Dinosaurs are a diverse group of animals of the clade
Dinosauria. They first appeared during the Triassic period, approximately 230 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65.5 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic Era. The fossil record indicates that birds evolved from theropod
dinosaurs during the Jurassic Period, and consequently they are
considered a subgroup of dinosaurs in modern classification systems.
[1][2]
Some birds survived the extinction event that occurred 65 million years
ago, and their descendants continue the dinosaur lineage to the present
day.
Dinosaurs are a varied group of animals from taxonomic, morphological
and ecological standpoints. Birds, at over 9,000 living species, are
the most diverse group of vertebrates besides
perciform fish.
[3] Using fossil evidence,
paleontologists have identified over 500 distinct
genera[4] and more than 1,000 different species of non-avian dinosaurs.
[5] Dinosaurs are represented on every continent by both
extant species and fossil remains.
[6] Some are herbivorous, others carnivorous. Most dinosaurs have been
bipedal, though many extinct groups included
quadrupedal
species, and some were able to shift between these body postures. Many
species possess elaborate display structures such as horns or crests,
and some prehistoric groups developed skeletal modifications such as
bony armor and
spines. Birds have been the planet's dominant flying vertebrate since the extinction of the
pterosaurs,
and evidence suggests that egg laying and nest building is a trait
shared by all dinosaurs. While many prehistoric dinosaurs were large
animals—the largest
sauropods
could reach lengths of almost 60 meters (200 feet) and were several
stories tall— the idea that non-avian dinosaurs were uniformly gigantic
is a misconception; many ancient species were nearly as small as birds
are today.
Although the word
dinosaur means "terrible lizard," the name is somewhat misleading, as dinosaurs are not
lizards.
Rather, they represent a separate group of reptiles with a distinct
upright posture not found in lizards. Through the first half of the 20th
century, before birds were recognized to be dinosaurs, most of the
scientific community believed dinosaurs to be sluggish and
cold-blooded. Most
research conducted since the 1970s, however, has indicated that ancient dinosaurs, particularly the carnivorous groups, were active animals with elevated
metabolisms and numerous adaptations for social interaction.
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 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 1842 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.
[7]:103 The term is derived from the
Greek words
δεινός (
deinos, meaning "terrible," "potent," or "fearfully great") and
σαῦρος (
sauros, meaning "lizard" or "reptile").
[7]:103[8]
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.
[9]
Definition
Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of
Triceratops,
Neornithes [modern birds], their
most recent common ancestor, and all descendants".
[10] It has also been suggested that Dinosauria be defined with respect to 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.
[11] Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria =
Ornithischia +
Saurischia", encompassing
theropods (mostly
bipedal carnivores and
birds),
ankylosaurians (armored herbivorous quadrupeds),
stegosaurians (plated herbivorous quadrupeds),
ceratopsians (herbivorous quadrupeds with horns and frills),
ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), and, perhaps,
sauropodomorphs (mostly large
herbivorous quadrupeds with long necks and tails).
The common
House Sparrow (
Passer domesticus) is often used to represent modern birds in definitions of the group Dinosauria
Many paleontologists note that the point at which sauropodomorphs and
theropods diverged may omit sauropodomorphs from the definition for
both saurischians and dinosaurs. To avoid instability, Dinosauria can be
more conservatively defined with respect to four anchoring nodes:
Triceratops horridus,
Saltasaurus loricatus, and
Passer domesticus,
their most recent common ancestor, and all descendants. This "safer"
definition can be expressed as "Dinosauria = Ornithischia +
Sauropodomorpha +
Theropoda".
[12]
There is near universal consensus among paleontologists that birds
are the descendants of theropod dinosaurs. In traditional taxonomy,
birds were considered a separate "
class" which had
evolved from dinosaurs. However, a majority of modern paleontologists reject the traditional style of classification in favor of
phylogenetic nomenclature,
which requires that all descendants of a single common ancestor must be
included in a group for that group to be natural. Birds are thus
considered by most modern scientists to
be dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup
Maniraptora, which are
coelurosaurs, which are theropods, which are
saurischians, which are dinosaurs.
[13]
General description
Using one of the above definitions, dinosaurs can be generally described as
archosaurs with
limbs held erect beneath the body.
[14] Many prehistoric animal groups are popularly conceived of as dinosaurs, such as
ichthyosaurs,
mosasaurs,
plesiosaurs,
pterosaurs, and
Dimetrodon, but are not classified scientifically as dinosaurs, and none had the erect limb posture characteristic of true dinosaurs.
