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The discovery in 1862 of
Archaeopteryx first suggested a close relationship between dinosaurs and birds, as only the fossilized feathers of
Archaeopteryx clearly distinguished it from contemporary small predatory dinosaurs such as
Compsognathus. Research since the 1970s indicates that
theropod dinosaurs are most likely the ancestors of
birds; in fact, most paleontologists regard birds as the only surviving dinosaurs and some believe dinosaurs and birds should be put together under one biological class.
Crocodilians are the other surviving close relatives of dinosaurs, and both groups are members of the
Archosauria, a group of
reptiles that first appeared in the very late
Permian and became dominant in the mid-Triassic.
For about the first half of the 20th century, both scientists and the general public regarded dinosaurs as slow, unintelligent
cold-blooded animals. However, the bulk of
research since the 1970s has supported the view that they were active animals with elevated metabolisms, and often with adaptations for social interactions. This change of view was strongly influenced by evidence of the descent of birds from theropod dinosaurs.
Since the first dinosaur
fossils were recognized in the early nineteenth century, mounted 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. They have been featured in best-selling books and films (notably
Jurassic Park), and new discoveries are regularly covered by the
media.
The term "dinosaur" was first coined 1841, by
Sir Richard Owen and derives from
Greek δεινός (
deinos) "terrible, stiff, masterly, skillful" +
σαῦρος (
sauros) "lizard". It 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 none of these were dinosaurs.
In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person, despite the dinosaurs' 160 million year reign and the global abundance and diversity of their descendants, the birds. This usage became common while dinosaurs were regarded as cold-blooded and sluggish.
What is a dinosaur?
Original definition
The
taxon Dinosauria was formally named in 1842 by
English palaeontologist 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. The term is derived from the
Greek words δεινός (
deinos meaning "terrible", "fearsome", or "formidable") and σαύρα (
saura meaning "lizard" or "reptile"). 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.
Modern definition
Under
phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the most recent common ancestor of
Triceratops and modern
birds. It has also been suggested that Dinosauria be defined as all of 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. Both definitions result in the same set of animals being defined as dinosaurs, including
theropods (mostly
bipedal carnivores),
sauropodomorphs (mostly large
herbivorous 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 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 must be included in a group for that group to be natural, birds
are 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.
From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" doesn't 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. It is also technically correct to refer to dinosaurs as a distinct group under the older
Linnaean classification system, which accepts
paraphyletic taxa that exclude some descendants of a single common ancestor.
General description
Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial
archosaurian
reptiles with
limbs held erect beneath the body, that existed from the
Late Triassic (first appearing in the
Carnian faunal stage) to the
Late Cretaceous (going extinct at the end of the
Maastrichtian). Many prehistoric animals are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and
Dimetrodon, but are not classified scientifically as dinosaurs. Marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs were neither terrestrial nor archosaurs; pterosaurs were archosaurs but not terrestrial; and
Dimetrodon was a
Permian animal more closely related to mammals. Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the
Jurassic and Cretaceous. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey. One notable exception is
Repenomamus giganticus, a
triconodont weighing between and that's known to have eaten small dinosaurs like young
Psittacosaurus.
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 1,850, nearly 75% of which remain to be discovered. An earlier study predicted that about 3,400 dinosaur genera existed, including many which wouldn't have been preserved in the fossil record. 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. 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.
A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of
archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as
diapsid reptiles, dinosaurs ancestrally had two pairs of
temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the
snout and lower jaw. Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated
scapula, or shoulder blade; a
sacrum composed of three or more fused
vertebrae (three are found in some other archosaurs, but only two are found in
Herrerasaurus); 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.
Dinosaurs stood erect in a manner similar to
most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. 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. Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that
surpassed those of "sprawling" reptiles. Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs. 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. but dinosaurs are now known to have formed a single group.
