Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.[27]

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.[34] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,[97] therizinosauroids,[98] dromaeosaurids,[99] and birds[11]). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).[26][100]

By contrast, ornithischians—"bird-hipped", from the Greek ornitheios (ὀρνίθειος) meaning "of a bird" and ischion (ἰσχίον) meaning "hip joint"—had a pelvis that superficially resembled a bird's pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species that were primarily herbivores.

Despite the terms "bird hip" (Ornithischia) and "lizard hip" (Saurischia), birds are not part of Ornithischia. Birds instead belong to Saurischia, the “lizard-hipped” dinosaurs—birds evolved from earlier dinosaurs with "lizard hips".[27]

The following is a simplified classification of dinosaur groups based on their evolutionary relationships, and organized based on the list of Mesozoic dinosaur species provided by Holtz (2007).[101] A more detailed version can be found at Dinosaur classification. The dagger (†) is used to signify groups with no living members.

• Dinosauria

• Saurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)

• †Herrerasauria (early bipedal carnivores)

• Theropoda (all bipedal; most were carnivorous)

• †Coelophysoidea (small, early theropods; includes Coelophysis and close relatives)

• †Ceratosauria (generally elaborately horned, the dominant southern carnivores of the Cretaceous)

• Tetanurae ("stiff tails"; includes most theropods)

• †Megalosauroidea (early group of large carnivores including the semiaquatic spinosaurids)

• †Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)

• †Megaraptora (group of medium to large sized theropods, often with large hand claws)

• Coelurosauria (feathered theropods, with a range of body sizes and niches)[63]

• †Compsognathidae (early coelurosaurs with reduced forelimbs)

• †Tyrannosauroidea (Tyrannosaurus and close relatives)

• †Ornithomimosauria ("bird-mimics"; ostrich-like; mostly toothless; carnivores to possible herbivores)

• †Alvarezsauroidea (small insectivores with reduced forelimbs each bearing one enlarged claw)

• Maniraptora ("hand snatchers"; had long, slender arms and fingers)

• †Therizinosauria (bipedal herbivores with large hand claws and small heads)

• †Oviraptorosauria (mostly toothless; their diet and lifestyle are uncertain)

Restoration of six dromaeosaurid theropods: from left to right Microraptor, Velociraptor, Austroraptor, Dromaeosaurus, Utahraptor, and Deinonychus

• †Deinonychosauria (small- to medium-sized; bird-like, with a distinctive toe claw)

• Avialae (modern birds and extinct relatives)

• †Archaeopterygidae (small, winged theropods or primitive birds)

• †Scansoriopterygidae (small primitive avialans with long third fingers)

• †Omnivoropterygidae (large, early short-tailed avialans)

• †Confuciusornithidae (small toothless avialans)

• †Enantiornithes (primitive tree-dwelling, flying avialans)

• Euornithes (advanced flying birds)

• †Yanornithiformes (toothed Cretaceous Chinese birds)

• †Hesperornithes (specialized aquatic diving birds)

• Aves (modern, beaked birds and their extinct relatives)

Restoration of four macronarian sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus

• †Sauropodomorpha (herbivores with small heads, long necks, long tails)

• †Guaibasauridae (small, primitive, omnivorous sauropodomorphs)

• †Plateosauridae (primitive, strictly bipedal "prosauropods")

• †Riojasauridae (small, primitive sauropodomorphs)

• †Massospondylidae (small, primitive sauropodomorphs)

• †Sauropoda (very large and heavy, usually over 15 m (49 ft) long; quadrupedal)

• †Vulcanodontidae (primitive sauropods with pillar-like limbs)

• †Eusauropoda ("true sauropods")

• †Cetiosauridae ("whale reptiles")

• †Turiasauria (group of Jurassic and Cretaceous sauropods)

• †Neosauropoda ("new sauropods")

• †Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)

• †Macronaria (boxy skulls; spoon- or pencil-shaped teeth)

• †Brachiosauridae (long-necked, long-armed macronarians)

• †Titanosauria (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

Restoration of six ornithopods; far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (large) Tenontosaurus.