[15] 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 cat, and were
generally rodent-sized carnivores of small prey.
[16] One notable exception is
Repenomamus giganticus, a
triconodont weighing between 12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young
Psittacosaurus.
[17]
Dinosaurs have always been an extremely varied group of animals;
according to a 2006 study, over 500 non-avialan 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.
[4]
An earlier study predicted that about 3400 dinosaur genera existed,
including many which would not have been preserved in the fossil record.
[18] By September 17, 2008, 1047 different species of dinosaurs had been named.
[5]
Some are herbivorous, others carnivorous. While most dinosaurs have
been bipeds, some prehistoric species were quadrupeds, and others, such
as
Ammosaurus and
Iguanodon,
could walk just as easily on two or four legs. Cranial modifications
like horns and crests are common among dinosaurs, 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 very small in size.
Dinosaurs today inhabit every continents, and fossils show that they had
achieved global distribution by at least the early Jurassic period.
[6]
Modern birds inhabit most available habitats, from terrestrial to
marine, and there is evidence that some non-avialan dinosaurs (such as
Microraptor) could fly or at least glide, and others, such as
spinosaurids, had semi-aquatic habits.
[19]
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. 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 features across a taxonomic group are
called
synapomorphies.
A detailed assessment of archosaur interrelations by S. Nesbitt
[20] confirmed or found the following 12 unambiguous synapomorphies, some previously known:
- in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra
- epipophyses present in anterior neck vertebrae (except atlas and axis)
- 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 shorter than 80% of humerus length
- fourth trochanter (projection where the caudofemoralis muscle attaches) on the femur (thigh bone) is a sharp flange
- fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft
- on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element
- exocciptials (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity
- proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface
- cnemial crest on the tibia (shinbone) arcs anterolaterally
- distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia
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 cenmial
crest on the tibia and of an ascending process on the astragalus,
[10] and many others.
Diagram of a typical diapsid skull
Hip joints and hindlimb postures of typical reptiles (left), dinosaurs and mammals (middle), and
rauisuchians (right)
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.
[21]
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);
[10] and an
acetabulum, or hip socket, with a hole at the center of its inside surface (closed in
Saturnalia, for example).
[22]
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.
[23]
Dinosaurs stand erect in a manner similar to
most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.
[24]
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.
[25]
Their erect posture enabled early dinosaurs to breathe easily while
moving, which likely permitted stamina and activity levels that
surpassed those of "sprawling" reptiles.
[26] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.
[27] 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.
[27]
Evolutionary history
Origins and early evolution
Skeleton of
Marasuchus lilloensis, a dinosaur-like
ornithodiran
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.
[28][29] Radiometric dating of the
rock formation that contained fossils from the early dinosaur
genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists think that
Eoraptor resembles the
common ancestor of all dinosaurs;
[30] if this is true, its traits suggest that the first dinosaurs were small, bipedal
predators.
[31] The discovery of primitive, dinosaur-like
ornithodirans such as
Marasuchus and
Lagerpeton in
Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.
When dinosaurs appeared, terrestrial habitats were occupied by various types of archosaurs and
therapsids, such as
aetosaurs,
cynodonts,
dicynodonts,
ornithosuchids,
rauisuchians, and
rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the
Carnian and
Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal
archosauromorphs, including the
prolacertiforms and rhynchosaurs, became extinct. This was followed by the
Triassic–Jurassic extinction event
(about 200 million years ago), that saw the end of most of the other
groups of early archosaurs, like aetosaurs, ornithosuchids,
phytosaurs, and rauisuchians. These losses left behind a land fauna of
crocodylomorphs, dinosaurs,
mammals,
pterosaurians, and
turtles.
[10] The first few lines of early dinosaurs
diversified through the
Carnian and
Norian stages of the Triassic, most likely by occupying the niches of the groups that became extinct.
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.
[32] 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.
[33]
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.
[32] 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.
[33] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from
maniraptoran coelurosaurians.
[13]
Earth during the
Jurassic,
the period in which dinosaurs gained global distribution. The
continents were in different locations from where they are today.
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.
[32]
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.
[33] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid
Nigersaurus.
[34]
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.
[32] Flowering plants were greatly radiating,
[33] with the first grasses appearing by the end of the Cretaceous.
