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 sometimes used, although wrongly, to describe any prehistoric reptile, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs, and mosasaurs, though none of these were technically dinosaurs.

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 portmanteau derived from the Greek words δεινός (deinos; "terrible", "fearfully great" or "formidable") and σαύρος (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

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

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. 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

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, and others kept their young in the middle of the herd for defense according to trackways at Davenport Ranch, Texas. Dinosaurs may have congregated in herds for defense, for migratory purposes, or to provide protection for their young.

Jack Horner's 1978 discovery of a Maiasaura ("good mother dinosaur") nesting ground in Montana demonstrated that parental care continued long after birth among the ornithopods. 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 northwestern Scotland. 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, 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 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, 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

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. 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.

The current "dinosaur renaissance"

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

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.

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

The dagger (†) is used to indicate taxa that are extinct.

Order Saurischia

• †(unranked) Herrerasauria
• Suborder Theropoda
  • †Superfamily Coelophysoidea
  • †Infraorder Ceratosauria
    • †Family Abelisauridae
  • (unranked) Tetanurae
    • †Superfamily Megalosauroidea
    • †Infraorder Carnosauria
    • Infraorder Coelurosauria
      • †Family Coeluridae
      • †Superfamily Tyrannosauroidea
      • †(unranked) Ornithomimosauria
      • (unranked) Maniraptora
        • †(unranked) Oviraptoriformes
          • †Superfamily Therizinosauria
          • †Superfamily Oviraptorosauria
        • †(unranked) Deinonychosauria
          • †Family Troodontidae
          • †Family Dromaeosauridae
        • Class Aves (birds)
• †Suborder Sauropodomorpha
  • †Thecodontosaurus
  • †Infraorder Prosauropoda
  • †Infraorder Sauropoda
    • †Superfamily Diplodocoidea
    • †(unranked) Macronaria
      • †Superfamily Titanosauriformes
        • †Family Brachiosauridae
        • †(unranked) Titanosauria

Order Ornithischia

• †Suborder Thyreophora
  • †Superfamily Stegosauria
  • †Superfamily Ankylosauria
• †(unranked) Cerapoda
  • †Family Heterodontosauridae
  • †Suborder Marginocephalia
    • †Superfamily Pachycephalosauria
    • †Superfamily Ceratopsia
      • †Family Psittacosauridae
      • †Family Protoceratopsidae
      • †Family Ceratopsidae
  • †Suborder Ornithopoda
    • †Family Hypsilophodontidae
    • †Superfamily Iguanodontia
      • †Family Iguanodontidae
      • †Family Hadrosauridae

Evolution

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.

Areas of debate

Warm-bloodedness

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.

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 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 during the Mesozoic period.

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 modern 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 stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in the Liaoning province in northeastern China, which was part of an island continent during the Cretaceous period. 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 by fossilization and thus are absent from the fossil record.

The feathered dinosaurs discovered so far include Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, Sinosauropteryx, and Jinfengopteryx. Dinosaur-like birds like Confuciusornis, which are 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.

Reproductive biology

A discovery of features in a Tyrannosaurus rex skeleton recently provided even 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 is rich in calcium, forms a layer inside the hard outer bone that is used to make eggshells. The presence of endosteally-derived bone tissues lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed the specimen to be female.

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

Lungs

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was 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 found in today's mammals and birds. A recently discovered troodont fossil demonstrates that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head 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. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths. Because a particular stone could have been swallowed at one location before being carried to another during migration, paleontologists sometimes use the stones found in dinosaur stomachs to establish possible 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. The formation in which the bone was discovered has been dated to the early Paleocene epoch approximately 64.5 million years ago. If the bone was not re-deposited into that stratum by weathering action, it would provide evidence that some dinosaur populations may have survived at least a half million years into the Cenozoic Era.

Bringing dinosaurs back to life

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.

