Dinosaurs are a diverse group of reptiles
of the clade Dinosauria.
They first appeared during the Triassic period,
between 243 and 233.23 million years ago,
although the exact origin and timing of the
evolution of dinosaurs is the subject of active
research.
They became the dominant terrestrial vertebrates
after the Triassic–Jurassic extinction event
201 million years ago; their dominance continued
through the Jurassic and Cretaceous periods.
Reverse genetic engineering and the fossil
record both demonstrate that birds are modern
feathered dinosaurs, having evolved from earlier
theropods during the late Jurassic Period.
As such, birds were the only dinosaur lineage
to survive the Cretaceous–Paleogene extinction
event 66 million years ago.
Dinosaurs can therefore be divided into avian
dinosaurs, or birds; and non-avian dinosaurs,
which are all dinosaurs other than birds.
This article deals primarily with non-avian
dinosaurs.
Dinosaurs are a varied group of animals from
taxonomic, morphological and ecological standpoints.
Birds, at over 10,000 living species, are
the most diverse group of vertebrates besides
perciform fish.
Using fossil evidence, paleontologists have
identified over 500 distinct genera and more
than 1,000 different species of non-avian
dinosaurs.
Dinosaurs are represented on every continent
by both extant species (birds) and fossil
remains.
Through the first half of the 20th century,
before birds were recognized to be dinosaurs,
most of the scientific community believed
dinosaurs to have been sluggish and cold-blooded.
Most research conducted since the 1970s, however,
has indicated that all dinosaurs were active
animals with elevated metabolisms and numerous
adaptations for social interaction.
Some were herbivorous, others carnivorous.
Evidence suggests that egg-laying and nest-building
are additional traits shared by all dinosaurs,
avian and non-avian alike.
While dinosaurs were ancestrally bipedal,
many extinct groups included quadrupedal species,
and some were able to shift between these
stances.
Elaborate display structures such as horns
or crests are common to all dinosaur groups,
and some extinct groups developed skeletal
modifications such as bony armor and spines.
While the dinosaurs' modern-day surviving
avian lineage (birds) are generally small
due to the constraints of flight, many prehistoric
dinosaurs (non-avian and avian) were large-bodied—the
largest sauropod dinosaurs are estimated to
have reached lengths of 39.7 meters (130 feet)
and heights of 18 meters (59 feet) and were
the largest land animals of all time.
Still, the idea that non-avian dinosaurs were
uniformly gigantic is a misconception based
in part on preservation bias, as large, sturdy
bones are more likely to last until they are
fossilized.
Many dinosaurs were quite small: Xixianykus,
for example, was only about 50 cm (20 in)
long.
Since the first dinosaur fossils were recognized
in the early 19th century, mounted fossil
dinosaur skeletons have been major attractions
at museums around the world, and dinosaurs
have become an enduring part of world culture.
The large sizes of some dinosaur groups, as
well as their seemingly monstrous and fantastic
nature, have ensured dinosaurs' regular appearance
in best-selling books and films, such as Jurassic
Park.
Persistent public enthusiasm for the animals
has resulted in significant funding for dinosaur
science, and new discoveries are regularly
covered by the media.
== Etymology ==
The taxon 'Dinosauria' was formally named
in 1841 by paleontologist Sir 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 Ancient Greek δεινός
(deinos), meaning 'terrible, potent or fearfully
great', and σαῦρος (sauros), 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.Other
prehistoric animals, including pterosaurs,
mosasaurs, ichthyosaurs, plesiosaurs, and
Dimetrodon, while often popularly conceived
of as dinosaurs, are not taxonomically classified
as dinosaurs.
Pterosaurs are distantly related to dinosaurs,
being members of the clade Ornithodira.
The other groups mentioned are, like dinosaurs
and pterosaurs, members of Sauropsida (the
reptile and bird clade), except Dimetrodon
(which is a synapsid).
== Definition ==
Under phylogenetic nomenclature, dinosaurs
are usually defined as the group consisting
of the most recent common ancestor (MRCA)
of Triceratops and Neornithes, and all its
descendants.
It has also been suggested that Dinosauria
be defined with respect to the MRCA 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: "Dinosauria
= Ornithischia + Saurischia", encompassing
ankylosaurians (armored herbivorous quadrupeds),
stegosaurians (plated herbivorous quadrupeds),
ceratopsians (herbivorous quadrupeds with
horns and frills), ornithopods (bipedal or
quadrupedal herbivores including "duck-bills"),
theropods (mostly bipedal carnivores and birds),
and sauropodomorphs (mostly large herbivorous
quadrupeds with long necks and tails).Birds
are now recognized as being the sole surviving
lineage of theropod dinosaurs.
In traditional taxonomy, birds were considered
a separate class that had evolved from dinosaurs,
a distinct superorder.
However, a majority of contemporary paleontologists
concerned with dinosaurs reject the traditional
style of classification in favor of phylogenetic
taxonomy; this approach requires that, for
a group to be natural, all descendants of
members of the group must be included in the
group as well.
Birds are thus considered to be dinosaurs
and dinosaurs are, therefore, not extinct.
Birds are classified as belonging to the subgroup
Maniraptora, which are coelurosaurs, which
are theropods, which are saurischians, which
are dinosaurs.Research by Matthew Baron, David
B. Norman, and Paul M. Barrett in 2017 suggested
a radical revision of dinosaurian systematics.
Phylogenetic analysis by Baron et al. recovered
the Ornithischia as being closer to the Theropoda
than the Sauropodomorpha, as opposed to the
traditional union of theropods with sauropodomorphs.
They resurrected the clade Ornithoscelida
to refer to the group containing Ornithischia
and Theropoda.
Dinosauria itself was re-defined as the last
common ancestor of Triceratops horridus, Passer
domesticus, Diplodocus carnegii, and all of
its descendants, to ensure that sauropods
and kin remain included as dinosaurs.
=== General description ===
Using one of the above definitions, dinosaurs
can be generally described as archosaurs with
hind limbs held erect beneath the body.