• †Ornithischia ("bird-hipped"; diverse bipedal and quadrupedal herbivores)

• †Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine-like teeth)

• †Thyreophora (armored dinosaurs; mostly quadrupeds)

• †Ankylosauria (scutes as primary armor; some had club-like tails)

• †Stegosauria (spikes and plates as primary armor)

• †Neornithischia ("new ornithischians")

• †Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)

  • †Hadrosauridae (large quadrupedal herbivores, with teeth merged into dental batteries)

• †Marginocephalia (characterized by a cranial growth)

• †Pachycephalosauria (bipeds with domed or knobby growth on skulls)

• †Ceratopsia (bipeds and quadrupeds with neck frills; many also had horns)

Timeline of major dinosaur groups per Holtz (2007).

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 other soft tissues.[64][65] 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).[102][103] Two topics of particular interest and study have been dinosaur size and behavior.[104]

Main article: Dinosaur size

Scale diagram comparing the average human to the longest known dinosaurs in five major clades:  Sauropoda (Supersaurus vivianae)  Ornithopoda (Shantungosaurus giganteus)  Theropoda (Spinosaurus aegyptiacus)  Thyreophora (Stegosaurus ungulatus)  Marginocephalia (Triceratops prorsus)

Current evidence suggests that dinosaur average size varied through the Triassic, Early Jurassic, Late Jurassic and Cretaceous.[79] Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the 100 to 1000 kg (220 to 2200 lb) category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the 10 to 100 kg (22 to 220 lb) category.[105] The mode of Mesozoic dinosaur body masses is between 1 to 10 metric tons (1.1 to 11.0 short tons).[106] This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb).[107]

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 was an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size.[108] 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.[109]

Scientists will probably never be certain of the largest and smallest dinosaurs to have ever existed. This is because only a tiny percentage of animals were ever fossilized 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.[110]

Comparative size of Argentinosaurus to the average human

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde in Berlin;[111] this mount is 12 meters (39 ft) tall and 21.8 to 22.5 meters (72 to 74 ft) long,[112][113] and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27 meters (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Museum of Natural History in 1907.[114] The longest dinosaur known from good fossil material is the Patagotitan: the skeleton mount in the American Museum of Natural History in New York is 37 meters (121 ft) long. The Museo Municipal Carmen Funes in Plaza Huincul, Argentina, has an Argentinosaurus reconstructed skeleton mount that is 39.7 meters (130 ft) long.[115]

An adult bee hummingbird, the smallest known dinosaur

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (90 to 110 short tons) and reached lengths of 30 to 40 meters (98 to 131 ft); some of the longest were the 33.5-meter (110 ft) long Diplodocus hallorum[109] (formerly Seismosaurus), the 33-to-34-meter (108 to 112 ft) long Supersaurus,[116] and 37-meter (121 ft) long Patagotitan; and the tallest, the 18-meter (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest dinosaur may have been Maraapunisaurus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed 122400 kg (270000 lb).[109] However, as no further evidence of sauropods of this size has been found, and the discoverer, Cope, had made typographic errors before, it is likely to have been an extreme overestimation.[117]

The largest carnivorous dinosaur was Spinosaurus, reaching a length of 12.6 to 18 meters (41 to 59 ft), and weighing 7 to 20.9 metric tons (7.7 to 23.0 short tons).[118][119] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.[119] Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest ornithischian dinosaur was probably the hadrosaurid Shantungosaurus giganteus which measured 16.6 meters (54 ft).[120] The largest individuals may have weighed as much as 16 metric tons (18 short tons).[121]

The smallest dinosaur known is the bee hummingbird,[122] with a length of only 5 centimeters (2.0 in) and mass of around 1.8 g (0.063 oz).[123] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.[124] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 g (3.9 oz)[125] and a total skeletal length of 34 centimeters (1.12 ft).[124][125] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 centimeters (2.0 ft) long each.[101][126]

A nesting ground of the hadrosaur Maiasaura peeblesorum was discovered in 1978

Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors that are common in birds, as well as in crocodiles (closest living relatives of birds), 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.[102]

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, ornithischians that were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.[127] Other mass-death sites have been discovered subsequently. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-billed (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,[128] although there is no evidence for specific herd structures.[129] Congregating into herds may have evolved for defense, for migratory purposes, or to provide protection for young. There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud.[130] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.[131][132] 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.[133]

Restoration of two Centrosaurus apertus engaged in intra-specific combat

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.[134]

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,[135] providing evidence that dinosaurs did indeed attack each other.[136] 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.[136] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.[137]