[35]
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.
[33]
The
Cretaceous–Paleogene extinction event,
which occurred approximately 65 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.
[36]
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,
mihirungs, and "
terror birds").
However, mammals were also rapidly diversifying during this time, and
out-competed the neornithines for dominance of most terrestrial niches.
[37]
Classification
Dinosaurs are
archosaurs, like 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.
[25] This basic form was modified by rotating the pubis backward to varying degrees in several groups (
Herrerasaurus,
[38] therizinosauroids,
[39] dromaeosaurids,
[40] and
birds[13]). 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
pubis bone was oriented
caudally
(rear-pointing). Unlike birds, the ornithischian pubis also usually had
an additional forward-pointing process. Ornithischia includes a variety
of species which were primarily herbivores. (
NB: the terms "lizard hip" and "bird hip" are misnomers – birds evolved from dinosaurs with "lizard hips".)
-
-
-
-
Edmontosaurus pelvis (showing ornithischian structure – left side)
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 (2008).
[41] A more detailed version can be found at
Dinosaur classification. The cross (†) is used to signify groups with no living members.
-
- Saurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)
-
-
-
-
-
-
-
-
-
-
-
-
-
- †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)
-
-
-
- †Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)
- †Marginocephalia (characterized by a cranial growth)
-
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.
[42][43] 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
Main article:
Dinosaur size
Scale diagram comparing the largest known dinosaurs in five major
clades and a human
Current evidence suggests that dinosaur average size varied through
the Triassic, early Jurassic, late Jurassic and Cretaceous periods.
[30] Theropod dinosaurs, when sorted by estimated weight into categories based on
order of magnitude, most often fall into the 100 to 1000 kilogram (220 to 2200 lb) category, whereas
recent predatory
carnivorans peak in the 10 to 100 kilogram (22 to 220 lb) category.
[44] The
mode of dinosaur body masses is between one and ten metric tonnes.
[45] This contrasts sharply with the size of
Cenozoic mammals, estimated by the
National Museum of Natural History as about 2 to 5 kilograms (5 to 10 lb).
[46]
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 the
Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size.
[47]
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.
[48]
Largest and smallest
Scientists will probably never be certain of the
largest and smallest dinosaurs.
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 1907–12. Bones from several similar-sized individuals were
incorporated into the skeleton now mounted and on display at the
Museum für Naturkunde Berlin;
[49] this mount is 12 meters (39 ft) tall and 22.5 meters (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-meter (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
80000 to
100000 kilograms (90 to 110 short tons); some of the longest were the 33.5 meters (110 ft) long
Diplodocus hallorum[48] (formerly
Seismosaurus) and the 33 meters (108 ft) long
Supersaurus;
[50] and the tallest, the 18 meters (59 ft) tall
Sauroposeidon, which could have reached a sixth-floor window. The heaviest and 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 meters (190 ft) long and weighed over
120000 kg (
260000 lb).
[48] The largest known
carnivorous dinosaur was
Spinosaurus, reaching a length of 16 to 18 meters (52 to 60 ft), and weighing in at 8150 kg (
18000 lb).
[51] Other large meat-eaters included
Giganotosaurus,
Carcharodontosaurus and
Tyrannosaurus.
[52]
Not including birds (
Avialae), the smallest known dinosaurs were about the size of a
pigeon.
[53] Not surprisingly, the smallest dinosaurs were theropods closely related to birds.
Anchiornis, for example, had a total skeletal length of under 35 centimeters (1.1 ft).
[53][54] Anchiornis is currently the smallest dinosaur described from an adult specimen, with an estimated weight of 110 grams.
[54] The smallest herbivorous non-avialan dinosaurs included
Microceratus and
Wannanosaurus, at about 60 cm (2 ft) long each.
[41][55]
Behavior
Many modern birds are highly social, often found living in flocks.
There is general agreement that some behaviors which 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 which were then thought to have perished together in
Bernissart,
Belgium, after they fell into a deep, flooded
sinkhole and drowned.
[56]
Other mass-death sites have been subsequently discovered. 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,
[57] although there is not evidence for specific herd structures.
[58] Congregated 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.
[59] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as
pack hunters working together to bring down large prey.
[60][61] 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.
[62]
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.
[63]
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,
[64] providing evidence that dinosaurs did indeed attack each other.
[65] 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.
[65] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod
Majungasaurus.
[66]
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.