Many prehistoric animal groups are popularly
conceived of as dinosaurs, such as ichthyosaurs,
mosasaurs, plesiosaurs, pterosaurs, and pelycosaurs
(especially Dimetrodon), but are not classified
scientifically as dinosaurs, and none had
the erect hind limb posture characteristic
of true dinosaurs.
Dinosaurs were the dominant terrestrial vertebrates
of the Mesozoic, especially the Jurassic and
Cretaceous periods.
Other groups of animals were restricted in
size and niches; mammals, for example, rarely
exceeded the size of a domestic cat, and were
generally rodent-sized carnivores of small
prey.Dinosaurs have always been an extremely
varied group of animals; according to a 2006
study, over 500 non-avian 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.
An earlier study predicted that about 3,400
dinosaur genera existed, including many that
would not have been preserved in the fossil
record.
By September 17, 2008, 1,047 different species
of dinosaurs had been named.In 2016, the estimated
number of dinosaur species that existed in
the Mesozoic era was estimated to be 1,543–2,468.
Some are herbivorous, others carnivorous,
including seed-eaters, fish-eaters, insectivores,
and omnivores.
While dinosaurs were ancestrally bipedal (as
are all modern birds), some prehistoric species
were quadrupeds, and others, such as Anchisaurus
and Iguanodon, could walk just as easily on
two or four legs.
Cranial modifications like horns and crests
are common dinosaurian traits, and some extinct
species had bony armor.
Although known for large size, many Mesozoic
dinosaurs were human-sized or smaller, and
modern birds are generally small in size.
Dinosaurs today inhabit every continent, and
fossils show that they had achieved global
distribution by at least the early Jurassic
period.
Modern birds inhabit most available habitats,
from terrestrial to marine, and there is evidence
that some non-avian dinosaurs (such as Microraptor)
could fly or at least glide, and others, such
as spinosaurids, had semiaquatic habits.
=== 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 archosaurian
skeleton, or are clear descendants of older
dinosaurs showing these modifications.
Although some later groups of dinosaurs featured
further modified versions of these traits,
they are considered typical for Dinosauria;
the earliest dinosaurs had them and passed
them on to their descendants.
Such modifications, originating in the most
recent common ancestor of a certain taxonomic
group, are called the synapomorphies of such
a group.A detailed assessment of archosaur
interrelations by Sterling Nesbitt confirmed
or found the following twelve unambiguous
synapomorphies, some previously known:
in the skull, a supratemporal fossa (excavation)
is present in front of the supratemporal fenestra,
the main opening in the rear skull roof
epipophyses, obliquely backward pointing processes
on the rear top corners, present in the anterior
(front) neck vertebrae behind the atlas and
axis, the first two neck vertebrae
apex of deltopectoral crest (a projection
on which the deltopectoral muscles attach)
located at or more than 30% down the length
of the humerus (upper arm bone)
radius, a lower arm bone, shorter than 80%
of humerus length
fourth trochanter (projection where the caudofemoralis
muscle attaches on the inner rear shaft) on
the femur (thighbone) is a sharp flange
fourth trochanter asymmetrical, with distal,
lower, margin forming a steeper angle to the
shaft
on the astragalus and calcaneum, upper ankle
bones, the proximal articular facet, the top
connecting surface, for the fibula occupies
less than 30% of the transverse width of the
element
exoccipitals (bones at the back of the skull)
do not meet along the midline on the floor
of the endocranial cavity, the inner space
of the braincase
in the pelvis, the proximal articular surfaces
of the ischium with the ilium and the pubis
are separated by a large concave surface (on
the upper side of the ischium a part of the
open hip joint is located between the contacts
with the pubic bone and the ilium)
cnemial crest on the tibia (protruding part
of the top surface of the shinbone) arcs anterolaterally
(curves to the front and the outer side)
distinct proximodistally oriented (vertical)
ridge present on the posterior face of the
distal end of the tibia (the rear surface
of the lower end of the shinbone)
concave articular surface for the fibula of
the calcaneum (the top surface of the calcaneum,
where it touches the fibula, has a hollow
profile)Nesbitt found a number of further
potential synapomorphies, and discounted a
number of synapomorphies previously suggested.
Some of these are also present in silesaurids,
which Nesbitt recovered as a sister group
to Dinosauria, including a large anterior
trochanter, metatarsals II and IV of subequal
length, reduced contact between ischium and
pubis, the presence of a cnemial crest on
the tibia and of an ascending process on the
astragalus, and many others.
A variety of other skeletal features are shared
by dinosaurs.
However, because they are 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 diapsids, 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);
and a perforate acetabulum, or hip socket,
with a hole at the center of its inside surface
(closed in Saturnalia, for example).
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 stand with their hind limbs erect
in a manner similar to most modern mammals,
but distinct from most other reptiles, whose
limbs sprawl out to either side.
This posture is 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 early 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.
== Evolutionary history ==
=== 
Origins and early evolution ===
Dinosaurs diverged from their archosaur ancestors
during the middle to late Triassic period,
roughly 20 million years after the Permian–Triassic
extinction event wiped out an estimated 95%
of all life on Earth.
Radiometric dating of the rock formation that
contained fossils from the early dinosaur
genus Eoraptor at 231.4 million years old
establishes its presence in the fossil record
at this time.
Paleontologists think that 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.
Dinosaurs may have appeared as early as 243
million years ago, as evidenced by remains
of the genus Nyasasaurus from that period,
though known fossils of these animals are
too fragmentary to tell if they are dinosaurs
or very close dinosaurian relatives.
Recently, it has been determined that Staurikosaurus
from the Santa Maria Formation dates to 233.23
Ma, making it older in geologic age than Eoraptor.When
dinosaurs appeared, they were not the dominant
terrestrial animals.
The terrestrial habitats were occupied by
various types of archosauromorphs and therapsids,
like cynodonts and rhynchosaurs.
Their main competitors were the pseudosuchia,
such as aetosaurs, ornithosuchids and rauisuchians,
which were more successful than the dinosaurs.
