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Dinosaur

2007 Schools Wikipedia Selection. Related subjects: Dinosaurs

   iDinosaurs

           Fossil range: Triassic – Cretaceous (excluding Aves)

   Replica of Tyrannosaurus rex at the Senckenberg Museum.
   Replica of Tyrannosaurus rex at the Senckenberg Museum.
                     Scientific classification

   Kingdom:    Animalia
   Phylum:     Chordata
   Class:      Sauropsida
   Subclass:   Diapsida
   Infraclass: Archosauromorpha
   Superorder: Dinosauria *
               Owen, 1842

                             Orders & Suborders

     * Saurischia
          + Sauropodomorpha
          + Theropoda
     * Ornithischia
          + Thyreophora
          + Ornithopoda
          + Marginocephalia

   Dinosaurs were vertebrate animals that dominated terrestrial ecosystems
   for over 160 million years, first appearing approximately 230 million
   years ago. At the end of the Cretaceous Period, 65 million years ago,
   dinosaurs experienced a catastrophic extinction, which ended their
   dominance on land. Taxonomists consider modern birds to be the direct
   descendants of theropod dinosaurs.

   Since the first dinosaur was recognized in the 19th century, mounted,
   fossilized dinosaur skeletons have become major attractions at museums
   around the world. Dinosaurs have become a part of world culture and
   remain consistently popular among children and adults alike. They have
   been featured in best-selling books and films such as Jurassic Park,
   and new discoveries are regularly covered by the media.

   The term dinosaur is sometimes used informally to describe other
   prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged
   pterosaurs and the aquatic ichthyosaurs, plesiosaurs and mosasaurs,
   although technically none of these were dinosaurs.

What is a dinosaur?

Definition

   Triceratops skeleton at the Smithsonian National Museum of Natural
   History.
   Enlarge
   Triceratops skeleton at the Smithsonian National Museum of Natural
   History.

   The taxon Dinosauria was formally named by the English palaeontologist
   Richard Owen in 1842 as "a distinct tribe or suborder of Saurian
   reptiles". The term is derived from the Greek words δεινός (deinos
   meaning "terrible", "fearsome" or "formidable") and σαύρα (saura
   meaning "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 an extremely varied group of animals; according to a
   2006 study, 527 dinosaur genera have been identified with certainty so
   far, and 1,844 genera are believed to have existed. Some were
   herbivorous, others carnivorous. Some dinosaurs were bipeds, some were
   quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk
   just as easily on two or four legs. Regardless of body type, nearly all
   known dinosaurs were well-adapted for a predominantly terrestrial,
   rather than aquatic or aerial, habitat.

   Dinosaur synapomorphies

   All dinosaurs so far discovered share certain modifications to the
   ancestral archosaurian skeleton. Although some later groups of
   dinosaurs featured further modified versions of these traits, they are
   considered typical across Dinosauria; the earliest dinosaurs had them
   and passed them on to all their descendants. Such common structures
   across a taxonomic group are called synapomorphies.

   Dinosaur synapomorphies include: reduced fourth and fifth digits on the
   manus ('hand'), reduced number of digits on the pes (foot) to three
   main toes, a sacrum (the region of the vertebral column to which the
   pelvis attaches, composed of three or more fused vertebrae) and an open
   or perforate acetabulum (hip socket with a hole at its centre).
   Dinosaurs are unique among all tetrapods in having this perforate
   acetabulum.

   Other shared anatomical features

   Scientists generally agree that a variety of other anatomical features
   were shared by most dinosaurs. These include forelimbs shorter and
   lighter than hind limbs, an unusual secondary palate that permitted
   dinosaurs to eat and breathe simultaneously, a relatively straight
   femur with medially-directed femoral head, two pairs of holes in the
   temporal region of the skull (i.e. a diapsid skull), rearward-pointing
   elbows in the front limbs and forward-pointing knees in the hind limbs.

   The hip joint arrangement described above allowed an erect stance, in
   which hind limbs were situated directly beneath the body or
   'underslung'. This stance is like that of most mammals today but unlike
   that of other reptiles, which have a less erect posture and limbs
   splayed out to either side. The vertical action of the limbs in
   dinosaurs allowed for more efficient and faster locomotion, compared to
   the clumsier and slower movement of other 'sprawled' reptiles. It also
   allowed many types of dinosaurs to become bipedal.
   Stegosaurus skeleton at the American Museum of Natural History in New
   York City.
   Enlarge
   Stegosaurus skeleton at the American Museum of Natural History in New
   York City.

