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
Main article:
Dinosaur size Scale diagram comparing the largest known dinosaurs in five major
clades and a human
While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. Even by dinosaur standards, the
sauropods were gigantic. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an
order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric
mammals such as the
Indricotherium and the Columbian
mammoth were dwarfed by the giant sauropods, and only a handful of modern aquatic animals approach or surpass them in size – most notably the
blue whale, which reaches up to
173000 kg (
381000 lb) and over 30 meters (100 ft) in length.
There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in
rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.
Most dinosaurs, however, were much smaller than the giant sauropods. Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods.
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.
The
mode of dinosaur body masses is between one and ten metric tonnes.
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).
Largest and smallest
Only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork. As a result, scientists will probably never be certain of the
largest and smallest dinosaurs.
The tallest and heaviest dinosaur known from good skeletons is
Giraffatitan brancai (previously classified as a species of
Brachiosaurus). Its remains were discovered in
Tanzania between 1907–12. Bones from multiple similar-sized individuals were incorporated into the skeleton now mounted and on display at the
Humboldt Museum of
Berlin;
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 (formerly
Seismosaurus) and the 33 meters (110 ft) long
Supersaurus;
and the tallest, the 18 meters (59 ft) tall
Sauroposeidon, which could have reached a sixth-floor window. The longest of them all may have been
Amphicoelias fragillimus, known only from a now lost partial vertebral
neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed over
120000 kg (
260000 lb).
The largest known
carnivorous dinosaur was
Spinosaurus, reaching a length of 16 to 18 meters (50 to 60 ft), and weighing in at 8150 kg (
18000 lb).
Other large meat-eaters included
Giganotosaurus,
Carcharodontosaurus and
Tyrannosaurus.
Not including modern birds, the smallest dinosaurs known were about the size of a
pigeon.
The theropods
Anchiornis and
Epidexipteryx both had a total skeletal length of under 35 centimeters (1.1 ft).
Anchiornis is currently the smallest dinosaur described from an adult specimen, with an estimated weight of 110 grams.
The smallest herbivorous dinosaurs included
Microceratus and
Wannanosaurus, at about 60 cm (2 ft) long each.
Behavior
A nesting ground of
Maiasaura was discovered in 1978
Interpretations of dinosaur behavior are generally based on the pose of body fossils and their
habitat,
computer simulations of their
biomechanics, and comparisons with modern animals in similar
ecological niches. As such, the current understanding of dinosaur behavior relies on speculation, and will likely remain controversial for the foreseeable future. However, there is general agreement that some behaviors which are common in crocodiles and birds, dinosaurs' closest living relatives, were also common among dinosaurs.
The first potential evidence of
herding behavior was the 1878 discovery of 31
Iguanodon dinosaurs which were then thought to have perished together in
Bernissart,
Belgium, after they fell into a deep, flooded
sinkhole and drowned.
Other mass-death sites have been subsequently discovered. Those, along with multiple trackways, suggest that gregarious behavior was common in many dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that
duck-bills (hadrosaurids) may have moved in great herds, like the
American Bison or the African
Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in
Oxfordshire, England,
although there is not evidence for specific herd structures.
Dinosaurs may have congregated in herds for defense, for
migratory purposes, or to provide protection for their young. There is evidence that many types of 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 twenty
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 the modern relatives of dinosaurs (
crocodiles and other reptiles, and
birds –
Harris's Hawk is a well-documented exception), and the
taphonomic evidence suggesting pack hunting in such theropods as
Deinonychus and
Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern
diapsid predators.
Jack Horner's 1978 discovery of a
Maiasaura ("good mother dinosaur")
nesting ground in
Montana demonstrated that parental care continued long after birth among the
ornithopods.
There is also evidence that other Cretaceous-era dinosaurs, like
Patagonian titanosaurian sauropods (1997 discovery), also nested in large groups.
The
Mongolian oviraptorid Citipati was discovered in a
chicken-like
brooding position in 1993, which may mean it was covered with an insulating layer of feathers that kept the
eggs warm.
Parental care is also implied by other finds. For example, the fossilized remains of a grouping of
Psittacosaurus has been found, consisting of one adult and 34 juveniles; in this case, the large number of juveniles may be due to communal nesting.
Additionally, a dinosaur embryo (pertaining to the
prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaur.
Trackways have also confirmed parental behavior among ornithopods from the
Isle of Skye in northwestern
Scotland.
Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, in a manner similar to modern birds and crocodiles.
Artist's rendering of two
Centrosaurus, herbivorous
ceratopsid dinosaurs from the late Cretaceous fauna of North America
The crests and
frills of some dinosaurs, like the
marginocephalians,
theropods and
lambeosaurines, may have been too fragile to be used for active defense, 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.
Based on current fossil evidence from dinosaurs such as
Oryctodromeus, some herbivorous species seem to have led a partially
fossorial (burrowing) lifestyle,
and some bird-like species may have been
arboreal (tree-climbing), most notably primitive
dromaeosaurids such as
Microraptor and the enigmatic
scansoriopterygids.
However, most dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of
biomechanics, in particular, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run,
whether
diplodocids could create
sonic booms via
whip-like tail snapping,
and whether sauropods could float.