Most of these other animals became extinct
in the Triassic, in one of two events.
First, at about 215 million years ago, a variety
of basal archosauromorphs, including the protorosaurs,
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.
Rhynchosaurs and dicynodonts survived (at
least in some areas) at least as late as early-mid
Norian and late Norian or earliest Rhaetian,
respectively, and the exact date of their
extinction is uncertain.
These losses left behind a land fauna of crocodylomorphs,
dinosaurs, mammals, pterosaurians, and turtles.
The first few lines of early dinosaurs diversified
through the Carnian and Norian stages of the
Triassic, possibly by occupying the niches
of the groups that became extinct.
Also notably, there was a heightened rate
of extinction during the Carnian Pluvial Event.
=== Evolution and paleobiogeography ===
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
Pangaea, and there was a worldwide dinosaur
fauna mostly composed of coelophysoid carnivores
and early sauropodomorph herbivores.
Gymnosperm plants (particularly conifers),
a potential food source, radiated in the late
Triassic.
Early sauropodomorphs did not have sophisticated
mechanisms for processing food in the mouth,
and 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.
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.
Another notable evolutionary event of the
Jurassic was the appearance of true birds,
descended from maniraptoran coelurosaurians.
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, iguanodontians, and brachiosaurids
through Europe, North America, and northern
Africa.
These were later supplemented or replaced
in Africa by large spinosaurid and carcharodontosaurid
theropods, and rebbachisaurid and titanosaurian
sauropods, also found in South America.
In Asia, maniraptoran coelurosaurians like
dromaeosaurids, troodontids, and oviraptorosaurians
became the common theropods, and ankylosaurids
and early ceratopsians like Psittacosaurus
became important herbivores.
Meanwhile, Australia was home to a fauna of
basal ankylosaurians, hypsilophodonts, and
iguanodontians.
The stegosaurians appear to have gone extinct
at some point in the late early Cretaceous
or early late Cretaceous.
A major change in the early Cretaceous, which
would be amplified in the late Cretaceous,
was the evolution of flowering plants.
At the same time, several groups of dinosaurian
herbivores evolved more sophisticated ways
to orally process food.
Ceratopsians developed a method of slicing
with teeth stacked on each other in batteries,
and iguanodontians refined a method of grinding
with tooth batteries, taken to its extreme
in hadrosaurids.
Some sauropods also evolved tooth batteries,
best exemplified by the rebbachisaurid Nigersaurus.
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.
Flowering plants were greatly radiating, with
the first grasses appearing by the end of
the Cretaceous.
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.The Cretaceous–Paleogene
extinction event, which occurred approximately
66 million years ago at the end of the Cretaceous
period, caused the extinction of all dinosaur
groups except for the neornithine birds.
Some other diapsid groups, such as crocodilians,
sebecosuchians, turtles, lizards, snakes,
sphenodontians, and choristoderans, also survived
the event.The surviving lineages of neornithine
birds, including the ancestors of modern ratites,
ducks and chickens, and a variety of waterbirds,
diversified rapidly at the beginning of the
Paleogene period, entering ecological niches
left vacant by the extinction of Mesozoic
dinosaur groups such as the arboreal enantiornithines,
aquatic hesperornithines, and even the larger
terrestrial theropods (in the form of Gastornis,
eogruiids, bathornithids, ratites, geranoidids,
mihirungs, and "terror birds").
It is often cited that mammals out-competed
the neornithines for dominance of most terrestrial
niches but many of these groups co-existed
with rich mammalian faunas for most of the
Cenozoic.
Terror birds and bathornithids occupied carnivorous
guilds alongside predatory mammals, and ratites
are still fairly successful as mid-sized herbivores;
eogruiids similarly lasted from the Eocene
to Pliocene, only becoming extinct very recently
after over 20 million years of co-existence
with many mammal groups.
== Classification ==
Dinosaurs belong to a group known as archosaurs,
which also includes modern crocodilians.
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
as a clade are divided into two primary branches,
Saurischia and Ornithischia.
Saurischia includes those taxa sharing a more
recent common ancestor with birds than with
Ornithischia, while Ornithischia includes
all taxa sharing a more recent common ancestor
with Triceratops than with Saurischia.
Anatomically, these two groups can be distinguished
most noticeably by their pelvic structure.
Early saurischians—"lizard-hipped", from
the Greek sauros (σαῦρος) meaning "lizard"
and ischion (ἰσχίον) meaning "hip joint"—retained
the hip structure of their ancestors, with
a pubis bone directed cranially, or forward.
This basic form was modified by rotating the
pubis backward to varying degrees in several
groups (Herrerasaurus, therizinosauroids,
dromaeosaurids, and birds).
Saurischia includes the theropods (exclusively
bipedal and with a wide variety of diets)
and sauropodomorphs (long-necked herbivores
which include advanced, quadrupedal groups).By
contrast, ornithischians—"bird-hipped",
from the Greek ornitheios (ὀρνίθειος)
meaning "of a bird" and ischion (ἰσχίον)
meaning "hip joint"—had a pelvis that superficially
resembled a bird's pelvis: the pubic bone
was oriented caudally (rear-pointing).
Unlike birds, the ornithischian pubis also
usually had an additional forward-pointing
process.
Ornithischia includes a variety of species
which were primarily herbivores.
(NB: the terms "lizard hip" and "bird hip"
are misnomers – birds evolved from dinosaurs
with "lizard hips".)
=== Taxonomy ===
The following is a simplified classification
of dinosaur groups based on their evolutionary
relationships, and organized based on the
list of Mesozoic dinosaur species provided
by Holtz (2007).
A more detailed version can be found at Dinosaur
classification.
The dagger (†) is used to signify groups
with no living members.
