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, a catastrophic
extinction event ended the dominance of dinosaurs on land. One
group of dinosaurs is known to have survived to the present day: taxonomists believe modern
birds are direct descendants of 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, 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 none of these were dinosaurs.
What is a dinosaur?
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.[1] 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.[2]
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.[3] An earlier study
predicted that about 3,400 dinosaur genera existed, including many which would not have been preserved in the fossil
record.[4] 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.
Distinguishing features of dinosaurs
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 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 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.[5]
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. Such shared features include a diapsid skull bearing two pairs of holes in
the temporal region; holes in the snout and lower jaw (two characteristics shared by other archosaurs); loss of the skull's
postfrontal bone; a long neck incorporating an S-shaped curve;[6] an elongated scapula, or shoulder blade; forelimbs shorter and
lighter than hind limbs, coupled to asymmetrical hands; a sacrum composed of three or more fused
vertebrae; and an acetabulum, or hip socket, with a hole at
the center of its inside surface.[7]
The open, or "perforate", hip joint described above had significant implications for dinosaur movement and behavior. Most
notably, it allowed dinosaur hind limbs to be "underslung", or situated directly beneath the animals' bodies; this, in turn,
allowed dinosaurs to stand erect in a manner similar to modern mammals, but distinct from most other reptiles, whose limbs sprawl
out to either side.[8] Vertical limb
configuration also enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that
surpassed those of "sprawling" reptiles.
Phylogenetic definition
Under phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the
most recent common ancestor of Triceratops and modern birds.[9] 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.[10] 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.[8] 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.
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.[11]
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.
Size
Comparative size of
Diplodocus; human figures provide scale.
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 over 30 m
(100 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.[12] 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.[13]
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.
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;[14] this
mount is 12 m (38 ft) tall, 22.5 m (74 ft) long, and would have belonged to an animal that 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 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 80,000–100,000 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 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 58 m (190 ft) long and weighed over 120,000 kg
(132 tons),[15] heavier than all known
dinosaurs except possibly the poorly known Bruhathkayosaurus, which could have
weighed 175,000–220,000 kg (193–243 tons). The largest known carnivorous dinosaur
was Spinosaurus, reaching a length of 16–18 meters (53–60 ft), and weighing in
at 9 tons.[16] 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 60 cm (2 ft) in length.
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 direct evidence of herding behavior 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.[17] Despite the
deposition of those skeletons being now regarded as more gradual,[18] other, well supported, mass death sites were 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,[19] 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.[20] 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.[21] Trackways have also confirmed parental behavior among sauropods and
ornithopods from the Isle of Skye in northwestern Scotland.[22] 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 marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, so they were likely used for sexual
or aggressive displays, though little is known about dinosaur mating and territorialism. The nature of dinosaur communication
also remains enigmatic, and is an active area of research. For example, recent evidence suggests that the hollow crests of the
lambeosaurines may have functioned as resonance chambers used for a wide range of
vocalizations.
From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a
Protoceratops,[23] proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise,[24] this too was confirmed by tooth marks from Madagascar in
2003.[25]
Based on current fossil evidence only a single dinosaur, Oryctodromeus
cubicularis, shows adaptations suggestive of a partially fossorial lifestyle, and
relatively few were arboreal, most notably the primitive dromaeosaurids such as Microraptor. Since the later mammalian
radiation in the Cenozoic produced numerous burrowing and tree-climbing species, e.g.,
rodents and primates, the lack of evidence for a similar
radiation of species among the dinosaurs is somewhat surprising. Because most dinosaur species 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,[26][27] whether diplodocids could create sonic booms via whip-like tail snapping,[28] whether giant theropods had to slow down when rushing for food to avoid fatal
injuries,[29] and whether sauropods could float.[30]
Evolution of dinosaurs
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.[31][32] 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;[33] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[34] 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.
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.[35][36] 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 identical and closely related dinosaur species have been found on different continents, in accordance with
the generally-accepted theory that all land masses were once connected in a super-continent called Pangaea.[37]
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, vicious predator that may have been warm-blooded, in marked contrast to the then-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
-
Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes, called
temporal fenestrae, located where the jaw muscles attach. 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
ornitheios (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".)
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Edmontosaurus pelvis (showing ornithischian structure - left side)
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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
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