[67]
Based on current fossil evidence from dinosaurs such as
Oryctodromeus, some ornithischian species seem to have led a partially
fossorial (burrowing) lifestyle.
[68] Many modern birds are
arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the
enantiornithines.
[69] While some early bird-like species may have already been arboreal as well (including
dromaeosaurids such as
Microraptor[70])
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,
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,
[71] whether
diplodocids could create
sonic booms via
whip-like tail snapping,
[72] and whether sauropods could float.
[73]
Communication
Modern birds are well known for
communicating
using primarily visual and auditory signals, and the wide diversity of
visual display structures among fossil dinosaur groups suggests that
visual communication has always been important to dinosaur biology.
However, the evolution of dinosaur vocalization is less certain. In
2008, paleontologist Phil Senter examined the evidence for vocalization
in Mesozoic animal life, including dinosaurs.
[74]
Senter found that, contrary to popular depictions of roaring dinosaurs
in motion pictures, it is likely that most Mesozoic dinosaurs were not
capable of creating any
vocalizations (though the hollow crests of the lambeosaurines could have functioned as
resonance chambers used for a wide range of vocalizations).
[75][76]
To draw this conclusion, Senter studied the distribution of vocal
organs in modern reptiles and birds. He found that vocal cords in the
larynx probably evolved multiple times among reptiles, including
crocodilians, which are able to produce guttural roars. Birds, on the other hand, lack a larynx. Instead, bird calls are produced by the
syrinx,
a vocal organ found only in birds, and which is not related to the
larynx, meaning it evolved independently from the vocal organs in
reptiles. The syrinx depends on the air sac system in birds to function;
specifically, it requires the presence of a
clavicular air sac
near the wishbone or collar bone. This air sac leaves distinctive marks
or opening on the bones, including a distinct opening in the upper arm
bone (
humerus). While extensive air sac systems are a unique
characteristic of saurischian dinosaurs, the clavicular air sac
necessary to vocalize does not appear in the fossil record until the
enantiornithines (one exception,
Aerosteon, probably evolved its clavicular air sac independently of birds for reasons other than vocalization).
[74]
The most primitive dinosaurs with evidence of a vocalizing syrinx are
the enantironithine birds. Any bird-line archosaurs more primitive than
this probably did not make vocal calls. Rather, several lines of
evidence suggest that early dinosaurs used primarily visual
communication, in the form of distinctive-looking (and possibly brightly
colored) horns, frills, crests, sails and feathers. This is similar to
some modern reptile groups such as lizards, in which many forms are
largely silent (though like dinosaurs they possess well-developed senses
of hearing) but use complex coloration and display behaviors to
communicate.
[74]
In addition, dinosaurs use other methods of producing sound for
communication. Other animals, including other reptiles, use a wide
variety of non-vocal sound communication, including hissing, jaw
grinding or clapping, use of environment (such as splashing), and wing
beating (possible in winged
maniraptoran dinosaurs).
[74]
Reproductive biology
All dinosaurs lay
amniotic eggs with hard shells made mostly of
calcium carbonate.
[citation needed] Eggs are usually laid in a
nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows.
[77] 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.
[78]
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
theropod
Allosaurus and the ornithopod
Tenontosaurus. Because the line of dinosaurs that includes
Allosaurus and
Tyrannosaurus diverged from the line that led to
Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.
[79]
Another widespread trait among modern birds 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, suggesting this behavior might have been common to all dinosaurs.
[80] There is also evidence that other non-theropod dinosaurs, like
Patagonian titanosaurian sauropods (1997 discovery), also nested in large groups.
[81] A specimen of the
Mongolian oviraptorid Citipati osmolskae was discovered in a
chicken-like
brooding position in 1993, which indicates that they had begun using an insulating layer of feathers to keep the eggs warm.
[82]
Parental care being a trait common to all dinosaurs is supported by
other finds. For example, the fossilized remains of a grouping of
Psittacosaurus
has been found, consisting of one adult and 34 juveniles; in this case,
the large number of juveniles may be due to communal nesting.
[83] Additionally, a dinosaur embryo (pertaining to the
prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaurs.
[84] Trackways have also confirmed parental behavior among ornithopods from the
Isle of Skye in northwestern
Scotland.
[85]
Nests and eggs have been found for most major groups of dinosaurs, and
it appears likely that all dinosaurs cared for their young to some
extent either before or shortly after hatching.
