User:Rdracr/Dinosaur

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Dinosaurs
Temporal range: Carnian to present
Mounted skeletons of Tyrannosaurus (left) and Apatosaurus (right) at the American Museum of Natural History
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum:
Vertebrata
Class:
Reptilia
Subclass:
Diapsida
Infraclass:
Archosauromorpha
Superorder:
Dinosauria

Owen, 1842
Orders and suborders

Dinosaurs ("terrible, powerful, or wondrous lizards") were the dominant

period, and most paleontologists regard them as the only clade of dinosaurs to have survived until the present day.[1]

Dinosaurs were a varied group of animals. Paleontologists have identified over 500 distinct genera[2] and more than 1,000 different species of dinosaur,[3] and remains have been found on every continent on Earth.[4] Some dinosaurs were herbivorous, others carnivorous. Some were bipedal, others quadrupedal, and others were able to shift between these body postures. Many species developed elaborate skeletal modifications such as bony armor, horns or crests. Although generally known for their large size, many dinosaurs were human-sized or even smaller. Most major groups of dinosaurs are known to have built nests and laid eggs, suggesting an oviparity similar to that of modern birds.

The term "dinosaur" was coined in 1842 by

Sir Richard Owen and derives from Greek δεινός (deinos) "terrible, powerful, wondrous" + σαῦρος (sauros) "lizard". Through the first half of the twentieth century, most of the scientific community believed dinosaurs to have been sluggish, unintelligent cold-blooded animals. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated metabolisms
and numerous adaptations for social interaction.

Since the first dinosaur

nineteenth century, mounted dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become a part of world culture. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media. As a result, the word "dinosaur" has entered the vernacular, although its use and meaning in colloquial speech may be inconsistent with modern science. In English, for example, "dinosaur" is commonly used to describe anything that is impractically large, slow-moving, obsolete, or bound for extinction.[5]

Etymology

The

paleontologist Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.[6] The term is derived from the Greek words δεινός (deinos meaning "terrible", "powerful", or "wondrous") and σαῦρος (sauros meaning "lizard" or "reptile").[7] Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.[8] In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person,[9]
despite the dinosaurs' 160 million year reign and the global abundance and diversity of their avian descendants: modern-day birds.

Modern definition

Triceratops skeleton at the American Museum of Natural History in New York City

Under

quadrupeds with long necks and tails), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), and ornithopods
(bipedal or quadrupedal herbivores including "duck-bills"). These definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term "dinosaur" means.

There is a wide consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds would thus be dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[12]

From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" does not include birds. Additionally, referring to dinosaurs that are not birds as "non-avian dinosaurs" is cumbersome. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur". The term "non-avian dinosaur" will be used for emphasis as needed.

General description

Field Museum, Chicago

Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial

triconodont weighing between 12 kg (26 lb) and 14 kg (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus.[16]

Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.

Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller. Dinosaur remains have been found on every continent on Earth, including Antarctica.[4]
No dinosaurs are known to have lived in marine or aerial habitats, although it is possible some feathered theropods were flyers.

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral

synapomorphies
.

Dinosaur synapomorphies include an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.[10]

Edmontonia was an "armored dinosaur" of the group Ankylosauria.

A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of

Saturnalia, for example).[19] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.[20]

Hip joints and hindlimb postures

Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.[21] Their posture was due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.[22] Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.[23] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.[24] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.[24]

Natural history

Origins and early evolution

Main article:
Evolution of dinosaurs
ornithodiran

For a long time many scientists thought dinosaurs were

polyphyletic with multiple groups of unrelated "dinosaurs" evolving due to similar pressures,[25][26][27] but dinosaurs are now known to have formed a single group.[10][28]

Dinosaurs diverged from their

ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic
strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.

