
• Palaeontologia Electronica From Coquina Press. Online technical journal.
• Impossible Dinosaurs Article on a gravity-based approach for the extinction by David Talbott and Wallace Thornhill.
• TeV scale gravity, mirror universe, and ... dinosaurs Article from Acta Physica Polonica B by Z.K. Silagadze.

Very technical

• DinoData Technical site, essays, classification, anatomy.
• Dinosauria On-Line Technical site, essays, pronunciation, dictionary.
• The Dinosauricon By T. Michael Keesey. Technical site, cladogram, illustrations and animations.
• Dinosauromorpha Cladogram From Palaeos. A detailed and wonderful amateur site about all things paleo.
• Dinobase AA dinosaur database with dinosaur lists, classification, pictures, and more.

Bird-dinosaur discussion

• DinoBuzz Are birds Dinosaurs?
• Dinosauria Site focussing on the Dino-Bird aspect.

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• Dinosaurs
• Paleontology
• Paleozoology
• Prehistoric reptiles
• Reptiles