Waste
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.
[86][87][88] However, at least some modern birds (such as
hummingbirds) can be facultatively
ammonotelic, excreting most of the nitrogenous wastes as ammonia.
[89] They also excrete
creatine, rather than
creatinine like mammals. This material, as well as the output of the intestines, emerges from the
cloaca.
[90][91] 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.
[92]
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.
[93]
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.
Originally, scientists broadly disagreed as to whether non-avian
dinosaurs or even early birds were capable of regulating their body
temperatures at all. More recently,
endothermy for all dinosaurs has become the consensus view, and debate has focused on the mechanisms of temperature regulation.
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.
[94]
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
(where they would have experienced a cold, dark six-month winter), and
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.
[95]
In the
saurischian
dinosaurs, higher metabolisms may have been supported by the evolution
of a bird-like 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.
[96]
Early dinosaurian respiratory systems with air sacs may have been
capable of sustaining higher activity levels than mammals of similar
size and build can sustain. 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.
[97]
Origin of birds
The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by
Thomas Henry Huxley.
[98] 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.
[99] However, as later discoveries showed, clavicles (or a single fused
wishbone, which derived from separate clavicles) were not actually absent;
[13] they had been found as early as 1924 in
Oviraptor, but misidentified as an
interclavicle.
[100] In the 1970s,
John Ostrom revived the dinosaur–bird theory,
[101] which gained momentum in the coming decades with the advent of cladistic analysis,
[102] and a great increase in the discovery of small theropods and early birds.
[21] 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.
[13]
Birds share over a hundred distinct anatomical features with theropod
dinosaurs, which are now generally accepted to have been their closest
ancient relatives.
[103] They are most closely allied with
maniraptoran coelurosaurs.
[13] 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,
[104] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only
convergent with dinosaurs.
[105]
Feathers
Archaeopteryx was the first fossil found which 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.
[106]
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. To this
point, protofeathers (thin, filament-like structures) are known from
dinosaurs at the base of Coelurosauria, such as
compsognathids like
Sinosauropteryx and
tyrannosauroids (
Dilong),
[107]
but barbed feathers are known only among the coelurosaur subgroup
Maniraptora, which includes oviraptorosaurs, troodontids,
dromaeosaurids, and birds.
[13][108]
The description of feathered dinosaurs has not been without
controversy; perhaps the most vocal critics have been Alan Feduccia and
Theagarten Lingham-Soliar, who have proposed that protofeathers are the
result of the decomposition of collagenous fiber that underlaid the
dinosaurs' integument,
[109][110][111] and that maniraptoran dinosaurs with barbed feathers were not actually dinosaurs, but
convergent with dinosaurs.
[105][110]
However, their views have for the most part not been accepted by other
researchers, to the point that the question of the scientific nature of
Feduccia's proposals has been raised.
[112]
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the
missing link
between birds and dinosaurs. However, the multiple skeletal features
also shared by the two groups represent another important line of
evidence for
paleontologists. Areas of the skeleton with important similarities include the neck,
pubis,
wrist (semi-lunate
carpal), arm and
pectoral girdle, furcula (wishbone), and
breast bone. Comparison of bird and dinosaur skeletons through
cladistic analysis strengthens the case for the link.
Soft anatomy
Large meat-eating dinosaurs had a complex system of air sacs similar
to those found in modern birds, according to an investigation which was
led by Patrick O'Connor of
Ohio University.
The lungs of theropod dinosaurs (carnivores that walked on two legs and
had 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.
[113] In a 2008 paper published in the online journal
PLoS ONE, 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.
[114][115]
Behavioral evidence
Fossils of the
troodonts Mei and
Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.
[116] 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.
[117]
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.
[118]
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.
[119]
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.
[120] However, all non-avian dinosaurs as well as many groups of birds did suddenly become
extinct approximately 65 million years ago. Many other groups of animals also became extinct at this time, including
ammonites (
nautilus-like
mollusks),
mosasaurs,
plesiosaurs,
pterosaurs, and many groups of
mammals.
[6] 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.
At the peak of the Mesozoic, there were no
polar ice caps,
and sea levels are estimated to have been from 100 to 250 meters (300
to 800 ft) higher than they are today. The planet's temperature was also
much more uniform, with only 25 °C (45 °F) separating average polar
temperatures from those at the equator. On average, atmospheric
temperatures were also much higher; the poles, for example, were 50 °C
(90 °F) warmer than today.