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.[138]

Based on fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.[139] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.[140] While some early bird-like species may have already been arboreal as well (including dromaeosaurids) such as Microraptor[141]) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, pioneered by Robert McNeill Alexander, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run,[102] whether diplodocids could create sonic booms via whip-like tail snapping,[142] and whether sauropods could float.[143]

Restoration of a striking and unusual visual display in a Lambeosaurus magnicristatus

Modern birds are known to communicate using visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups, such as horns, frills, crests, sails, and feathers, suggests that visual communication has always been important in dinosaur biology.[144] Reconstruction of the plumage color of Anchiornis, suggest the importance of color in visual communication in non-avian dinosaurs.[145] The evolution of dinosaur vocalization is less certain. Paleontologist Phil Senter has suggested that non-avian dinosaurs relied mostly on visual displays and possibly non-vocal acoustic sounds like hissing, jaw grinding or clapping, splashing and wing beating (possible in winged maniraptoran dinosaurs). He states they were unlikely to have been capable of vocalizing since their closest relatives, crocodilians and birds, use different means to vocalize, the former via the larynx and the latter through the unique syrinx, suggesting they evolved independently and their common ancestor was mute.[144]

The earliest remains of a syrinx, which has enough mineral content for fossilization, was found in a specimen of the duck-like Vegavis iaai dated 69 –66 million years ago, and this organ is unlikely to have existed in non-avian dinosaurs. However, in contrast to Senter, other researchers have suggested that dinosaurs could vocalize and that the syrinx-based vocal system of birds evolved from a larynx-based one, rather than the two systems evolving independently.[146] A 2016 study suggests that some dinosaurs produced closed mouth vocalizations like cooing, hooting and booming. These occur in both reptiles and birds and involve inflating the esophagus or tracheal pouches. Such vocalizations evolved independently in extant archosaurs numerous times, following increases in body size.[147] The crests of the Lambeosaurini and nasal chambers of ankylosaurids have been suggested to have functioned in vocal resonance,[148][149] though Senter stated that the presence of resonance chambers in some dinosaurs is not necessarily evidence of vocalization as modern snakes have such chambers which intensify their hisses.[144]

See also: Dinosaur egg

Nest of a plover (Charadrius)

All dinosaurs laid amniotic eggs. Dinosaur eggs were usually laid in a nest. Most species create somewhat elaborate nests which can be cups, domes, plates, beds scrapes, mounds, or burrows.[150] 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.[151]

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 skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs.[152]

Fossil interpreted as a nesting oviraptorid Citipati at the American Museum of Natural History. Smaller fossil far right showing inside one of the eggs.

Another widespread trait among modern birds (but see below in regards to fossil groups and extant megapodes) is parental care for young after hatching. Jack Horner's 1978 discovery of a Maiasaura ("good mother lizard") nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods.[153] A specimen of the oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993,[154] which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.[155] An embryo of the basal sauropodomorph Massospondylus was found without teeth, indicating that some parental care was required to feed the young dinosaurs.[156] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.[157]

However, there is ample evidence of precociality or superprecociality among many dinosaur species, particularly theropods. For instance, non-ornithuromorph birds have been abundantly demonstrated to have had slow growth rates, megapode-like egg burying behavior and the ability to fly soon after birth.[158][159][160][161] Both Tyrannosaurus and Troodon had juveniles with clear superprecociality and likely occupying different ecological niches than the adults.[151] Superprecociality has been inferred for sauropods.[162]

Genital structures are unlikely to fossilize as they lack scales that may allow preservation via pigmentation or residual calcium phosphate salts. In 2021, the best preserved specimen of a dinosaur's cloacal vent exterior was described for Psittacosaurus, demonstrating lateral swellings similar to crocodylian musk glands used in social displays by both sexes and pigmented regions which could also reflect a signalling function. However, this specimen on its own does not offer enough information to determine whether this dinosaur had sexual signalling functions; it only supports the possibility. Cloacal visual signalling can occur in either males or females in living birds, making it unlikely to be useful to determine sex for extinct dinosaurs.[163]