Soft tissue in dinosaur fossils

One of the best examples of soft tissue impressions in a fossil dinosaur was discovered in Petraroia, Italy. The discovery was reported in 1998, and described the specimen of a small, very young Coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.

In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual 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 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 mass extinction of the non-avian dinosaurs, which occurred around 65 million years ago, is one of the most intriguing mysteries in paleontology. Many other groups of animals 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 extensively studied since the 1970s. At present, several related theories are broadly supported by paleontologists.

Asteroid collision

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

Multiple collisions—the Oort cloud

While similar to Alvarez's impact theory (which involved a single asteroid or comet), this theory proposes that a stream of comets was dislodged from the Oort cloud due to the gravitational disruption caused by a passing star. One or more of these objects then collided with the Earth at approximately the same time, causing the worldwide extinction. As with the impact of a single asteroid, the end result of this comet bombardment would have been a sudden drop in global temperatures, followed by a protracted cool period.

Environment changes

At the peak of the dinosaur era, there were no polar ice caps, and sea levels are estimated to have been from 100 to 250 metres (330 to 820 feet) higher than they are today. The planet's temperature was also 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 Cretaceous, 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 of for millennia, although 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. Iguanodon was 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, a professor of geology at Oxford University, unearthed the fossilized bones of Megalosaurus bucklandii near Oxford. Buckland then became the first person to describe his find in a scientific journal.

The study of these "great fossil lizards" soon 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, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of Prince Albert of Saxe-Coburg-Gotha, the 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 found and it was clearly a bipedal creature. This was a revolutionary discovery, as until that point most scientists had believed dinosaurs walked on four feet like other lizards. Foulke's discoveries sparked a wave of dinosaur mania in the United States.

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. The feud probably originated when Marsh publicly pointed out that Cope's reconstruction of an Elasmosaurus skeleton was flawed; Cope had inadvertently placed the plesiosaur's head at what should have been the animal's tail end. The fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh won the contest primarily because he was better funded through a relationship with the US Geological Survey. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair's rough methods; for example, their diggers often used dynamite to unearth bones (a method modern paleontologists would find appalling). Despite the pair's unrefined methods, their 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 on display at the Peabody Museum of Natural History at Yale University.

Since 1897, the search for dinosaur fossils has extended to every continent, including Antarctica. The first Antarctic dinosaur to be discovered, the ankylosaurid Ankylosaurus, was found on Ross Island in 1986, although it was 1994 before an Antarctic species, the Cryolophosaurus ellioti, was formally named and described in a scientific journal.

Current dinosaur "hot spots" 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 fossilization.

In popular culture

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 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 models and stop motion animation. 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 revolution in the depiction of dinosaurs on film. Perhaps the most prominent example of CGI dinosaurs remains the film Jurassic Park, directed by Stephen Spielberg and featuring special effects by ILM. The success of Jurassic Park and its two sequels, The Lost World: Jurassic Park and Jurassic Park III, demonstrates the continued popularity of dinosaurs. The falling cost of computer-generated effects has also recently allowed the production of documentaries for television; the award-winning 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 1980s' Dino-Riders, the 1990s' Dinosaurs and 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 did not appear until tens of 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 entitled 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".

Many computer and console games have also featured dinosaurs as characters. The Jurassic Park films inspired multiple computer games. Crash Bandicoot: Warped, Ape Escape, the Turok series, and even Zoo Tycoon have involved dinosaurs in their storylines.

Because dinosaurs were a highly successful, imposing group of creatures that abruptly and completely became extinct, they are often evoked in metaphor. People, styles and ideas that are perceived to be out of date and on the wane are often referred to as "dinosaurs". For example, members of the punk movement derided the "progressive" bands that preceded them as "dinosaur groups".

Religious views

Various religious groups have views about dinosaurs that differ from those that are generally accepted as fact by scientists. While many mainstream scientists respect these views as faith positions, they argue that religiously-inspired interpretations of dinosaurs do not withstand serious scientific scrutiny.