[121][122]
The atmosphere's composition during the Mesozoic is a matter for
debate. While some academics argue that oxygen levels were much higher
than today, others argue that biological adaptations seen in birds and
dinosaurs indicate that respiratory systems evolved beyond what would be
necessary if oxygen levels were high.
[123]
By the late Cretaceous, the environment was changing dramatically.
Volcanic activity was decreasing, which led to a cooling trend as levels
of atmospheric carbon dioxide dropped. Oxygen levels in the atmosphere
also started to fluctuate and would ultimately fall considerably. Some
scientists hypothesize that climate change, combined with lower oxygen
levels, might have led directly to the demise of many species. If the
dinosaurs had respiratory systems similar to those commonly found in
modern birds, it may have been particularly difficult for them to cope
with reduced respiratory efficiency, given the enormous oxygen demands
of their very large bodies.
[6]
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 65.5 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.
[124] The bulk of the evidence now suggests that a
bolide 5 to 15 kilometers (3 to 9 mi) 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.
[125][126] Scientists are not certain whether dinosaurs were thriving or declining before the
impact event.
Some scientists propose that the meteorite caused a long and unnatural
drop in Earth's atmospheric temperature, while others claim that it
would have instead created an unusual heat wave. 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.
[127]
In September 2007, U.S. researchers led by William Bottke of the
Southwest Research Institute in
Boulder, Colorado, and
Czech scientists used
computer simulations to identify the probable source of the Chicxulub impact. They calculated a 90% probability that an asteroid named
Baptistina, approximately 160 km (99 mi) in diameter, orbiting in the asteroid belt which lies between
Mars and
Jupiter,
was struck by a smaller unnamed asteroid about 55 km (35 mi) in
diameter about 160 million years ago. The impact shattered Baptistina,
creating a cluster which still exists today as the
Baptistina family.
Calculations indicate that some of the fragments were sent hurtling
into earth-crossing orbits, one of which was the 10 km (6.2 mi) wide
meteorite which struck
Mexico's
Yucatan peninsula 65 million years ago, creating the Chicxulub crater.
[128] In 2011, new data from the
Wide-field Infrared Survey Explorer revised the date of the collision which created the
Baptistina family
to about 80 million years ago. This makes an asteroid from this family
highly improbable to be the asteroid that created the Chicxulub Crater,
as typically the process of resonance and collision of an asteroid takes
many tens of millions of years.
[129]
A similar but more controversial explanation proposes that "passages of the [hypothetical] solar companion star
Nemesis through the
Oort comet cloud would trigger comet showers."
[130]
One or more of these comets then collided with the Earth at
approximately the same time, causing the worldwide extinction. As with
the impact of a single asteroid, the end result of this comet
bombardment would have been a sudden drop in global temperatures,
followed by a protracted cool period.
[130]
Deccan Traps
Main article:
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 65.5 million years ago, and so these eruptions would have caused a
fairly rapid extinction, possibly over a period of thousands of years,
but still longer than would be expected from a single impact event.
[131][132]
The Deccan Traps could have caused extinction through several
mechanisms, including the release into the air of dust and sulphuric
aerosols, which might have blocked sunlight and thereby reduced
photosynthesis in plants. In addition, Deccan Trap volcanism might have
resulted in carbon dioxide emissions, which would have increased the
greenhouse effect when the dust and aerosols cleared from the atmosphere.
[132] 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]."
[131][132]
In the years when the Deccan Traps theory 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.
[133]
Failure to adapt to changing conditions
Lloyd
et al. (2008) noted that, in the Mid Cretaceous, the flowering,
angiosperm plants became a major part of terrestrial
ecosystems, which had previously been dominated by
gymnosperms such as conifers. Dinosaur
coprolite–fossilized
dung–indicate that, while some ate angiosperms, most herbivorous
dinosaurs ate mainly gymnosperms. Statistical analysis by Lloyd
et al. concluded that, contrary to earlier studies, dinosaurs did not diversify very much in the Late Cretaceous. Lloyd
et al. suggested that dinosaurs' failure to diversify as ecosystems were changing doomed them to extinction.
[134]
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.
[135] Other evidence includes the finding of dinosaur remains in the
Hell Creek Formation up to 1.3 meters (51 in) above (
40000 years later than) the Cretaceous–Paleogene boundary. Similar reports have come from other parts of the world, including China.
