Dinosaur

For other uses, see Dinosaur (disambiguation).

? Dinosaur Conservation status: Fossil Fossil range: Triassic – Cretaceous
Scientific classification
Domain: {{{domain}}} Superkingdom: {{{superregnum}}} Kingdom: Animalia Subkingdom: {{{subregnum}}} Superdivision: {{{superdivisio}}} Superphylum: {{{superphylum}}} Division: {{{divisio}}} Phylum: Chordata Subdivision: {{{subdivisio}}} Subphylum: {{{subphylum}}} Infraphylum: {{{infraphylum}}} Microphylum: {{{microphylum}}} Nanophylum: {{{nanophylum}}} Superclass: {{{superclassis}}} Class: Reptilia Sublass: {{{subclassis}}} Infraclass: {{{infraclassis}}} Superorder: Dinosauria Owen, 1842 Order: {{{ordo}}} Suborder: {{{subordo}}} Infraorder: {{{infraordo}}} Superfamily: {{{superfamilia}}} Family: {{{familia}}} Subfamily: {{{subfamilia}}} Supertribe: {{{supertribus}}} Tribe: {{{tribus}}} Subtribe: {{{subtribus}}} Genus: {{{genus}}} Subgenus: {{{subgenus}}} Section: {{{sectio}}} Series: {{{series}}} Species: {{{species}}} Supspecies: {{{subspecies}}}
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Orders
Saurischia Sauropodomorpha Theropoda Ornithischia
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Synonyms
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Dinosaurs are vertebrates that have ranged from reptile-like to bird-like. Dinosaurs dominated the terrestrial ecosystem for over 160 million years from around 230 million years ago until 65 million years ago, at the end of the Cretaceous period, when all non-avian dinosaurs became extinct. Dinosaurs still exist today in the line of birds (avian dinosaurs). Knowledge about dinosaurs comes from both fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.^[1][2] Dinosaur remains have been found on every continent on Earth, including Antarctica. This shows that all land masses were at one time connected in a super continent, called Pangaea, due to the fact that numerous fossils of the same species have been found on completely different continents. Pangaea began to break up during the Triassic period around 230 Mya.^[3]

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 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 on-going dinosaur renaissance 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 (homoeothermic), in marked contrast to the prevailing image of dinosaurs as sluggish and cold-blooded. Vertebrate paleontology has also become a global science, with major new discoveries in previously unexploited regions, including South America, Madagascar, Antarctica, and most significantly the amazingly well-preserved feathered dinosaurs in China, which 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, which are still known from an incomplete fossil record.

What is a dinosaur?

Definition

Triceratops skeleton at the Smithsonian National Museum of Natural History.

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 formidable arsenal. Dinosaurs are extremely varied. Some were herbivorous, others carnivorous. Some dinosaurs were bipedal, others quadrupedal, while others could walk easily on both two and 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 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 under the older Linnaean classification system, which accepts taxa that exclude some descendants of a single common ancestor (paraphyletic taxa). Paleontologists mostly use cladistics in their classifications, which classifies birds as dinosaurs, 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. Reconstructing a skeleton by comparing the size and morphology of bones to those of similar, better-known species is inexact, and restoring the muscles and other organs is, at best, educated guesswork.

A statue of Diplodocus carnegiei, outside the Carnegie Museum of Natural History.

Size of a human compared to a Tyrannosaurus rex.

While the largest and smallest dinosaurs will probably remain unknown, and a comparison between existing specimens is 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 (30–70 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 they are only known from a scant number of fossil samples. The largest specimens on record all date from the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000–100,000 kg (90–110 tons); the longest, the 40 m (130 ft) long Supersaurus; and the tallest, the 18 m (60 ft) Sauroposeidon, which could have reached a sixth-floor window.

Dinosaurs were the largest of all terrestrial animals. The largest elephant on record weighed 12,000 kg (13.5 tons), and the tallest giraffe was 6 m (20 ft) tall. Even the giant prehistoric mammals such as the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods. Only a small handful of aquatic animals approach it in size, of which the blue whale is largest, reaching up to 190,000 kg (210 tons) and 33.5 m (110 ft) in length.

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. In fact, most dinosaurs were much smaller than we would expect, with the average size of a dinosaur being around the size of a large sheep.

Behavior

Interpretations of behavior based on the pose of a body fossil and its habitat, computer simulations of their biomechanics, and comparison with modern animals in similar ecological niches rely on speculation, and promise to generate controversy for the foreseeable future. However, it is likely that at least the behaviors common in both of their closest living relatives, crocodiles and birds, are also common among dinosaurs. It should be of note 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 that perished together in Bernissart, Belgium, after they fell down a deep ravine, drowning as the latter was filled with rainwater. Similar mass deaths and trackways suggest that herd or pack behavior was common among 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 they traveled in groups composed of several different species, at least in Oxford, England,^[4] 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.