DinosauriaSaurischia ("lizard-hipped"; includes
Theropoda and Sauropodomorpha)†Herrerasauria
(early bipedal carnivores)
Theropoda (all bipedal; most were carnivorous)
†Coelophysoidea (small, early theropods;
includes Coelophysis and close relatives)
†Dilophosauridae (early crested and carnivorous
theropods)
†Ceratosauria (generally elaborately horned,
the dominant southern carnivores of the Cretaceous)
Tetanurae ("stiff tails"; includes most theropods)†Megalosauroidea
(early group of large carnivores including
the semiaquatic spinosaurids)
†Carnosauria (Allosaurus and close relatives,
like Carcharodontosaurus)
Coelurosauria (feathered theropods, with a
range of body sizes and niches)†Compsognathidae
(common early coelurosaurs with reduced forelimbs)
†Tyrannosauridae (Tyrannosaurus and close
relatives; had reduced forelimbs)
†Ornithomimosauria ("ostrich-mimics"; mostly
toothless; carnivores to possible herbivores)
†Alvarezsauroidea (small insectivores with
reduced forelimbs each bearing one enlarged
claw)
Maniraptora ("hand snatchers"; had long, slender
arms and fingers)†Therizinosauria (bipedal
herbivores with large hand claws and small
heads)
†Oviraptorosauria (mostly toothless; their
diet and lifestyle are uncertain)
†Archaeopterygidae (small, winged theropods
or primitive birds)
†Deinonychosauria (small- to medium-sized;
bird-like, with a distinctive toe claw)
Avialae (modern birds and extinct relatives)†Scansoriopterygidae
(small primitive avialans with long third
fingers)
†Omnivoropterygidae (large, early short-tailed
avialans)
†Confuciusornithidae (small toothless avialans)
†Enantiornithes (primitive tree-dwelling,
flying avialans)
Euornithes (advanced flying birds)†Yanornithiformes
(toothed Cretaceous Chinese birds)
†Hesperornithes (specialized aquatic diving
birds)
Aves (modern, beaked birds and their extinct
relatives)
†Sauropodomorpha (herbivores with small
heads, long necks, long tails)†Guaibasauridae
(small, primitive, omnivorous sauropodomorphs)
†Plateosauridae (primitive, strictly bipedal
"prosauropods")
†Riojasauridae (small, primitive sauropodomorphs)
†Massospondylidae (small, primitive sauropodomorphs)
†Sauropoda (very large and heavy, usually
over 15 m (49 ft) long; quadrupedal)†Vulcanodontidae
(primitive sauropods with pillar-like limbs)
†Eusauropoda ("true sauropods")†Cetiosauridae
("whale reptiles")
†Turiasauria (European group of Jurassic
and Cretaceous sauropods)
†Neosauropoda ("new sauropods")†Diplodocoidea
(skulls and tails elongated; teeth typically
narrow and pencil-like)
†Macronaria (boxy skulls; spoon- or pencil-shaped
teeth)†Brachiosauridae (long-necked, long-armed
macronarians)
†Titanosauria (diverse; stocky, with wide
hips; most common in the late Cretaceous of
southern continents)
†Ornithischia ("bird-hipped"; diverse bipedal
and quadrupedal herbivores)†Heterodontosauridae
(small basal ornithopod herbivores/omnivores
with prominent canine-like teeth)
†Thyreophora (armored dinosaurs; mostly
quadrupeds)†Ankylosauria (scutes as primary
armor; some had club-like tails)
†Stegosauria (spikes and plates as primary
armor)†Neornithischia ("new ornithischians")†Ornithopoda
(various sizes; bipeds and quadrupeds; evolved
a method of chewing using skull flexibility
and numerous teeth)
†Marginocephalia (characterized by a cranial
growth)†Pachycephalosauria (bipeds with
domed or knobby growth on skulls)
†Ceratopsia (quadrupeds with frills; many
also had horns)
== Biology ==
Knowledge about dinosaurs is derived from
a variety of fossil and non-fossil records,
including fossilized bones, feces, trackways,
gastroliths, feathers, impressions of skin,
internal organs and soft tissues.
Many fields of study contribute to our understanding
of dinosaurs, including physics (especially
biomechanics), chemistry, biology, and the
earth sciences (of which paleontology is a
sub-discipline).
Two topics of particular interest and study
have been dinosaur size and behavior.
=== Size ===
Current evidence suggests that dinosaur average
size varied through the Triassic, early Jurassic,
late Jurassic and Cretaceous periods.
Predatory theropod dinosaurs, which occupied
most terrestrial carnivore niches during the
Mesozoic, most often fall into the 100 to
1000 kg (220 to 2200 lb) category when sorted
by estimated weight into categories based
on order of magnitude, whereas recent predatory
carnivoran mammals peak in the 10 to 100 kg
(22 to 220 lb) category.
The mode of Mesozoic dinosaur body masses
is between one and ten metric tonnes.
This contrasts sharply with the average size
of Cenozoic mammals, estimated by the National
Museum of Natural History as about 2 to 5
kg (4.4 to 11.0 lb).The sauropods were the
largest and heaviest dinosaurs.
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 Paraceratherium
(the largest land mammal ever) were dwarfed
by the giant sauropods, and only modern whales
approach or surpass them in size.
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.
==== Largest and smallest ====
Scientists will probably never be certain
of the largest and smallest dinosaurs to have
ever existed.
This is because 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.
The tallest and heaviest dinosaur known from
good skeletons is Giraffatitan brancai (previously
classified as a species of Brachiosaurus).
Its remains were discovered in Tanzania between
1907 and 1912.
Bones from several similar-sized individuals
were incorporated into the skeleton now mounted
and on display at the Museum für Naturkunde
Berlin; this mount is 12 meters (39 ft) tall
and 21.8–22.5 meters (72–74 ft) long,
and would have belonged to an animal that
weighed between 30000 and 60000 kilograms
(70000 and 130000 lb).
The longest complete dinosaur is the 27 meters
(89 feet) long Diplodocus, which was discovered
in Wyoming in the United States and displayed
in Pittsburgh's Carnegie Natural History Museum
in 1907.
The longest dinosaur known from good fossil
material is the Patagotitan: the skeleton
mount in the American Museum of Natural History
is 37 meters (121 ft) long.
The Carmen Funes Museum has an Argentinosaurus
reconstructed skeleton mount 39.7 metres (130
ft) long.