   Taxonomic definition

   Under phylogenetic taxonomy, dinosaurs are defined as all descendants
   of the most recent common ancestor of Triceratops and modern birds.
   They are divided into Ornithischia (bird-hipped) and Saurischia
   (lizard-hipped), depending upon pelvic structure. Ornithischian
   dinosaurs had a four-pronged pelvic configuration, incorporating a
   caudally-directed (rear-pointing) pubis bone with (most commonly) a
   forward-pointing process. By contrast, the pelvic structure of
   saurischian dinosaurs was three-pronged, and featured a pubis bone
   directed cranially, or forwards, only. Ornithischia includes all taxa
   sharing a more recent common ancestor with Triceratops than with
   Saurischia, while Saurischia includes those 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, because these
   were two of the three genera cited by Richard Owen when he recognized
   the Dinosauria.

   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 cumbersome. Birds are still
   referred to as 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, but some biologists of the older
   generation 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). The term 'non-avian dinosaur' will be
   used for emphasis as needed. 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

   While the evidence is incomplete, it is clear that, as a group,
   dinosaurs were large. Even 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. Giant prehistoric mammals such as the Indricotherium
   and the Columbian mammoth were dwarfed by the giant sauropods, and only
   a handful of modern aquatic animals approach or surpass 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.

   Most dinosaurs, however, were much smaller than the giant sauropods.
   Current evidence suggests that dinosaur average size varied through the
   Triassic, early Jurassic, late Jurassic and Cretaceous periods.
   According to paleontologist Bill Erickson, estimates of median dinosaur
   weight range from 500 kg to 5 tonnes; a recent study of 63 dinosaur
   genera yielded an average weight greater than 850 kg — comparable to
   the weight of a grizzly bear — and a median weight of nearly 2 tons, or
   about as much as a giraffe. This contrasts sharply with the size of
   modern mammals; on average, mammals weigh only 863 grams, or about as
   much as a large 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.
   A statue of Diplodocus carnegiei, outside the Carnegie Museum of
   Natural History.
   Enlarge
   A statue of Diplodocus carnegiei, outside the Carnegie Museum of
   Natural History.

   Largest and smallest dinosaurs

   Only a tiny percentage of animals ever fossilize, and most of these
   remain buried in the earth. 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. As a
   result, scientists will probably never be certain of the largest and
   smallest dinosaurs.
   Size of a human compared to a Tyrannosaurus rex.
   Enlarge
   Size of a human compared to a Tyrannosaurus rex.

   The tallest and heaviest dinosaur known from a complete skeleton is the
   Brachiosaurus specimen that 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 herbivorous
   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. The largest known carnivorous
   dinosaur was Spinosaurus, reaching a length of 16-18 meters (53-60 ft),
   and weighing in at 9 tons. Other large meat-eaters included
   Giganotosaurus, Mapusaurus, Tyrannosaurus rex and Carcharodontosaurus.

   Not including modern birds like the bee hummingbird, the smallest
   dinosaurs known were about the size of a crow or a chicken. The
   theropods Microraptor, Parvicursor, and Saltopus were all under 60 cm
   (2 ft) in length.

Behaviour

   A nesting ground of Maiasaura was discovered in 1978.
   Enlarge
   A nesting ground of Maiasaura was discovered in 1978.

   Interpretations of dinosaur behaviour 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 behaviour 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 perceived direct evidence of herding behaviour was the 1878
   discovery of 31 Iguanodon dinosaurs which were thought to have perished
   together in Bernissart, Belgium, after they fell into a deep, flooded
   sinkhole and drowned. Despite the deposition of those skeletons being
   now regarded as more gradual, other, well supported, mass death sites
   were subsequently discovered. Those, along with multiple trackways,
   suggest that herd or pack behaviour 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 behaviour 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 behavioural 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 behaviour among
   theropods is no surprise, this too was confirmed by tooth marks from
   Madagascar in 2003.