Radiometric dating of the
rock formation that contained 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. The discovery of primitive, dinosaur-like
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
therapsids, such as
aetosaurs,
cynodonts,
dicynodonts,
ornithosuchids,
rauisuchias, and
rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the
Carnian and
Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal
archosauromorphs, including the
prolacertiforms and rhynchosaurs, became extinct. This was followed by the
Triassic-Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids,
phytosaurs, and rauisuchians. These losses left behind a land fauna of
crocodylomorphs, dinosaurs,
mammals,
pterosaurians, and
turtles.
therizinosauroids,
dromaeosaurids, and
birds
Gymnosperm plants (particularly
conifers), a potential food source, radiated in the Late Triassic. Prosauropods didn't have sophisticated mechanisms for processing food in the mouth, so must have employed other means of breaking down food farther along the digestive tract. The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of
ceratosaurians,
spinosauroids, and
carnosaurians, and herbivores consisting of
stegosaurian ornithischians and large sauropods. Examples of this include the
Morrison Formation of North America and
Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized
sinraptorid theropods and unusual, long-necked sauropods like
Mamenchisaurus.
There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were
tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and
pachycephalosaurians. In the southern continents that had made up the now-splitting
Gondwana,
abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids,
rhabdodontid iguanodontians,
nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent. Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with
therizinosaurians and
ornithomimosaurians becoming common.
Paleobiology
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. There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in
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.
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. A rough estimate for average dinosaur weight is about . This contrasts sharply with the size of
Cenozoic mammals, estimated by the same source (the
National Museum of Natural History) as about 2 to 5 kilograms (5 to 10 lb).
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.
The tallest and heaviest dinosaur known from good skeletons is
Brachiosaurus brancai (also known as
Giraffatitan). Its remains were discovered in
Tanzania between 1907–12. Bones from multiple similarly-sized individuals were incorporated into the skeleton now mounted and on display at the
Humboldt Museum of
Berlin; this mount is tall and long, and would have belonged to an animal that weighed between 30,000 and 60,000 kilograms (70,000 and 130,000 lb). 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 fragmentary fossils. Most of 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 to 100,000 kilograms (90 to 110 short tons); some of the longest, the long
Diplodocus hallorum and the tallest, the
Sauroposeidon, which could have reached a sixth-floor window. The longest of them all may have been
Amphicoelias fragillimus, known only from a now lost partial vertebral
neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been long and weighed over, 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
crow or a
chicken. The theropods
Microraptor and
Parvicursor were both under 0.6 meters (2 ft) in length.
Behavior
Interpretations of dinosaur behavior are generally based on the pose of body fossils and their
habitat,
computer simulations of their
biomechanics, and comparisons with modern animals in similar
ecological niches. As such, the current understanding of dinosaur behavior relies on speculation, and will likely remain controversial for the foreseeable future. However, there's general agreement that some behaviors which are common in crocodiles and birds, dinosaurs' closest living relatives, were also common among dinosaurs.
The first direct evidence of
herding behavior was the 1878 discovery of 31
Iguanodon dinosaurs which were then thought to have perished together in
Bernissart,
Belgium, after they fell into a deep, flooded
sinkhole and drowned. Other mass death sites have been subsequently discovered. Those, along with multiple trackways, suggest that
herd or
pack behavior was common in many dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that
duck-bills (hadrosaurids) may have moved in great herds, like the
American Bison or the African
Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in
Oxford, England, although there isn't evidence for specific herd structures. Dinosaurs may have congregated in herds for defense, for
migratory purposes, or to provide protection for their young. The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as
pack hunters working together to bring down large prey. However, this lifestyle is uncommon among the modern relatives of dinosaurs (
crocodiles and other reptiles, and
birds -
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.
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
Patagonian
titanosaurian sauropods (1997 discovery), also nested in large groups. The
Mongolian
oviraptorid Citipati 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. Parental care is also implied by other finds. For example, the fossilized remains of a grouping of
Psittacosaurus has been found, consisting of one adult and 34 juveniles; in this case, the large number of juveniles may be due to communal nesting. Additionally, a dinosaur embryo (pertaining to the
prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaur. Trackways have also confirmed parental behavior among 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. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations. 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.
From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the
Gobi Desert in 1971. It included a
Velociraptor attacking a
Protoceratops, providing evidence that dinosaurs did indeed attack each other. Additional evidence for attacking live prey is the partially-healed tail of an
Edmontosaurus, a hadrosaurid dinosaur; the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived.
Based on current fossil evidence from dinosaurs such as
Oryctodromeus, some herbivorous species seem to have led a partially
fossorial (burrowing) lifestyle, and some bird-like species may have been
arboreal (tree-climbing), most notably primitive
dromaeosaurids such as
Microraptor and the enigmatic
scansoriopterygids. However, most dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of
biomechanics, in particular, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run, whether
diplodocids could create
sonic booms via
whip-like tail snapping, and whether sauropods could float.