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 discovered in the 1970s or
later, and include the massive Argentinosaurus,
which may have weighed 80000 to 100000 kilograms
(90 to 110 short tons) and reached length
of 30–40 metres (98–131 ft); some of the
longest were the 33.5 meters (110 ft) long
Diplodocus hallorum (formerly Seismosaurus),
the 33–34 meters (108–112 ft) long Supersaurus
and 37 metres (121 ft) long Patagotitan; and
the tallest, the 18 meters (59 ft) tall Sauroposeidon,
which could have reached a sixth-floor window.
The heaviest and longest dinosaur 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 meters (190
ft) long and weighed 122400 kg (270000 lb).
However, as no further evidence of sauropods
of this size has been found, and the discoverer,
Edward Cope, had made typographic errors before,
it is likely to have been an extreme overestimation.The
largest carnivorous dinosaur was Spinosaurus,
reaching a length of 12.6 to 18 meters (41
to 59 ft), and weighing 7–20.9 tonnes (7.7–23
short tons).
Other large carnivorous theropods included
Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.
Therizinosaurus and Deinocheirus were among
the tallest of the theropods.
The largest Ornithischian dinosaur was probably
the hadrosaurid Shantungosaurus which measured
16 metres (52 ft) and weighed about 13 tonnes
(29,000 lb).
The smallest dinosaur known is the bee hummingbird,
with a length of only 5 cm (2.0 in) and mass
of around 1.8 g (0.063 oz).
The smallest known non-avialan dinosaurs were
about the size of pigeons and were those theropods
most closely related to birds.
For example, Anchiornis huxleyi is currently
the smallest non-avialan dinosaur described
from an adult specimen, with an estimated
weight of 110 grams and a total skeletal length
of 34 cm (1.12 ft).
The smallest herbivorous non-avialan dinosaurs
included Microceratus and Wannanosaurus, at
about 60 cm (2.0 ft) long each.
=== Behavior ===
Many modern birds are highly social, often
found living in flocks.
There is general agreement that some behaviors
that are common in birds, as well as in crocodiles
(birds' closest living relatives), were also
common among extinct dinosaur groups.
Interpretations of behavior in fossil species
are generally based on the pose of skeletons
and their habitat, computer simulations of
their biomechanics, and comparisons with modern
animals in similar ecological niches.The first
potential evidence for herding or flocking
as a widespread behavior common to many dinosaur
groups in addition to birds was the 1878 discovery
of 31 Iguanodon bernissartensis, ornithischians
that 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 discovered
subsequently.
Those, along with multiple trackways, suggest
that gregarious behavior was common in many
early 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 Oxfordshire, England,
although there is no evidence for specific
herd structures.
Congregating into herds may have evolved for
defense, for migratory purposes, or to provide
protection for young.
There is evidence that many types of slow-growing
dinosaurs, including various theropods, sauropods,
ankylosaurians, ornithopods, and ceratopsians,
formed aggregations of immature individuals.
One example is a site in Inner Mongolia that
has yielded the remains of over 20 Sinornithomimus,
from one to seven years old.
This assemblage is interpreted as a social
group that was trapped in mud.
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
modern birds, crocodiles, and other reptiles,
and the taphonomic evidence suggesting mammal-like
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.
The crests and frills of some dinosaurs, like
the marginocephalians, theropods and 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.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.
Cannibalism amongst some species of dinosaurs
was confirmed by tooth marks found in Madagascar
in 2003, involving the theropod Majungasaurus.Comparisons
between the scleral rings of dinosaurs and
modern birds and reptiles have been used to
infer daily activity patterns of dinosaurs.
Although it has been suggested that most dinosaurs
were active during the day, these comparisons
have shown that small predatory dinosaurs
such as dromaeosaurids, Juravenator, and Megapnosaurus
were likely nocturnal.
Large and medium-sized herbivorous and omnivorous
dinosaurs such as ceratopsians, sauropodomorphs,
hadrosaurids, ornithomimosaurs may have been
cathemeral, active during short intervals
throughout the day, although the small ornithischian
Agilisaurus was inferred to be diurnal.Based
on current fossil evidence from dinosaurs
such as Oryctodromeus, some ornithischian
species seem to have led a partially fossorial
(burrowing) lifestyle.
Many modern birds are arboreal (tree climbing),
and this was also true of many Mesozoic birds,
especially the enantiornithines.
While some early bird-like species may have
already been arboreal as well (including dromaeosaurids
such as Microraptor) most non-avialan 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, pioneered
by Robert McNeill Alexander, 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.
=== Communication ===
Modern birds are known to communicate using
visual and auditory signals, and the wide
diversity of visual display structures among
fossil dinosaur groups, such as horns, frills,
crests, sails and feathers, suggests that
visual communication has always been important
in dinosaur biology.
Reconstruction of the plumage color of Anchiornis
huxleyi, suggest the importance of color in
visual communication in non-avian dinosaurs.
The evolution of dinosaur vocalization is
less certain.
Paleontologist Phil Senter suggests that non-avian
dinosaurs relied mostly on visual displays
and possibly non-vocal acoustic sounds like
hissing, jaw grinding or clapping, splashing
and wing beating (possible in winged maniraptoran
dinosaurs).
He states they were unlikely to have been
capable of vocalizing since their closest
relatives, crocodilians and birds, use different
means to vocalize, the former via the larynx
and the latter through the unique syrinx,
suggesting they evolved independently and
their common ancestor was mute.The earliest
remains of a syrinx, which has enough mineral
content for fossilization, was found in a
specimen of the duck-like Vegavis iaai dated
69-66 million year ago, and this organ is
unlikely to have existed in non-avian dinosaurs.
However, in contrast to Senter, the researchers
have suggested that dinosaurs could vocalize
and that the syrinx-based vocal system of
birds evolved from a larynx-based one, rather
than the two systems evolving independently.
A 2016 study suggests that dinosaurs produced
closed mouth vocalizations like cooing, which
occur in both crocodilians and birds as well
as other reptiles.