   There seem to have been no burrowing species of dinosaur and few
   climbing 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.

Evolution of dinosaurs

   Image:Eoraptor.jpg
   A reconstruction of Eoraptor, an early dinosaur.

   Dinosaurs diverged from their archosaur ancestors approximately 230
   million years ago during the Middle to Late Triassic period, roughly 20
   million years after the Permian-Triassic extinction event wiped out an
   estimated 95% of all life on Earth. Radiometric dating of fossils from
   the early dinosaur genus Eoraptor establishes its presence in the
   fossil record at this time. Paleontologists believe Eoraptor resembles
   the common ancestor of all dinosaurs; if this is true, its traits
   suggest that the first dinosaurs were small, bipedal predators. Such a
   notion is corroborated by the more primitive dinosaur-like
   ornithodirans hailing from Middle Triassic strata of Argentina such as
   Marasuchus and Lagerpeton, which were also small bipedal, perhaps
   leaping, predators.

   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 occurred 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.

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, 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 consolidated the link between dinosaurs
   and their conjectured 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

   Main article: Dinosaur 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.

   Collectively, dinosaurs are usually regarded as a superorder or an
   unranked clade. They are divided into two orders, the Saurischia and
   the Ornithischia, on the basis of their hip structure. Saurischians
   ('lizard-hipped', from the Greek sauros (σαυρος) meaning 'lizard' and
   ischion (ισχιον) meaning 'hip joint') are dinosaurs that originally
   retained the hip structure of their ancestors. They include all the
   theropods (bipedal carnivores) and sauropods (long-necked herbivores).
   Ornithischians ('bird-hipped', from the Greek ornitheos (ορνιθειος)
   meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint') is the
   other dinosaurian order, most of which were quadrupedal herbivores.
   (NB: the terms "lizard hip" and "bird-hip" are misnomers — birds
   evolved from dinosaurs with "lizard hips".)

   Saurischian pelvis structure (left side)

   Tyrannosaurus pelvis (showing saurischian structure - left side)

   Ornithischian pelvis structure (left side).

   Edmontosaurus pelvis (showing ornithischian structure - left side)

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

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

Order Saurischia

     * †Infraorder Herrerasauria
     * Suborder Theropoda
          + †Superfamily Coelophysoidea
          + †Infraorder Ceratosauria
               o †Family Abelisauridae
          + (unranked) Tetanurae
               o †Superfamily Megalosauroidea
               o †Infraorder Carnosauria
               o Infraorder Coelurosauria
                    # †Family Coeluridae
                    # †Superfamily Tyrannosauroidea
                    # †(unranked) Ornithomimosauria
                    # (unranked) Maniraptora
                         @ †(unranked) Oviraptoriformes
                              - †(unranked) Therizinosauria
                              - †(unranked) Oviraptorosauria
                         @ †(unranked) Deinonychosauria
                              - †Family Troodontidae
                              - †Family Dromaeosauridae
                         @ Class Aves (birds)
     * †Suborder Sauropodomorpha
          + † Thecodontosaurus
          + †Infraorder Prosauropoda
          + †Infraorder Sauropoda
               o †Superfamily Diplodocoidea
               o †(unranked) Macronaria
                    # †Family Brachiosauridae
                    # †Family Titanosauridae

Order Ornithischia

     * †Suborder Thyreophora
          + †Infraorder Stegosauria
          + †Infraorder Ankylosauria
     * †(unranked) Cerapoda
          + †Family Heterodontosauridae
          + †Suborder Marginocephalia
               o †Infraorder Pachycephalosauria
               o †Infraorder Ceratopsia
                    # †Family Psittacosauridae
                    # †Family Protoceratopsidae
                    # †Family Ceratopsidae
          + †Suborder Ornithopoda
               o †Family Hypsilophodontidae
               o †Infraorder Iguanodontia
                    # †Family Iguanodontidae
                    # †Family Hadrosauridae

Areas of debate

Warm-bloodedness

   Dinosaur models at the Royal Ontario Museum.
   Enlarge
   Dinosaur models at the Royal Ontario Museum.

   A vigorous debate on the subject of temperature regulation in dinosaurs
   has been ongoing since the 1960s. Originally, scientists broadly
   disagreed as to whether dinosaurs were capable of regulating their body
   temperatures at all. More recently, dinosaur endothermy has become the
   consensus view, and debate has focused on the mechanisms of temperature
   regulation.