When dinosaurs appeared, terrestrial habitats were occupied by various types of basal archosaurs and

pterosaurians, and turtles.[10]

Full skeleton of an early carnivorous dinosaur, displayed in a glass case in a museum
The early forms Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull

The first few lines of primitive dinosaurs

stages of the Triassic, most likely by occupying the niches of groups that became extinct. Traditionally, dinosaurs were thought to have replaced the variety of other Triassic land animals by proving superior through a long period of competition. This now appears unlikely, for several reasons. Dinosaurs do not show a pattern of steadily increasing in diversity and numbers, as would be predicted if they were competitively replacing other groups; instead, they were very rare through the Carnian, making up only 1–2% of individuals present in faunas. In the Norian, however, after the extinction of several other groups, they became significant components of faunas, representing 50–90% of individuals. Also, what had been viewed as a key adaptation of dinosaurs, their erect stance, is now known to have been present in several contemporaneous groups that were not as successful (aetosaurs, ornithosuchids, rauisuchians, and some groups of crocodylomorphs). Finally, the Late Triassic itself was a time of great upheaval in life, with shifts in plant life, marine life, and climate.[10] Crurotarsans, today represented only by crocodilians but in the Late Triassic also encompassing such now-extinct groups as aetosaurs, phytosaurs, ornithosuchians, and rauisuchians, were actually more diverse in the Late Triassic than dinosaurs, indicating that the survival of dinosaurs had more to do with luck than superiority.[33]

Low diversification in the Cretaceous

Statistical analyses based on raw data suggest that dinosaurs diversified, i.e. the number of species increased, in the Late

crocodilians and birds also diversified at the same time. Lloyd et al. suggest that dinosaurs' failure to diversify as ecosystems were changing doomed them to extinction.[34]

Classification

Main article: Dinosaur classification

Dinosaurs (including birds) are

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 crocodilians sprawl out to either side.

Collectively, dinosaurs are usually regarded as a

therizinosauroids,[36] dromaeosaurids,[37] and birds[12]). Saurischia includes the theropods (bipedal and mostly carnivores, except for birds) and sauropodomorphs (long-necked quadrupedal herbivores
).

By contrast, ornithischians ("bird-hipped", from the Greek ornitheios (ορνιθειος) meaning "of a bird" and ischion (ισχιον) meaning "hip joint") had a pelvis that superficially resembled a bird's pelvis: the pubis bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of herbivores. (NB: the terms "lizard hip" and "bird hip" are misnomers – birds evolved from dinosaurs with "lizard hips".)

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

List of dinosaur classifications
.


Sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus
heterodontosaurid. Far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (small): Heterodontosaurus, far right (large) Tenontosaurus
.
  • Dinosauria
  • Tyrannosauroids (small to gigantic, often with reduced forelimbs)
  • Ornithomimosaurians ("ostrich-mimics"; mostly toothless; carnivores to possible herbivores)
  • Therizinosauroids
    (bipedal herbivores with large hand claws and small heads)
  • Oviraptorosaurians (mostly toothless; their diet and lifestyle are uncertain)
  • Dromaeosaurids (popularly known as "raptors"; bird-like carnivores)
  • Troodontids (similar to dromaeosaurids, but more lightly built, and possibly omnivorous)
  • Avialans (flying dinosaurs, including modern birds: the only living dinosaurs)
  • Sauropodomorphs (quadrupedal herbivores with small heads, long necks and tails, and elephant-like bodies)
  • "
    Prosauropods
    " (early relatives of sauropods; small to quite large; some possibly omnivorous; bipeds and quadrupeds)
  • Sauropods (very large, usually over 15 meters long [49 ft])
  • Diplodocoids (skulls and tails elongated; teeth typically narrow and pencil-like)
  • Macronarians (boxy skulls; spoon-shaped or pencil-shaped teeth)
  • Brachiosaurids (very long necks; forelimbs longer than hindlimbs)
  • Titanosaurians (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)
  • scutes
    as primary armor; some had club-like tails)
  • Stegosaurians (spikes and plates as primary armor)
  • Ornithopods (diverse, from meter- or yard-scale bipeds to 12-meter (39 ft) animals that could move as both bipeds and quadrupeds; evolved a method of chewing using skull flexibility and large numbers of teeth)
  • Hadrosaurids
    ("duckbilled dinosaurs")
  • Pachycephalosaurians ("bone-heads"; bipeds with domed or knobby growth on skulls)
  • Ceratopsians (dinosaurs with horns and frills, although most early forms had only the beginnings of these features)

Evolution and paleobiogeography

Main article:
Evolution of dinosaurs

Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass

sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[38] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.[39] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[12]

An illustration of 18 species of basal ceratopsia to scale

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians,

hadrosaurids.[39] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[40]

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were

therizinosaurians and ornithomimosaurians becoming common.[39]

The

sphenodontians, and choristoderans, also survived the event.[42]

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including

earth sciences (of which paleontology
is a sub-discipline). Two topics of particular interest and study have been dinosaur size and behavior.