Physiology
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
endotherm 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
endotherm in dinosaurs includes the discovery of
polar dinosaurs in Australia and
Antarctica (where they'd 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're hairless, and have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud wallowing). These behaviors increase cooling through evaporation.
Large dinosaurs would presumably have had to deal with similar issues; their body size 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. The idea is controversial within the scientific community, coming under fire for bad anatomical science or simply wishful thinking. 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), so dinosaurs probably had them as well.
Soft tissue and DNA
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.
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 related modern species such as reptiles and birds. In addition, several
proteins have putatively been detected in dinosaur fossils, including hemoglobin.
Even if dinosaur DNA could be reconstructed, it would be exceedingly difficult to clone and "grow" dinosaurs using current technology since no closely related species exist to provide
zygotes or a suitable environment for
embryonic development.
Feathers and the origin of birds
The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by
Thomas Henry Huxley. After the work of
Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of generalized
thecodont ancestors, with the key piece of evidence being the supposed lack of
clavicles in dinosaurs. However, as later discoveries showed, clavicles (or a single fused
wishbone, which derived from separate clavicles) were not actually absent; In the 1970s,
John Ostrom revived the dinosaur-bird theory, which gained momentum in the coming decades with the advent of cladistic analysis, and a great increase in the discovery of small theropods and early birds.
They are most closely allied with
maniraptoran coelurosaurs. or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only
convergent with dinosaurs.
Feathers
Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the
Solnhofen limestone in southern Germany, which is a
lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils.
Archaeopteryx is a
transitional fossil, with features clearly intermediate between those of modern reptiles and birds. Brought to light just two years after Darwin's seminal
The Origin of Species, its discovery spurred the nascent debate between proponents of
evolutionary biology and
creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was 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
lagerstätte of the Yixian Formation,
Liaoning, northeastern
China, which was part of an island continent during the Cretaceous. Though feathers have been found only in a few locations, it's 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. To this point, protofeathers (thin, filament-like structures) are known from dinosaurs at the base of Coelurosauria, such as
compsognathids like
Sinosauropteryx and
tyrannosauroids (
Dilong), but barbed feathers are only known among the coelurosaur subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds. The description of feathered dinosaurs hasn't 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, and that maniraptoran dinosaurs with barbed feathers were not actually dinosaurs, but
convergent with dinosaurs.
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the
missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for
paleontologists. Areas of the skeleton with important similarities include the neck,
pubis,
wrist (semi-lunate
carpal), arm and
pectoral girdle, furcula (wishbone), and
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
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.
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.
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 in their limbs between the hard outer bone and the
marrow. 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. Further research has found medullary bone in the theropod
Allosaurus and 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.
Behavioral evidence
A recently discovered
troodont fossil demonstrates that some dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
Extinction
extinct approximately 65 million years ago. 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. This
mass extinction is known as the
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're 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 warmer; the poles, for example, were 50 °C (90 °F) warmer than today.
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. The bulk of the evidence now suggests that a 5 to 15 kilometer (3 to 9 mi) wide
bolide hit in the vicinity of the
Yucatán Peninsula, creating the 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 can't 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 of 2007, U.S. researchers led by
William Bottke of the
Southwest Research Institute in
Boulder, Colorado, and
Czech scientists used
computer simulations to identify the probable source of the Chicxulub impact. They calculated a 90% probability that a giant asteroid named
Baptistina, approximately in diameter, orbiting in the asteroid belt which lies between
Mars and
Jupiter, was struck by a smaller unnamed asteroid about 55 kilometers (35 mi) in diameter about 160 million years ago. The impact shattered Baptistina, creating a cluster which still exists today as the
Baptistina family. Calculations indicate that some of the fragments were sent hurtling into earth-crossing orbits, one of which was the wide
meteorite which struck
Mexico's
Yucatan peninsula 65 million years ago, creating the
Chicxulub crater .
While similar to Alvarez's impact theory (which involved a single asteroid or comet), this theory proposes that "passages of the
solar companion star Nemesis through the
Oort comet cloud would trigger comet showers." 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.
The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulphuric aerosols into the air which might have blocked sunlight and thereby reducing photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the
greenhouse effect when the dust and aerosols cleared from the atmosphere.