A nesting ground of Maiasaura was discovered in 1978.

Jack Horner's 1978 discovery of a Maiasaura ("good mother dinosaur") nesting ground in Montana demonstrated parental care long after birth among the ornithopods,^[5][6] and similar nesting behavior and even huge nesting colonies like those of penguins have been discovered of other Cretaceous dinosaurs like the Patagonian sauropod Saltasaurus (in 1997). The Mongolian maniraptoran Oviraptor was even 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.^[7] Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in the United Kingdom.^[8] 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.

One of the most valuable fossils, a Velociraptor attacking a Protoceratops, was discovered in the Gobi Desert in 1971,^[9] proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise,^[10] it was confirmed by tooth marks from Madagascar in 2003.^[11]

Compared to the later mammalian radiation in the Cenozoic, there seem to be no burrowing and few climbing dinosaurs. Biomechanics has given insight into how fast dinosaurs could run,^[12][13] whether diplodocids could create sonic booms by snapping their tails like a whip,^[14] whether giant theropods had to slow down when rushing for food to avoid fatal injuries,^[15] and if sauropods could float.^[16]

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. These are set apart by having diapsid skulls, having 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 also have teeth that grow from sockets (an archosaur characteristic) 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. 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: these include 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 major orders, the Saurischia and the Ornithischia, on the basis of hip structure. Saurischians (from the Greek meaning "lizard hip") are dinosaurs that retained the hip structure of their ancestors. They include all the theropods (bipedal carnivores) and sauropods (long-necked herbivores). Ornithischians (from the Greek meaning "bird-hip") is the other dinosaurian order, most of which were quadrupedal herbivores.

Saurischian pelvis structure.

Ornithischian pelvis structure.

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 Mya during the Triassic period, about 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 dinosaur species Eoraptor appear in the fossil record a this time, paleontologists believe the Eoraptor resembles the common ancestor of all dinosaurs.^[17] A few lines of primitive dinosaurs diversified rapidly after the Triassic, and quickly expanded until they filled most of the vacant ecological niches. Some of the earliest dinosaurs were the Eoraptor and the primative Lagosuchus. A slightly later dinosaur, but still in the early Triassic period, was the tiny Saltopus, barely larger than a human hand. During the reign of the dinosaurs, 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 Mya 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?

Dinosaur models at the Royal Ontario Museum.

Scientists have waged a constant and vigorous debate over the temperature regulation of dinosaur blood; at first over its possibility, then over its method, a debate first popularized by Robert T. "Bob" Bakker. From the first discovery of dinosaurs, paleontologists posited that they were ectothermic creatures: "terrible lizards" as their name suggested. This axiomatic expectation implied that dinosaurs were mostly slow, sluggish organisms, comparable to modern reptiles, which need the sun to heat their bodies. However, new evidence of dinosaurs in chilly temperate climates, of polar dinosaurs in Australia and Antarctica where they experienced a six-month chilly and dark winter, of feathered dinosaurs whose feathers provided regulatory insulation, and analysis of blood-vessel structures that are typical of endotherms within dinosaur bone, confirmed the possibility that some dinosaurs regulated their body temperature by internal biological methods, some aided partly by their very bulk. Skeletal structures suggest active lifestyles for theropods and other creatures, behavior more suitable for an endothermic cardiovascular system. Sauropods exhibit fewer endothermic characters. Perhaps some dinosaurs were endothermic and others not. Scientific debate over the details continues, although many paleontologists would now agree that endothermic systems are more likely.^[18]

Complicating this debate, warm-bloodedness can emerge from more than one mechanism. Most discussions of dinosaur endothermia compare them to average birds or mammals, which 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 surface area to volume ratio (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 and mammals. However, so far this theory fails to explain the vast multitudes of dog- and goat-sized dinosaurs, which made up the bulk of the ecosystem in the mesozoic.

Feathered dinosaurs and the bird connection

Main article: Feathered dinosaurs

A number of similiarities occur between birds and non-avian dinosaurs, in fact over a hundred distinct anatomical features are shared by avian dinosaurs and theropod dinosaurs.^[19]

Feathers

A model of Archaeopteryx lithographica on display at the Oxford University Museum of Natural History.

The first good specimen of a "feathered dinosaur" was the 1861 discovery of the Archaeopteryx in Germany, in the Solnhofen limestone, which is a lagerstätte; one of the rare and remarkable geological formations known for their superbly detailed fossils. Coming just two years after Darwin's seminal The Origin of Species, the evidence of a transitional fossil between reptiles and birds spurred the debates between evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, the specimens are commonly mistaken for Compsognathus.

Since the 1990s, a number of feathered dinosaurs have been found, providing clear evidence of the close relationship between dinosaurs and birds. Most of these specimens were local to Liaoning province in northeastern China, which was part of an island continent in the Cretaceous. However, the feathers were only preserved by the lagerstätte of the Yixian Formation; 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 Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, and Sinosauropteryx, and potentially Adasaurus; and dinosaur-like birds like Confuciusornis; all of which come from the same area and 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

Tyrannosaurus rex skeleton at the Field Museum of Natural History.