Size

Main article: Dinosaur size
Scale diagram comparing the largest known dinosaurs in four suborders and a human

While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. Even by dinosaur standards, the

rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.[46]

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.[31] Theropod dinosaurs, when sorted by estimated weight into categories based on order of magnitude, most often fall into the 100 to 1000 kilogram (220 to 2200 lb) category, whereas recent predatory carnivorans peak in the 10 to 100 kilogram (22 to 220 lb) category.[47] The mode of dinosaur body masses is between one and ten metric tonnes.[48] This contrasts sharply with the size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kilograms (5 to 10 lb).[49]

Largest and smallest

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
.

Comparative size of Giraffatitan

The tallest and heaviest dinosaur known from good skeletons is

Carnegie Natural History Museum
in 1907.

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest

neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 metres (190 ft) long and weighed over 120000 kg (260000 lb),[46] heavier than all known dinosaurs except possibly the poorly known Bruhathkayosaurus, which could have weighed 175000 to 220000 kilograms (400000 to 500000 lb). The largest known carnivorous dinosaur was Spinosaurus, reaching a length of 16 to 18 meters (50 to 60 ft), and weighing in at 8150 kg (18000 lb).[52] Other large meat-eaters included Giganotosaurus, Mapusaurus, Tyrannosaurus rex and Carcharodontosaurus
.

Not including modern birds, the smallest dinosaurs known were about the size of a

pigeon.[53] The theropods Anchiornis and Epidexipteryx both had a total skeletal length of under 35 centimeters (1.1 ft).[54][53] Anchiornis is currently the smallest dinosaur described from an adult specimen, with an estimated weight of 110 grams.[54] The smallest herbivorous dinosaurs included Microceratus and Wannanosaurus, at about 60 cm (2 ft) long each.[55][56]

Behavior

A nesting ground of Maiasaura was discovered in 1978.

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 potential evidence of

Harris's Hawk is a well-documented exception), and the taphonomic evidence suggesting pack hunting in such theropods as Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.[63]

Fossilized egg of the oviraptorid Citipati, American Museum of Natural History

prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaur.[68] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.[69]
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.

Artist's rendering of two Centrosaurus, herbivorous ceratopsid dinosaurs from the late Cretaceous fauna of North America

The crests and frills of some dinosaurs, like the

lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.[70] The nature of dinosaur communication also remains enigmatic, and is an active area of research. For example, recent studies suggest that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.[71][72]

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the

Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.[75]

Based on current fossil evidence from dinosaurs such as

diplodocids could create sonic booms via whip-like tail snapping,[80] and whether sauropods could float.[81]

Physiology

Main article: Physiology of dinosaurs
Tyrannosaurus rex skull and upper vertebral column, Palais de la Découverte, Paris

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

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 within dinosaur bone that are typical of endotherms. 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.[82]

Eubrontes, a dinosaur footprint in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah

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 with average-sized 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 have little hair as adults, 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 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 numerous dog- and goat-sized dinosaur species, or the young of larger species.

Modern

computerized tomography (CT) scans of a dinosaur's chest cavity (conducted in 2000) found the apparent remnants of a four-chambered heart, much like those found in today's mammals and birds.[83] The idea is controversial within the scientific community, coming under fire for bad anatomical science[84] or simply wishful thinking.[85] The question of how this find reflects on metabolic rate and dinosaur internal anatomy may be moot, though, regardless of the object's identity: both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), and so dinosaurs probably had them as well.[86]

Soft tissue and DNA

Museo Civico di Storia Naturale
, in Milan, Italy

One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in Petraroia,

coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.[43]

In the March 2005 issue of Science, the paleontologist 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.[44]