[136]
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.
[137][138]
However, 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.
[139]
If correct, the presence of a handful of dinosaurs in the early
Paleocene would not change the underlying facts of the extinction.
[137]
History of study
Dinosaur fossils have been known for millennia, although their true
nature was not recognized. The Chinese, whose modern word for dinosaur
is
konglong (恐龍, or "terrible dragon"), considered them to be
dragon bones and documented them as such. For example,
Hua Yang Guo Zhi, a book written by Zhang Qu during the
Western Jin Dynasty, reported the discovery of dragon bones at Wucheng in
Sichuan Province.
[140]
Villagers in central China have long unearthed fossilized "dragon
bones" for use in traditional medicines, a practice that continues
today.
[141] In Europe, dinosaur fossils were generally believed to be the remains of
giants and other creatures killed by the
Great Flood.
Marsh's 1896 illustration of the bones of
Stegosaurus, a dinosaur he described and named in 1877.
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,
[142] was recovered from a limestone quarry at Cornwell near
Chipping Norton,
Oxfordshire, England, 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",
[143][144] that had been found in Caswell, near Witney, Oxfordshire.
[145]
Between 1815 and 1824, the Rev
William Buckland, a professor of
geology at
Oxford University, collected more fossilized bones of
Megalosaurus and became the first person to describe a dinosaur in a
scientific journal.
[142][146] 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.
[147][148]
The study of these "great fossil lizards" soon became of great
interest to European and American scientists, and in 1842 the English
paleontologist
Richard Owen coined the term "dinosaur". He recognized that the remains that had been found so far,
Iguanodon,
Megalosaurus and
Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of
Prince Albert of Saxe-Coburg-Gotha, the husband of
Queen Victoria, Owen established the
Natural History Museum in
South Kensington,
London, to display the national collection of dinosaur fossils and other biological and geological exhibits.
In 1858, the first known American dinosaur was discovered, in
marl pits in the small town of
Haddonfield, New Jersey (although fossils had been found before, their nature had not been correctly discerned). The creature was named
Hadrosaurus foulkii. It was an extremely important find:
Hadrosaurus was one of the first nearly complete dinosaur skeletons found (
the first was in 1834, in Maidstone, Kent, 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.
[149]
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
Ross Island in 1986, although it was 1994 before an Antarctic species, the theropod
Cryolophosaurus ellioti, was formally named and described in a scientific journal.
Current dinosaur "hot spots" include southern South America (especially
Argentina) and
China. China in particular has produced many exceptional
feathered dinosaur
specimens due to the unique geology of its dinosaur beds, as well as an
ancient arid climate particularly conducive to fossilization.
The "dinosaur renaissance"
The field of dinosaur research has enjoyed a surge in activity that
began in the 1970s and is ongoing. This was triggered, in part, by
John Ostrom's discovery of
Deinonychus, an active predator that may have been
warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and
cold-blooded.
Vertebrate paleontology has become a global
science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including
India, South America,
Madagascar,
Antarctica, and most significantly
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.
Soft tissue and DNA
One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in Petraroia,
Italy. The discovery was reported in 1998, and described the specimen of a small, very young
coelurosaur,
Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.
[42]
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.
[43]
When the fossilized bone was treated over several weeks to remove
mineral content from the fossilized bone-marrow cavity (a process called
demineralization), Schweitzer found evidence of intact structures such
as
blood vessels,
bone matrix, and connective tissue (bone fibers). Scrutiny under the
microscope further revealed that the putative dinosaur soft tissue had
retained fine structures (microstructures) even at the cellular level.
The exact nature and composition of this material, and the implications
of Schweitzer's discovery, are not yet clear; study and interpretation
of the material is ongoing.
[43]
The successful extraction of ancient DNA from dinosaur fossils has
been reported on two separate occasions, but, upon further inspection
and
peer review, neither of these reports could be confirmed.
[150] However, a functional visual
peptide
of a theoretical dinosaur has been inferred using analytical
phylogenetic reconstruction methods on gene sequences of related modern
species such as reptiles and birds.
[151] In addition, several
proteins, including hemoglobin,
[152] have putatively been detected in dinosaur fossils.
[153][154]
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.
[155]
Public enthusiasm for dinosaurs first developed in
Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, the famous
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.
[156]
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.
[157]
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,
[158] 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.
[159] Even
fine artists have featured dinosaurs in their works.
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