Saurischian pelvis structure (left side)

Tyrannosaurus pelvis (showing saurischian structure - left side)

Ornithischian pelvis structure (left side).

Edmontosaurus pelvis (showing ornithischian structure - left side)

The following is a simplified classification of dinosaur families. A more detailed version can be found at List of dinosaur classifications.

Assemblage of Tetanuran theropods: Torvosaurus left above, Eustreptospondylus left below, Piatnitzkysaurus in cental position, Megalosaurus right below, Afrovenator right above, and Cryolophosaurus behind it

Several macronarian Sauropods, from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus

Various ornithopod dinosaurs and one heterodontosaurid. Far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (small): Heterodontosaurus, far right (large) Tenontosaurus.

• Dinosauria

• Saurischia (theropods and sauropods)

• Herrerasaurians (early bipedal predators)
• Theropods (all bipedal; most were carnivores)

• Coelophysoids (Coelophysis and close relatives)
• Ceratosaurians (Ceratosaurus and abelisaurids - the latter were important Late Cretaceous predators in southern continents)
• Spinosauroids (long bodies, short arms, some with crocodile-like skulls and bony "sails" in their backs)
• Carnosaurians (Allosaurus and close relatives, like Carcharodontosaurus)
• Coelurosaurians (diverse, with a range of body sizes and niches)

• Tyrannosauroids (small to gigantic, often with reduced forelimbs)
• Ornithomimosaurians ("ostrich-mimics", toothless, carnivores to herbivores)
• Therizinosauroids (bipedal herbivores with long arms and small heads)
• Oviraptorosaurians (toothless; their diet and lifestyle are uncertain)
• Dromaeosaurids (like the "raptors" in Jurassic Park)
• Troodontids (similar to dromaeosaurids, but more lightly built, and possibly omnivorous)
• Birds (the only living dinosaurs)

• Sauropodomorphs (quadrupedal herbivores with small heads and long necks and tails, and elephant-like bodies)

• "Prosauropods" (early relatives of sauropods; small to quite large; some possibly omnivorous; bipeds and quadrupeds)
• Sauropods (very large, usually over 15 meters long [49 ft])

• Diplodocoids (skulls and tails elongated; teeth typically narrow and pencil-like)
• Macronarians (boxy skulls; spoon-shaped or pencil-shaped teeth)

• Brachiosaurids (very long necks; forelimbs longer than hindlimbs)
• Titanosaurians (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

• Ornithischians (diverse bipedal and quadrupedal herbivores)

• Heterodontosaurids (meter- or yard-scale herbivores or omnivores with prominent canine teeth)
• Thyreophorans (armored dinosaurs, mostly quadrupeds)

• Ankylosaurians (scutes as primary armor; some had club-like tails)
• Stegosaurians (spikes and plates as primary armor)

• Ornithopods (diverse, from meter- or yard-scale bipeds to 12 meter (39 ft) animals that could moves as both bipeds and quadrupeds, evolved a method of chewing using skull flexibility and large numbers of teeth)

• Hadrosaurids ("duckbilled dinosaurs")

• Pachycephalosaurians ("bone-heads", bipeds with domed or knobby growth on skulls)
• Ceratopsians (dinosaurs with horns and frills, although most early forms had only the beginnings of these features)

Evolution and paleobiogeography

Main article: Evolution of dinosaurs

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, there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and prosauropod herbivores.^[38] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Prosauropods did not have sophisticated mechanisms for processing food in the mouth, so must have employed other means of breaking down food farther along the digestive tract.^[39] 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.^[38] 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.^[39] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.^[8]

An Illustration of 18 species of basal ceratopsia to scale.

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.^[38] 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.^[39] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.^[40]

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.^[38] Flowering plants were greatly radiating,^[39] with the first grasses appearing by the end of the Cretaceous.^[41] 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.^[39]

The Cretaceous–Tertiary extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the birds. Some other diapsid groups, such as crocodylians, lizards, snakes, sphenodontians, and choristoderans, also survived the event.^[42]

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.^[43][44] 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 four suborders 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 173,000 kilograms (381,000 lb) and over 30 meters (100 ft) in length.^[45] 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.^[46]

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.^[31] Theropod dinosaurs, when sorted by estimated weight into categories based on order of magnitude, most often fall into the 100 to 1,000 kilogram (220 to 2,200 lb) category, whereas recent predatory carnivorans peak in the 10 to 100 kilogram (22 to 220 lb) category.^[47] The mode of dinosaur body masses is between one and ten metric tonnes.^[48] 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).^[49]

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.

Comparative size of Brachiosaurus.