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, and the implications of Schweitzer's discovery, are not yet clear; study and interpretation of the material is ongoing.[44]

Newer research, published in PloS One (30 July 2008), has challenged the claims that the material found is the soft tissue of Tyrannosaurus. Thomas Kaye of the

ammonite. In the ammonite they found the spheres in a place where the iron they contain could not have had any relationship to the presence of blood.[88]

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.[89] However, a functional visual peptide of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds.[90] In addition, several proteins, including hemoglobin,[91] have putatively been detected in dinosaur fossils.[92]

Feathers and the origin of birds

Main article: Origin of birds

The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by

thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs.[94] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;[12] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.[95] In the 1970s, John Ostrom revived the dinosaur–bird theory,[96] which gained momentum in the coming decades with the advent of cladistic analysis,[97] and a great increase in the discovery of small theropods and early birds.[18] Of particular note have been the fossils of the Yixian Formation, where a variety of theropods and early birds have been found, often with feathers of some type.[12] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.[98]
They are most closely allied with maniraptoran coelurosaurs.[12] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann's basal archosaur proposal,[99] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.[100]

Feathers

Main article:
Feathered dinosaurs
The famous Berlin Specimen of Archaeopteryx lithographica

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, at least one specimen was mistaken for Compsognathus.[101]

Since the 1990s, a number of additional

Dilong),[102] but barbed feathers are known only among the coelurosaur subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.[12][103] The description of feathered dinosaurs has not been without controversy; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that protofeathers are the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument,[104][105][106] and that maniraptoran dinosaurs with barbed feathers were not actually dinosaurs, but convergent with dinosaurs.[100][105] However, their views have for the most part not been accepted by other researchers, to the point that the question of the scientific nature of Feduccia's proposals has been raised.[107]

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the

breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis
strengthens the case for the link.

Soft anatomy

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

CT-scanning revealed the evidence of air sacs within the body cavity of the Aerosteon skeleton.[109][110]

Another piece of evidence that birds and dinosaurs are closely related is the use by both of gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.[111]

Reproductive biology

A discovery of features in a Tyrannosaurus rex skeleton recently provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, 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.[112] Further research has found medullary bone in the theropod Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.[113]

Behavioral evidence

A recently discovered troodont fossil demonstrates that some dinosaurs slept with their heads tucked under their arms.[114] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.

Extinction

Main articles:
K–T boundary

Non-avian dinosaurs suddenly became

Cretaceous–Tertiary extinction event
. The nature of the event that caused this mass extinction has been extensively studied since the 1970s; at present, several related theories are supported by paleontologists. Though the general consensus is that an impact event was the primary cause of dinosaur extinction, some scientists cite other possible causes, or support the idea that a confluence of several factors was responsible for the sudden disappearance of dinosaurs from the fossil record.

At the peak of the Mesozoic, there were no polar ice caps, and sea levels are estimated to have been from 100 to 250 meters (300 to 800 ft) higher than they are today. The planet's temperature was also much more uniform, with only 25 °C (45 °F) separating average polar temperatures from those at the equator. On average, atmospheric temperatures were also much higher; the poles, for example, were 50 °C (90 °F) warmer than today.[115][116]

The atmosphere's composition during the Mesozoic 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.[4]

Impact event

Main article: Chicxulub crater
Chicxulub Crater at the tip of the Yucatán Peninsula
; the impactor that formed this crater may have caused the dinosaur extinction.

The asteroid collision theory, which was brought to wide attention in 1980 by

mass extinction.[118][119] 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).

In September 2007, U.S. researchers led by William Bottke of the

Yucatan peninsula 65 million years ago, creating the Chicxulub crater.[120]

A similar but more controversial explanation proposes that "passages of the [hypothetical] solar companion star

Nemesis through the Oort comet cloud would trigger comet showers."[121] One or more of these comets 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.[121]

Deccan Traps

Main article: Deccan Traps

Before 2000, arguments that the

million years ago and lasted for over 2 million years. However, there is evidence that two-thirds of the Deccan Traps were created in only 1 million years about 65.5 million years ago, and so these eruptions would have caused a fairly rapid extinction, possibly over a period of thousands of years, but still longer than would be expected from a single impact event.[122][123]