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 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 some 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 ostrich and other ratites.

Comparisons 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, wrists (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle and breast bone.

Reproduction biology

Tyrannosaurus rex skull at Carnegie Museum Natural History.

A discovery 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 sex a dinosaur. When laying eggs, female birds have a special type of bone, called a medullary bone, that grows in their limbs, forming a layer inside the hard outer bone. It is rich in calcium and 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 elements 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, possibly the adult dinosaur regurgitated nutrition into the young dinosaur's mouth. This behavior is seen in numerous modern-day bird species; the parent birds regurgitated food into the hatchling's mouth.

Lungs

Big meat-eating dinosaurs had a complex system of air sacs similar to the setup 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.^[20]

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.^[21] This would allow the head to be kept warm as is shown by modern birds.

Gizzard

Another piece of evidence that birds and dinosaurs are closely connected is that both birds and dinosaurs have used gizzard stones. The 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, they are called gastroliths. Paleontologists use the stones found in the dinosaur's stomach to determine migration routes, for example, the stone could have been swallowed at a certain point before being carried to another point during migration.

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.^[22]

Bringing dinosaurs back to life

A worker on scaffolding services the head of a full-size animatronic model of Tyrannosaurus rex.

There has been much speculation about the availability of technology to bring dinosaurs back to life. The idea proposed in Michael Crichton's book Jurassic Park, using blood from fossilized mosquitos that have been suspended in tree sap since the Mesozoic and then filling in the gaps with frog genes to create the DNA of a dinosaur, is probably impossible. A problem with this theory is that DNA decays over time by exposure to air, water and radiation, thus depleting the chances of salvaging any useful DNA. Decay can be measured by a racemization test.

There have been two claims about the successful extraction of ancient DNA from dinosaur fossils, but upon further inspection, neither of these reports could be confirmed.^[23] However, a working visual peptide of a (theoretical) dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of still-living related species (reptiles and birds).^[24]

Discovery of probable soft tissue from dinosaur fossils

In the March 2005 issue of Science, Dr. Schweitzer announced material, after rehydrating, that resembled soft tissue was discovered inside a Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana, from about 68 million years ago. When the fossilized bone was treated over several weeks to remove mineral content (demineralize) from the fossilized bone marrow cavity, Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under microscope further revealed the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. It has not been made clear of what this flexible material is actually composed, although many news reports immediately linked it with the movie "Jurassic Park", and the interpretation of the artifact as well as the relative importance of Dr. Schweitzer's discovery is still undecided.^[25]

Extinction theories

Main article: Cretaceous-Tertiary extinction event

The extinction of the non-avian dinosaurs is one of the most intriguing problems in paleontology. Only since the 1970s has the nature of this extinction become researched in detail, showing some possible causes of the dinosaur extinction.

Asteroid collision

The Chicxulub Crater at the tip of the Yucatán Peninsula, the impact of which may have caused the Dinosaur extinction.

The theory first proposed by Walter Alvarez in the late 1970s, linked the extinction event at the end of the Cretaceous period to a bolide 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 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 in steady decline or still thriving before the meteor struck. Some scientists state that the meteor would have caused an unnatural winter, 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 the extinction was extremely rapid. It appears to have been caused by heat from the meteorite impact and the matter ejected from the crater reentering the Earth's atmosphere around the world.

The Oort cloud

Similar to Alvarez's theory, which involved a single comet, the Oort cloud suggests that a vast shower of comets that were dislodged in an astral phenomenon hit the Earth at the same time, causing world wide extinction. The end result would again be an unnatural winter, ultimately freezing the dinosaurs.^[26]

Environment changes

The environment during the late Cretaceous was changing dramatically. Volcanic activity was decreasing, which led to a cooling trend as the levels of carbon dioxide diminished. At the era's peak, sea levels are estimated to have been between 100 metres (330 feet) to 250 metres (820 feet) higher than now with no polar ice caps. The planet's temperature was much more uniform, with only a 25 degrees C difference from the polar regions to the equator and much warmer with the poles 50 degrees C warmer than today.

The atmosphere's composition had carbon dioxide levels 12 times higher than today's levels, and oxygen formed 32 to 35 percent of the atmosphere, as compared with 21 percent today. Toward the end of the Cretaceous, however, these levels started to fluctuate wildly. Some hypothesize that climate change combined with the fall of oxygen levels might have led to many species demise, especially if the dinosaurs had a respiratory system commonly found in today's birds - something that would be difficult for an animal as large as a dinosaur with lower oxygen levels to breathe in.

Other groups besides dinosaurs became extinct at the same time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most kinds of birds, and many groups of mammals.^[27]

History of discovery

Dinosaur fossils have been known about 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