Such vocalizations evolved independently in
extant archosaurs numerous times, following
increases in body size.
The crests of the Lambeosaurini and nasal
chambers of ankylosaurids have been suggested
to function in vocal resonance, though Senter
states that the presence of resonance chambers
in some dinosaurs is not necessarily evidence
of vocalization as modern snakes have such
chambers which intensify their hisses.
=== Reproductive biology ===
All dinosaurs lay amniotic eggs with hard
shells made mostly of calcium carbonate.
Eggs are usually laid in a nest.
Most species create somewhat elaborate nests,
which can be cups, domes, plates, beds scrapes,
mounds, or burrows.
Some species of modern bird have no nests;
the cliff-nesting common guillemot lays its
eggs on bare rock, and male emperor penguins
keep eggs between their body and feet.
Primitive birds and many non-avialan dinosaurs
often lay eggs in communal nests, with males
primarily incubating the eggs.
While modern birds have only one functional
oviduct and lay one egg at a time, more primitive
birds and dinosaurs had two oviducts, like
crocodiles.
Some non-avialan dinosaurs, such as Troodon,
exhibited iterative laying, where the adult
might lay a pair of eggs every one or two
days, and then ensured simultaneous hatching
by delaying brooding until all eggs were laid.When
laying eggs, females 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.
A discovery of features in a Tyrannosaurus
rex skeleton provided evidence of medullary
bone in extinct dinosaurs and, for the first
time, allowed paleontologists to establish
the sex of a fossil dinosaur specimen.
Further research has found medullary bone
in the carnosaur 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 the production of medullary tissue is
a general characteristic of all dinosaurs.
Another widespread trait among modern birds
(but see below in regards to fossil groups
and extant megapodes) is parental care for
young after hatching.
Jack Horner's 1978 discovery of a Maiasaura
("good mother lizard") nesting ground in Montana
demonstrated that parental care continued
long after birth among ornithopods.
A specimen of the Mongolian oviraptorid Citipati
osmolskae was discovered in a chicken-like
brooding position in 1993, which may indicate
that they had begun using an insulating layer
of feathers to keep the eggs warm.
A dinosaur embryo (pertaining to the prosauropod
Massospondylus) was found without teeth, indicating
that some parental care was required to feed
the young dinosaurs.
Trackways have also confirmed parental behavior
among ornithopods from the Isle of Skye in
northwestern Scotland.However, there is ample
evidence of supreprecociality among many dinosaur
species, particularly theropods.
For instance, non-ornithuromorph birds have
been abundantly demonstrated to have had slow
growth rates, megapode-like egg burying behaviour
and the ability to fly soon after birth.
Both Tyrannosaurus rex and Troodon formosus
display juveniles with clear supreprecociality
and likely occupying different ecological
niches than the adults.
Superprecociality has been inferred for sauropods.
=== Physiology ===
Because both modern crocodilians and birds
have four-chambered hearts (albeit modified
in crocodilians), it is likely that this is
a trait shared by all archosaurs, including
all dinosaurs.
While all modern birds have high metabolisms
and are "warm blooded" (endothermic), a vigorous
debate has been ongoing since the 1960s regarding
how far back in the dinosaur lineage this
trait extends.
Scientists disagree as to whether non-avian
dinosaurs were endothermic, ectothermic, or
some combination of both.After non-avian dinosaurs
were discovered, paleontologists first posited
that they were ectothermic.
This supposed "cold-bloodedness" was used
to imply that the ancient dinosaurs were relatively
slow, sluggish organisms, even though many
modern reptiles are fast and light-footed
despite relying on external sources of heat
to regulate their body temperature.
The idea of dinosaurs as ectothermic and sluggish
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 even non-avian dinosaurs and birds thrived
in cooler temperate climates, and that at
least some early species must have regulated
their body temperature by internal biological
means (aided by the animals' bulk in large
species and feathers or other body coverings
in smaller species).
Evidence of endothermy in Mesozoic dinosaurs
includes the discovery of polar dinosaurs
in Australia and Antarctica as well as analysis
of blood-vessel structures within fossil bones
that are typical of endotherms.
Scientific debate continues regarding the
specific ways in which dinosaur temperature
regulation evolved.
In saurischian dinosaurs, higher metabolisms
were supported by the evolution of the avian
respiratory system, characterized by an extensive
system of air sacs that extended the lungs
and invaded many of the bones in the skeleton,
making them hollow.
Early avian-style respiratory systems with
air sacs may have been capable of sustaining
higher activity levels than those of mammals
of similar size and build.
In addition to providing a very efficient
supply of oxygen, the rapid airflow would
have been an effective cooling mechanism,
which is essential for animals that are active
but too large to get rid of all the excess
heat through their skin.Like other reptiles,
dinosaurs are primarily uricotelic, that is,
their kidneys extract nitrogenous wastes from
their bloodstream and excrete it as uric acid
instead of urea or ammonia via the ureters
into the intestine.
In most living species, uric acid is excreted
along with feces as a semisolid waste.
However, at least some modern birds (such
as hummingbirds) can be facultatively ammonotelic,
excreting most of the nitrogenous wastes as
ammonia.
They also excrete creatine, rather than creatinine
like mammals.
This material, as well as the output of the
intestines, emerges from the cloaca.
In addition, many species regurgitate pellets,
and fossil pellets that may have come from
dinosaurs are known from as long ago as the
Cretaceous period.
== 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 the idea of their being descendants of
generalized thecodonts, 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; they had been found as early as 1924
in Oviraptor, but misidentified as an interclavicle.
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.
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.
Birds share over a hundred distinct anatomical
features with theropod dinosaurs, which are
now generally accepted to have been their
closest ancient relatives.
They are most closely allied with maniraptoran
coelurosaurs.
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, or
that maniraptoran theropods are the ancestors
of birds but themselves are not dinosaurs,
only convergent with dinosaurs.
=== Feathers ===
Feathers are one of the most recognizable
characteristics of modern birds, and a trait
that was shared by all other dinosaur groups.