Communication and vocalization
The nature of dinosaur
communication remains enigmatic, and is an active area of research. In 2008, paleontologist Phil Senter examined the evidence for vocalization in Mesozoic animal life, including dinosaurs.
Senter found that, contrary to popular depictions of roaring dinosaurs in motion pictures, it is likely that most dinosaurs were not capable of creating any
vocalizations. To draw this conclusion, Senter studied the distribution of vocal organs in reptiles and birds. He found that vocal chords 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 many dinosaurs show evidence of extensive air sac systems, almost none possess the clavicular air sac necessary to vocalize (one exception,
Aerosteon, probably evolved its clavicular air sac independently of birds for reasons other than vocalization).
The most primitive animals 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 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.
Also, though they may not have been able to vocalize, some dinosaurs may have used other methods of producing sound for communication. Modern animals, including reptiles and birds, use a wide variety of non-vocal sound communication, including hissing, jaw grinding or clapping, use of environment (such as splashing), and wing beating (which would have been possible in winged
maniraptoran dinosaurs).
Some studies have suggested that the hollow crests of the lambeosaurines may have functioned as
resonance chambers used for a wide range of vocalizations.
However, Senter (2008) noted that such chambers are also used in modern non-vocal animals to accentuate or deepen non-vocal sounds like hissing. For example, many snakes, which lack vocal chords, have resonating chambers in the skull.
Physiology
A vigorous debate on the subject of temperature regulation in dinosaurs has been ongoing since the 1960s. Originally, scientists broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, dinosaur
endothermy has become the consensus view, and debate has focused on the mechanisms of temperature regulation.
After dinosaurs were discovered, paleontologists first posited that they were
ectothermic creatures: "terrible
lizards" as their name suggests. This supposed cold-bloodedness implied that dinosaurs were relatively slow, sluggish organisms, comparable to modern reptiles, which need external sources of heat in order to regulate their body temperature. Dinosaur ectothermy remained a prevalent view until
Robert T. "Bob" Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.
Modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaur species must have regulated their body temperature by internal biological means (perhaps aided by the animals' bulk). Evidence of
endothermy in dinosaurs includes the discovery of
polar dinosaurs in Australia and
Antarctica (where they would have experienced a cold, dark six-month winter), the discovery of dinosaurs whose feathers may have provided regulatory insulation, and analysis of blood-vessel structures within dinosaur bone that are typical of endotherms. Skeletal structures suggest that theropods and other dinosaurs had active lifestyles better suited to an endothermic cardiovascular system, while sauropods exhibit fewer endothermic characteristics. It is certainly possible that some dinosaurs were endothermic while others were not. Scientific debate over the specifics continues.
Eubrontes, a dinosaur footprint in the Lower
Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah
Complicating the debate is the fact that warm-bloodedness can emerge based on more than one mechanism. Most discussions of dinosaur endothermy tend to compare them with average-sized birds or mammals, which expend energy to elevate body temperature above that of the environment. Small birds and mammals also possess
insulation, such as
fat,
fur, or
feathers, which slows down heat loss. However, large mammals, such as elephants, face a different problem because of their relatively small ratio of surface area to volume (
Haldane's principle). This ratio compares the volume of an animal with the area of its skin: as an animal gets bigger, its surface area increases more slowly than its volume. At a certain point, the amount of heat radiated away through the skin drops below the amount of heat produced inside the body, forcing animals to use additional methods to avoid overheating. In the case of elephants, they have little hair as adults, have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud-wallowing). These behaviors increase cooling through evaporation.
Large dinosaurs would presumably have had to deal with similar issues; their body size suggest they lost heat relatively slowly to the surrounding air, and so could have been what are called
inertial homeotherms, animals that are warmer than their environments through sheer size rather than through special adaptations like those of birds or mammals. However, so far this theory fails to account for the numerous dog- and goat-sized dinosaur species, or the young of larger species.
Modern
computerized tomography (CT) scans of a
dinosaur's chest cavity (conducted in 2000) found the apparent remnants of a four-chambered heart, much like those found in today's mammals and birds.
The idea is controversial within the scientific community, coming under fire for bad anatomical science
or simply wishful thinking.
The question of how this find reflects on metabolic rate and dinosaur internal anatomy may be moot, though, regardless of the object's identity: both modern
crocodilians and
birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), and so dinosaurs probably had them as well.
Soft tissue and DNA
One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in Petraroia,
Italy. The discovery was reported in 1998, and described the specimen of a small, very young
coelurosaur,
Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.
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; study and interpretation of the material is ongoing.
Newer research, published in PloS One (30 July 2008), has challenged the claims that the material found is the soft tissue of
Tyrannosaurus. Thomas Kaye of the
University of Washington and his co-authors contend that what was really inside the tyrannosaur bone was slimy
biofilm created by bacteria that coated the voids once occupied by blood vessels and cells.
The researchers found that what previously had been identified as remnants of blood cells, because of the presence of iron, were actually
framboids, microscopic mineral spheres bearing iron. They found similar spheres in a variety of other fossils from various periods, including an
ammonite. In the ammonite they found the spheres in a place where the iron they contain could not have had any relationship to the presence of blood.
The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but, upon further inspection and
peer review, neither of these reports could be confirmed.
However, a functional visual
peptide of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds.
In addition, several
proteins, including hemoglobin,
have putatively been detected in dinosaur fossils.