   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 because of 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 behavioural
   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 suggest they lost heat relatively slowly to the
   surrounding air, and so could have been what are called inertial
   homeotherms, 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
   A model of Archaeopteryx lithographica on display at the Oxford
   University Museum of Natural History.
   Enlarge
   A model of Archaeopteryx lithographica on display at the Oxford
   University Museum of Natural History.

   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 non-avian
   dinosaurs elsewhere in the world were also feathered. The lack of
   widespread fossil evidence for feathered non-avian 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
   Tyrannosaurus rex skeleton at the Field Museum of Natural History.
   Enlarge
   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 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
   Tyrannosaurus rex skull and upper vertebral column, Palais de la
   Découverte, Paris.
   Enlarge
   Tyrannosaurus rex skull and upper vertebral column, Palais de la
   Découverte, Paris.
   Euoplocephalus was a typical "armored dinosaur" of the Ankylosauria
   superfamily.
   Enlarge
   Euoplocephalus was a typical "armored dinosaur" of the Ankylosauria
   superfamily.

   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 behaviour 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 behaviour, 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 in the San Juan Basin, New Mexico.
   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

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

   There has been much speculation about the use of technology to bring
   dinosaurs back to life. In Michael Crichton's book Jurassic Park (later
   adapted into a movie), 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,
   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.

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, 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

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

   The asteroid collision theory, which was first proposed by Walter
   Alvarez in the late 1970s, links the extinction event at the end of the
   Cretaceous period to a bolide impact approximately 65.5 million years
   ago. Alvarez proposed that a sudden increase in iridium levels,
   recorded around the world in the period's rock stratum, was direct
   evidence of the impact. The bulk of the evidence now suggests that a
   5-15 km wide bolide hit in the vicinity of the Yucatán Peninsula,
   creating the 170 km-wide Chicxulub Crater and triggering the mass
   extinction. Scientists are not certain whether dinosaurs were thriving
   or declining before the impact event. Some scientists propose that the
   meteorite caused a long and unnatural drop in Earth's atmospheric
   temperature, while others claim that it would have instead created an
   unusual heat wave.

   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% of the atmosphere, as
   compared to 21% 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 for millennia, although their true
   nature was not recognized. The Chinese, whose own word for dinosaur is
   konglong (恐龍, or "terrible dragon"), considered them to be dragon bones
   and documented them as such. For example, Hua Yang Guo Zhi, a book
   written by Zhang Qu during the Western Jin Dynasty, reported the
   discovery of dragon bones at Wucheng in Sichuan Province. In Europe,
   dinosaur fossils were generally believed to be the remains of giants
   and other creatures killed by the Great Flood.
   William Buckland
   Enlarge
   William Buckland

   Megalosaurus was the first dinosaur to be formally described, in 1677,
   when part of a bone was recovered from a limestone quarry at Cornwell
   near Oxford, England. This bone fragment was identified correctly as
   the lower extremity of the femur of an animal larger than anything
   living in modern times. The second dinosaur species to be identified,
   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 more fossilized
   bones of Megalosaurus and became the first person to describe dinosaurs
   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.
   Othniel Charles Marsh, (19th century photograph).
   Enlarge
   Othniel Charles Marsh, (19th century photograph).
   Edward Drinker Cope, (19th century photograph).
   Enlarge
   Edward Drinker Cope, (19th century photograph).

   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 Antarctopelta oliveroi, was found on Ross
   Island in 1986, although it was 1994 before an Antarctic species, the
   theropod 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

   Pink dinosaur model at Vernal, Utah.
   Enlarge
   Pink dinosaur model at Vernal, Utah.

   By human standards, dinosaurs were creatures of fantastic appearance
   and often enormous size. As such, they have captured people's
   imagination and become an enduring part of human popular culture.
   Dinosaur exhibitions, parks and museum exhibits around the world both
   cater to and reinforce the public's interest. The popular preoccupation
   with dinosaurs is also reflected in a broad array of 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.

Religious views

   Various religious groups have views about dinosaurs that differ from
   those held 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. See the referenced article for specific examples and further
   context.

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