The tallest and heaviest dinosaur known from good skeletons is Brachiosaurus brancai (also known as Giraffatitan). Its remains were discovered in Tanzania between 1907–12. Bones from multiple similarly-sized individuals were incorporated into the skeleton now mounted and on display at the Humboldt Museum of Berlin;^[50] 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 30,000 and 60,000 kilograms (70,000 and 130,000 lb). The longest complete dinosaur is the 27 m (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Natural History Museum in 1907.

Comparative size of Eoraptor.

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000 to 100,000 kilograms (90 to 110 short tons); some of the longest, the 33.5 meters (110 ft) long Diplodocus hallorum^[46] (formerly Seismosaurus) and the 33 meters (110 ft) long Supersaurus;^[51] and the tallest, the 18 meters (59 ft) Sauroposeidon, which could have reached a sixth-floor window. The longest of them all may have been Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed over 120,000 kilograms (260,000 lb),^[46] heavier than all known dinosaurs except possibly the poorly known Bruhathkayosaurus, which could have weighed 175,000 to 220,000 kilograms (400,000 to 500,000 lb). The largest known carnivorous dinosaur was Spinosaurus, reaching a length of 16 to 18 meters (50 to 60 ft), and weighing in at 8,150 kilograms (18,000 lb).^[52] Other large meat-eaters included Giganotosaurus, Mapusaurus, Tyrannosaurus rex and Carcharodontosaurus.

Not including modern birds, the smallest dinosaurs known were about the size of a crow or a chicken. The theropods Microraptor and Parvicursor were both under 0.6 meters (2 ft) in length.

Behavior

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.^[53] 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,^[54] although there is not evidence for specific herd structures.^[55] 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.^[56] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.^[57][58] 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.^[59]

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Fossilized egg of the oviraptorid Citipati, American Museum of Natural History.

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.^[60] There is also evidence that other Cretaceous-era dinosaurs, like Patagonian titanosaurian sauropods (1997 discovery), also nested in large groups.^[61] 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.^[62] 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.^[63] 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.^[64] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.^[65] Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that dinosaurs communicated with their young, in a manner similar to modern birds and crocodiles.

Artist's rendering of two Centrosaurus, herbivorous ceratopsid dinosaurs from the late Cretaceous fauna of North America.

The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.^[66] The nature of dinosaur communication also remains enigmatic, and is an active area of research. For example, recent studies suggest that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.^[67][68]

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,^[69] providing evidence that dinosaurs did indeed attack each other.^[70] 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.^[70] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.^[71]

Based on current fossil evidence from dinosaurs such as Oryctodromeus, some herbivorous species seem to have led a partially fossorial (burrowing) lifestyle,^[72] and some bird-like species may have been arboreal (tree-climbing), most notably primitive dromaeosaurids such as Microraptor^[73] and the enigmatic scansoriopterygids.^[74] 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,^[75] whether diplodocids could create sonic booms via whip-like tail snapping,^[76] and whether sauropods could float.^[77]

Physiology

Main article: Physiology of dinosaurs

Tyrannosaurus rex skull and upper vertebral column, Palais de la Découverte, Paris.

A vigorous debate on the subject of temperature regulation in dinosaurs has been ongoing since the 1960s. Originally, scientists broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, dinosaur endotherm has become the consensus view, and debate has focused on the mechanisms of temperature regulation.

After dinosaurs were discovered, paleontologists first posited that they were ectothermic creatures: "terrible lizards" as their name suggests. This supposed cold-bloodedness implied that dinosaurs were relatively slow, sluggish organisms, comparable to modern reptiles, which need external sources of heat in order to regulate their body temperature. Dinosaur ectothermy remained a prevalent view until Robert T. "Bob" Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.

Modern evidence indicates that dinosaurs thrived in cooler temperate climates, and that at least some dinosaur species must have regulated their body temperature by internal biological means (perhaps aided by the animals' bulk). Evidence of endotherm in dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica (where they 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 that are typical of endotherms within dinosaur bone. Skeletal structures suggest that theropods and other dinosaurs had active lifestyles better suited to an endothermic cardiovascular system, while sauropods exhibit fewer endothermic characteristics. It is certainly possible that some dinosaurs were endothermic while others were not. Scientific debate over the specifics continues.^[78]

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 to average birds or mammals, which expend energy to elevate body temperature above that of the environment. Small birds and mammals also possess insulation, such as fat, fur, or feathers, which slows down heat loss. However, large mammals, such as elephants, face a different problem because of their relatively small ratio of surface area to volume (Haldane's principle). This ratio compares the volume of an animal with the area of its skin: as an animal gets bigger, its surface area increases more slowly than its volume. At a certain point, the amount of heat radiated away through the skin drops below the amount of heat produced inside the body, forcing animals to use additional methods to avoid overheating. In the case of elephants, they have little hair as adults, and have large ears which increase their surface area, and have behavioral adaptations as well (such as using the trunk to spray water on themselves and mud wallowing). These behaviors increase cooling through evaporation.