Possible Paleocene survivors
Nonavian dinosaur remains are occasionally found above the K-T boundary. In 2002, paleontologists Zielinski and Budahn reported the discovery of a single
hadrosaur leg bone fossil in the San Juan Basin, New Mexico and described it as evidence of
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 wasn't 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. Other evidence includes the finding of dinosaur remains in the
Hell Creek Formation up to 1.3 meters (51 in) above (40,000 years later than) the K-T boundary. Similar reports have come from other parts of the world, including China. Many scientists, however, dismiss the "Paleocene dinosaurs" as re-worked, for example washed out of their original locations and then re-buried in much later sediments, or find that, if correct, the presence of a handful of dinosaurs in the early Paleocene wouldn't change the underlying facts of the extinction. Villagers in central China have been digging up dinosaur bones for decades, thinking they were from dragons, to make traditional medicine. In Europe, dinosaur fossils were generally believed to be the remains of
giants and other creatures killed by the
Great Flood.
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
Chipping Norton,
Oxfordshire,
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 genus 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 hadn't been correctly discerned). The creature was named
Hadrosaurus foulkii. It was an extremely important find;
Hadrosaurus was the one of the first nearly complete dinosaur skeletons found and it was clearly a
bipedal creature. (
The first was in 1834, in Maidstone, Kent, England) This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke's discoveries sparked a wave of dinosaur mania in the
United States.
Dinosaur mania was exemplified by the fierce rivalry between
Edward Drinker Cope and
Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the
Bone Wars. The feud probably originated when Marsh publicly pointed out that Cope's reconstruction of an
Elasmosaurus skeleton was flawed; Cope had inadvertently placed the
plesiosaur's head at what should have been the animal's tail end. The fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh 'won' the contest primarily because he was better funded through a relationship with the
US Geological Survey. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair's rough methods; for example, their diggers often used
dynamite to unearth bones (a method modern paleontologists would find appalling). Despite their unrefined methods, the contributions of Cope and Marsh 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.
The "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 predator that may have been
warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and
cold-blooded.
Vertebrate paleontology 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.
Cultural depictions
By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they've captured the public imagination and become an enduring part of human culture. Only three decades after the first scientific descriptions of dinosaur remains, the famous
dinosaur sculptures were erected in
Crystal Palace Park in
London. These sculptures excited the public so strongly that smaller replicas were sold, one of the first examples of
tie-in merchandising. Since Crystal Palace, dinosaur exhibitions have opened at parks and
museums around the world, both catering to, and reinforcing, the public interest. Dinosaur popularity has long had a reciprocal effect on dinosaur science, as well. The competition between museums for public attention led directly to the
Bone Wars waged between Marsh and Cope, each striving to return with more spectacular fossil remains than the other, and the resulting contribution to dinosaur science was enormous.
Dinosaurs hold an integral place in modern culture. The word "dinosaur" itself has entered the English
lexicon as an expression describing anything that's impractically large, slow-moving, or obsolete, bound for extinction. The public preoccupation with dinosaurs led to their inevitable entrance into worldwide
popular culture. Beginning with a passing mention of
Megalosaurus in the first paragraph of
Charles Dickens'
Bleak House in 1852, dinosaurs have been featured in a broad array of
fictional works.
Sir Arthur Conan Doyle's 1912 book
The Lost World, the iconic 1933
film King Kong, the 1954 introduction of
Godzilla and its many subsequent sequels, the best-selling 1990 novel
Jurassic Park by
Michael Crichton and its 1993
film version, briefly the
highest-grossing film of all time, are just a few prominent examples of the long tradition of dinosaurs in fiction.
Non-fiction authors, including some prominent paleontologists, have also sought to take advantage of dinosaur popularity, especially among children, to educate readers about dinosaurs in particular and science in general. Dinosaurs are ubiquitous in
advertising, with numerous
companies seeking to utilize dinosaurs to sell their own products or to characterize their rivals as slow-moving or obsolete.
Religious views
Various religious groups have views about dinosaurs that differ from those held by the vast majority of scientists, usually due to conflicts with
creation stories in their scriptures. However, most of the
scientific community rejects these religiously-inspired interpretations of dinosaurs.
Further Information
Get more info on 'Dinosaurs'.
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