Based on the current distribution of fossil
evidence, it appears that feathers were an
ancestral dinosaurian trait, though one that
may have been selectively lost in some species.
Direct fossil evidence of feathers or feather-like
structures has been discovered in a diverse
array of species in many non-avian dinosaur
groups, both among saurischians and ornithischians.
Simple, branched, feather-like structures
are known from heterodontosaurids, primitive
neornithischians and theropods, and primitive
ceratopsians.
Evidence for true, vaned feathers similar
to the flight feathers of modern birds has
been found only in the theropod subgroup Maniraptora,
which includes oviraptorosaurs, troodontids,
dromaeosaurids, and birds.
Feather-like structures known as pycnofibres
have also been found in pterosaurs, suggesting
the possibility that feather-like filaments
may have been common in the bird lineage and
evolved before the appearance of dinosaurs
themselves.
Research into the genetics of American alligators
has also revealed that crocodylian scutes
do possess feather-keratins during embryonic
development, but these keratins are not expressed
by the animals before hatching.Archaeopteryx
was the first fossil found that revealed a
potential connection between dinosaurs and
birds.
It is considered a transitional fossil, in
that it displays features of both groups.
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 in only a
few locations, it is possible that non-avian
dinosaurs elsewhere in the world were also
feathered.
The lack of widespread fossil evidence for
feathered non-avian dinosaurs may be because
delicate features like skin and feathers are
not often preserved by fossilization and thus
are absent from the fossil record.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 some purported feather-like
fossils are the result of the decomposition
of collagenous fiber that underlaid the dinosaurs'
skin, and that maniraptoran dinosaurs with
vaned feathers were not actually dinosaurs,
but convergent with dinosaurs.
However, their views have for the most part
not been accepted by other researchers, to
the point that the scientific nature of Feduccia's
proposals has been questioned.In 2016, it
was reported that a dinosaur tail with feathers
had been found enclosed in amber.
The fossil is about 99 million years old.
=== 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 a 2005 investigation
led by Patrick M. O'Connor.
The lungs of theropod dinosaurs (carnivores
that walked on two legs and had bird-like
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.
In 2008, scientists described Aerosteon riocoloradensis,
the skeleton of which supplies the strongest
evidence to date of a dinosaur with a bird-like
breathing system.
CT-scanning of Aerosteon's fossil bones revealed
evidence for the existence of air sacs within
the animal's body cavity.
=== Behavioral evidence ===
Fossils of the troodonts Mei and Sinornithoides
demonstrate that some dinosaurs slept with
their heads tucked under their arms.
This behavior, which may have helped to keep
the head warm, is also characteristic of modern
birds.
Several deinonychosaur and oviraptorosaur
specimens have also been found preserved on
top of their nests, likely brooding in a bird-like
manner.
The ratio between egg volume and body mass
of adults among these dinosaurs suggest that
the eggs were primarily brooded by the male,
and that the young were highly precocial,
similar to many modern ground-dwelling birds.Some
dinosaurs are known to have used gizzard stones
like modern birds.
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.
== Extinction of major groups ==
The discovery that birds are a type of dinosaur
showed that dinosaurs in general are not,
in fact, extinct as is commonly stated.
However, all non-avian dinosaurs, estimated
to have been 628-1078 species, as well as
many groups of birds did suddenly become extinct
approximately 66 million years ago.
It has been suggested that because small mammals,
squamata and birds occupied the ecological
niches suited for small body size, non-avian
dinosaurs never evolved a diverse fauna of
small-bodied species, which led to their downfall
when large-bodied terrestrial tetrapods were
hit by the mass extinction event.
Many other groups of animals also became extinct
at this time, including ammonites (nautilus-like
mollusks), mosasaurs, plesiosaurs, pterosaurs,
and many groups of mammals.
Significantly, the insects suffered no discernible
population loss, which left them available
as food for other survivors.
This mass extinction is known as the Cretaceous–Paleogene
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 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.
=== Impact event ===
The asteroid collision theory, which was brought
to wide attention in 1980 by Walter Alvarez
and colleagues, links the extinction event
at the end of the Cretaceous period to a bolide
impact approximately 66 million years ago.
Alvarez et al. proposed that a sudden increase
in iridium levels, recorded around the world
in the period's rock stratum, was direct evidence
of the impact.
The bulk of the evidence now suggests that
a bolide 5 to 15 kilometers (3.1 to 9.3 miles)
wide hit in the vicinity of the Yucatán Peninsula
(in southeastern Mexico), creating the approximately
180 km (110 mi) 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
impact caused a long and unnatural drop in
Earth's atmospheric temperature, while others
claim that it would have instead created an
unusual heat wave.
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).
Although the speed of extinction cannot be
deduced from the fossil record alone, various
models suggest that the extinction was extremely
rapid, being down to hours rather than years.
=== Deccan Traps ===
Before 2000, arguments that the Deccan Traps
flood basalts caused the extinction were usually
linked to the view that the extinction was
gradual, as the flood basalt events were thought
to have started around 68 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 66 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.The
Deccan Traps in India could have caused extinction
through several mechanisms, including the
release into the air of dust and sulfuric
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 greenhouse effect
when the dust and aerosols cleared from the
atmosphere.
Before the mass extinction of the dinosaurs,
the release of volcanic gases during the formation
of the Deccan Traps "contributed to an apparently
massive global warming.
Some data point to an average rise in temperature
of 8 °C (14 °F) in the last half million
years before the impact [at Chicxulub]."In
the years when the Deccan Traps hypothesis
was linked to a slower extinction, Luis Alvarez
(who died in 1988) replied that paleontologists
were being misled by 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.
=== Possible Paleocene survivors ===
Non-avian dinosaur remains are occasionally
found above the Cretaceous–Paleogene boundary.
In 2001, 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 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.
Other evidence includes the finding of dinosaur
remains in the Hell Creek Formation up to
1.3 m (51 in) above the Cretaceous–Paleogene
boundary, representing 40000 years of elapsed
time.
Similar reports have come from other parts
of the world, including China.