The Deccan Traps could have caused extinction through several mechanisms, including the release into the air of dust and sulphuric aerosols, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions, which would have increased the

impact [at Chicxulub]."[122][123]

In the years when the Deccan Traps theory was linked to a slower extinction,

sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.[124]

Failure to adapt to changing conditions

Lloyd et al. (2008) noted that, in the Mid Cretaceous, the flowering,

angiosperm plants became a major part of terrestrial ecosystems, which had previously been dominated by gymnosperms such as conifers. Dinosaur coprolites – fossilized dung – indicate that, while some ate angiosperms, most herbivorous dinosaurs mainly ate gymnosperms. Statistical analysis by Lloyd et al. concluded that, contrary to earlier studies, dinosaurs did not diversify very much in the Late Cretaceous. Lloyd et al. suggested that dinosaurs' failure to diversify as ecosystems were changing doomed them to extinction.[34]

Possible Paleocene survivors

Main article:
Paleocene dinosaurs

Non-avian dinosaur remains are occasionally found above the

Paleocene dinosaurs. 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.[125] Other evidence includes the finding of dinosaur remains in the Hell Creek Formation up to 1.3 meters (51 in) above (40000 years later than) the K-T boundary. Similar reports have come from other parts of the world, including China.[126] Many scientists, however, dismiss the "Paleocene dinosaurs" as re-worked, i.e. washed out of their original locations and then re-buried in much later sediments,[127][128] or find that, if correct, the presence of a handful of dinosaurs in the early Paleocene would not change the underlying facts of the extinction.[127]

History of discovery

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese, whose modern word for dinosaur is konglong (恐龍, or "terrible dragon"), considered them to be

Great Flood
.

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in

Rutellum implicatum"[132][133], that had been found in Caswell, near Witney, Oxfordshire.[134]

William Buckland

Between 1815 and 1824, the Rev William Buckland, a professor of geology at Oxford University, collected more fossilized bones of Megalosaurus and became the first person to describe a dinosaur in a scientific journal.[131][135] The second dinosaur genus to be identified, Iguanodon, was discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825.[136][137]

The study of these "great fossil lizards" soon became of great interest to European and American scientists, and in 1842 the English paleontologist

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

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
Edward Drinker Cope, 19th century photograph

Dinosaur mania was exemplified by the fierce rivalry between

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 their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, 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.[138]

Since 1897, the search for dinosaur fossils has extended to every continent, including

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.

The "dinosaur renaissance"

Main article: 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

fossil record
.

Cultural depictions

Main article: Cultural depictions of dinosaurs
A Megalosaurus stalks Crystal Palace Park in London.

By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. Entry of the word "dinosaur" into the common vernacular reflects the animals' cultural importance: in English, "dinosaur" is commonly used to describe anything that is impractically large, slow-moving, obsolete, or bound for extinction.[5]

Public enthusiasm for dinosaurs first developed in

dinosaur sculptures were unveiled in London's Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.[139] Dinosaurs' enduring popularity, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions.[140]

The popular preoccupation with dinosaurs has ensured their appearance in

companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted or obsolete.[142]

See also

Notes and references

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  141. ^ "London. Michaelmas term lately over, and the Lord Chancellor sitting in Lincoln's Inn Hall. Implacable November weather. As much mud in the streets, as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet a Megalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborne Hill." From page 1 of Dickens, Charles J.H. (1852). Bleak House. London: Bradbury & Evans.
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    )

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  • Dinosaurnews (www.dinosaurnews.org) The dinosaur-related headlines from around the world. Recent news on dinosaurs, including finds and discoveries, and many links.
  • Dinosauria From UC Berkeley Museum of Paleontology Detailed information – scroll down for menu.
  • LiveScience.com All about dinosaurs, with current featured articles.
  • Zoom Dinosaurs (www.enchantedlearning.com) From Enchanted Learning. Kids' site, info pages and stats, theories, history.
  • Dinosaur genus list contains data tables on nearly every published dinosaur genus.
  • LiveScience.com Giant Dinosaurs Get Downsized by LiveScience, June 21, 2009
Technical
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