Main article: Physiology of dinosaurs

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.[164] While all modern birds have high metabolisms and are endothermic ("warm-blooded"), a vigorous debate has been ongoing since the 1960s regarding how far back in the dinosaur lineage this trait extended. Various researchers have supported dinosaurs as being endothermic, ectothermic ("cold-blooded"), or somewhere in between.[165] An emerging consensus among researchers is that, while different lineages of dinosaurs would have had different metabolisms, most of them had higher metabolic rates than other reptiles but lower than living birds and mammals,[166] which is termed mesothermy by some.[167] Evidence from crocodiles and their extinct relatives suggests that such elevated metabolisms could have developed in the earliest archosaurs, which were the common ancestors of dinosaurs and crocodiles.[168][169]

This 1897 restoration of Brontosaurus as an aquatic, tail-dragging animal, by Charles R. Knight, typified early views on dinosaur lifestyles.

After non-avian dinosaurs were discovered, paleontologists first posited that they were ectothermic. This 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 remained a prevalent view until Robert T. Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968. Bakker specifically used anatomical and ecological evidence to argue that sauropods, which had hitherto been depicted as sprawling aquatic animals with their tails dragging on the ground, were endotherms that lived vigorous, terrestrial lives. In 1972, Bakker expanded on his arguments based on energy requirements and predator-prey ratios. This was one of the seminal results that led to the Dinosaur Renaissance (see § "Dinosaur renaissance").[58][59][61][170]

One of the greatest contributions to the modern understanding of dinosaur physiology has been paleohistology, the study of microscopic tissue structure in dinosaurs.[171][172] From the 1960s forward, Armand de Ricqlès suggested that the presence of fibrolamellar bone—bony tissue with an irregular, fibrous texture and filled with blood vessels—was indicative of consistently fast growth and therefore endothermy. Fibrolamellar bone was common in both dinosaurs and pterosaurs,[173][174] though not universally present.[175][176] This has led to a significant body of work in reconstructing growth curves and modeling the evolution of growth rates across various dinosaur lineages,[177] which has suggested overall that dinosaurs grew faster than living reptiles.[172] Other lines of evidence suggesting endothermy include the presence of feathers and other types of body coverings in many lineages (see § Feathers); more consistent ratios of the isotope oxygen-18 in bony tissue compared to ectotherms, particularly as latitude and thus air temperature varied, which suggests stable internal temperatures[178][179] (although these ratios can be altered during fossilization[180]); and the discovery of polar dinosaurs, which lived in Australia, Antarctica, and Alaska when these places would have had cool, temperate climates.[181][182][183][184]

Comparison between the air sacs of an abelisaur and a bird

In saurischian dinosaurs, higher metabolisms were supported by the evolution of the avian respiratory system, characterized by an extensive system of air sacs that extended the lungs and invaded many of the bones in the skeleton, making them hollow.[185] Such respiratory systems, which may have appeared in the earliest saurischians,[186] would have provided them with more oxygen compared to a mammal of similar size, while also having a larger resting tidal volume and requiring a lower breathing frequency, which would have allowed them to sustain higher activity levels.[108] The rapid airflow would also have been an effective cooling mechanism, which in conjunction with a lower metabolic rate[187] would have prevented large sauropods from overheating. These traits may have enabled sauropods to grow quickly to gigantic sizes.[188][189] Sauropods may also have benefitted from their size—their small surface area to volume ratio meant that they would have been able to thermoregulate more easily, a phenomenon termed gigantothermy.[108][190]

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. This would have helped them to conserve water.[166] In most living species, uric acid is excreted along with feces as a semisolid waste.[191][192] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[193] This material, as well as the output of the intestines, emerges from the cloaca.[194][195] In addition, many species regurgitate pellets,[196] and fossil pellets are known as early as the Jurassic from Anchiornis.[197]

The size and shape of the brain can be partly reconstructed based on the surrounding bones. In 1896, Marsh calculated ratios between brain weight and body weight of seven species of dinosaurs, showing that the brain of dinosaurs was proportionally smaller than in today's crocodiles, and that the brain of Stegosaurus was smaller than in any living land vertebrate. This contributed to the widespread public notion of dinosaurs as being sluggish and extraordinarily stupid. Harry Jerison, in 1973, showed that proportionally smaller brains are expected at larger body sizes, and that brain size in dinosaurs was not smaller than expected when compared to living reptiles.[198] Later research showed that relative brain size progressively increased during the evolution of theropods, with the highest intelligence – comparable to that of modern birds – calculated for the troodontid Troodon.[199]