Many scientists, however, dismissed the supposed
Paleocene dinosaurs as re-worked, that is,
washed out of their original locations and
then re-buried in much later sediments.
Direct dating of the bones themselves has
supported the later date, with U–Pb dating
methods resulting in a precise age of 64.8
± 0.9 million years ago.
If correct, the presence of a handful of dinosaurs
in the early Paleocene would not change the
underlying facts of the extinction.
== History of study ==
Dinosaur fossils have been known for millennia,
although their true nature was not recognized.
The Chinese considered them to be dragon bones
and documented them as such.
For example, Hua Yang Guo Zhi, a book written
by Chang Qu during the Western Jin Dynasty
(265–316), reported the discovery of dragon
bones at Wucheng in Sichuan Province.
Villagers in central China have long unearthed
fossilized "dragon bones" for use in traditional
medicines, a practice that continues today.
In Europe, dinosaur fossils were generally
believed to be the remains of giants and other
biblical creatures.Scholarly descriptions
of what would now be recognized as dinosaur
bones first appeared in the late 17th century
in England.
Part of a bone, now known to have been the
femur of a Megalosaurus, was recovered from
a limestone quarry at Cornwell near Chipping
Norton, Oxfordshire, in 1676.
The fragment was sent to Robert Plot, Professor
of Chemistry at the University of Oxford and
first curator of the Ashmolean Museum, who
published a description in his Natural History
of Oxfordshire in 1677.
He correctly identified the bone as the lower
extremity of the femur of a large animal,
and recognized that it was too large to belong
to any known species.
He therefore concluded it to be the thigh
bone of a giant human similar to those mentioned
in the Bible.
In 1699, Edward Lhuyd, a friend of Sir Isaac
Newton, was responsible for the first published
scientific treatment of what would now be
recognized as a dinosaur when he described
and named a sauropod tooth, "Rutellum implicatum",
that had been found in Caswell, near Witney,
Oxfordshire.
Between 1815 and 1824, the Rev William Buckland,
a professor of geology at Oxford, collected
more fossilized bones of Megalosaurus and
became the first person to describe a dinosaur
in a scientific journal.
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.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, the husband
of Queen Victoria, Owen established the Natural
History Museum, London, to display the national
collection of dinosaur fossils and other biological
and geological exhibits.In 1858, William Parker
Foulke discovered the first known American
dinosaur, in marl pits in the small town of
Haddonfield, New Jersey.
(Although fossils had been found before, their
nature had not been correctly discerned.)
The creature was named Hadrosaurus foulkii.
It was an extremely important find: Hadrosaurus
was one of the first nearly complete dinosaur
skeletons found (the first was in 1834, in
Maidstone, England), and it was clearly a
bipedal creature.
This was a revolutionary discovery as, until
that point, most scientists had believed dinosaurs
walked on four feet, like other lizards.
Foulke's discoveries sparked a wave of dinosaur
mania in the United States.
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, 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.After 1897, the
search for dinosaur fossils extended to every
continent, including Antarctica.
The first Antarctic dinosaur to be discovered,
the ankylosaurid Antarctopelta oliveroi, was
found on James 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.
=== "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 China (the
amazingly well-preserved feathered dinosaurs
in China have further consolidated the link
between dinosaurs and their living descendants,
modern birds).
The widespread application of cladistics,
which rigorously analyzes the relationships
between biological organisms, has also proved
tremendously useful in classifying dinosaurs.
Cladistic analysis, among other modern techniques,
helps to compensate for an often incomplete
and fragmentary fossil record.
=== Soft tissue and DNA ===
One of the best examples of soft-tissue impressions
in a fossil dinosaur was discovered in Pietraroia,
Italy.
The discovery was reported in 1998, and described
the specimen of a small, very young coelurosaur,
Scipionyx samniticus.
The fossil includes portions of the intestines,
colon, liver, muscles, and windpipe of this
immature dinosaur.In the March 2005 issue
of Science, 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.
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.In 2009, a team including
Schweitzer announced that, using even more
careful methodology, they had duplicated their
results by finding similar soft tissue in
a duck-billed dinosaur, Brachylophosaurus
canadensis, found in the Judith River Formation
of Montana.
This included even more detailed tissue, down
to preserved bone cells that seem even to
have visible remnants of nuclei and what seem
to be red blood cells.
Among other materials found in the bone was
collagen, as in the Tyrannosaurus bone.
The type of collagen an animal has in its
bones varies according to its DNA and, in
both cases, this collagen was of the same
type found in modern chickens and ostriches.The
extraction of ancient DNA from dinosaur fossils
has been reported on two separate occasions;
upon further inspection and peer review, however,
neither of these reports could be confirmed.
However, a functional peptide involved in
the vision 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, including hemoglobin,
have putatively been detected in dinosaur
fossils.In 2015, researchers reported finding
structures similar to blood cells and collagen
fibers, preserved in the bone fossils of six
Cretaceous dinosaur specimens, which are approximately
75 million years old.
== Cultural depictions ==
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, obsolete, or bound for extinction.Public
enthusiasm for dinosaurs first developed in
Victorian England, where in 1854, three decades
after the first scientific descriptions of
dinosaur remains, a menagerie of lifelike
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.
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.The popular
preoccupation with dinosaurs has ensured their
appearance in literature, film, and other
media.
Beginning in 1852 with a passing mention in
Charles Dickens' Bleak House, dinosaurs have
been featured in large numbers of fictional
works.
Jules Verne's 1864 novel Journey to the Center
of the Earth, Sir Arthur Conan Doyle's 1912
book The Lost World, the iconic 1933 film
King Kong, the 1954 Godzilla and its many
sequels, the best-selling 1990 novel Jurassic
Park by Michael Crichton and its 1993 film
adaptation are just a few notable examples
of dinosaur appearances in fiction.
Authors of general-interest non-fiction works
about dinosaurs, including some prominent
paleontologists, have often sought to use
the animals as a way to educate readers about
science in general.
Dinosaurs are ubiquitous in advertising; numerous
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.
== See also ==
== Notes
