Dinosaurs' size

While the evidence is incomplete, it is clear that, as a group, dinosaurs were large. By dinosaur standards the sauropods were gigantic. 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 walked the Earth since.

The tallest and heaviest dinosaur known from a complete skeleton is the Brachiosaurus, which was discovered in Tanzania between 1907–12. It is now mounted and on display at the Humboldt Museum of Berlin and is 12 m (38 ft) tall and probably weighed between 30,000–60,000 kg (33–66 short tons). 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.

There were larger dinosaurs, but knowledge of them is based entirely on a small number of incomplete fossil samples. The largest specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80,000–100,000 kg (88–121 tons); the longest, the 40 m (130 ft) long Supersaurus; and the tallest, the 18 m (60 ft) Sauroposeidon, which could have reached a sixth-floor window.

Dinosaurs were the largest of all terrestrial animals. The largest elephant on record weighed 12,000 kg (13.2 tons), while the tallest giraffe was 6 m (20 ft) tall. Even giant prehistoric mammals such as the Indricotherium and the Columbian mammoth were dwarfed by the giant sauropods. Only a handful of modern aquatic animals approach them in size, most notably the blue whale (which reaches up to 190,000 kg (209 tons) and 33.5 m (110 ft) in length).

Not including modern birds like the bee hummingbird, the smallest dinosaurs known were about the size of a crow or a chicken. The Microraptor, Parvicursor, and Saltopus were all under 60 cm (2 ft) in length.

Average size

The meaning of "dinosaur average size" is debatable. However it is defined, current evidence suggests different values for average size in the Triassic, early Jurassic, late Jurassic and Cretaceous periods. According to Bill Erickson, "Estimates of median dinosaur mass range from 500 kg to 5 metric tons, Eighty percent of the biomass from the Late Jurassic Morrison formation of the western United States consisted of stegosaurs and sauropods; the latter averaged 20 tons. The typically large size of the dinosaurs, and the comparatively small size of modern mammals, has been quantified by Nicholas Hotton. Based on 63 dinosaur genera, Hotton's data yield an average generic mass in excess of 850 kg (about the size of an average grizzly bear) and a median generic mass of nearly 2 tons (which is comparable to a giraffe). This contrasts sharply with extant mammals (788 genera) whose average generic mass is 863 grams (a large rodent) and a median mass of 631 grams (a smaller rodent). The smallest dinosaur was bigger than two-thirds of all current mammals; the majority of dinosaurs were bigger than all but 2% of living mammals.

BEHAVIOR

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 direct evidence of herding behavior was the 1878 discovery of 31 Iguanodon dinosaurs which perished together in Bernissart, Belgium, after they fell into a deep, flooded ravine and drowned. Similar mass deaths and trackways suggest that herd or pack 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 Oxford, England, and others kept their young in the middle of the herd for defense according to trackways at Davenport Ranch, Texas. Dinosaurs may have congregated in herds for defense, for migratory purposes, or to provide protection for their young.

A nesting ground of Maiasaura was discovered in 1978. 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 the Patagonian sauropod Saltasaurus (1997 discovery), had similar nesting behaviors, and that the animals congregated in huge nesting colonies like those of penguins. The Mongolian maniraptoran Oviraptor 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. Trackways have also confirmed parental behavior among sauropods and ornithopods from the Isle of Skye in the United Kingdom. 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.

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. The nature of dinosaur communication also remains enigmatic, and is an active area of research. For example, recent evidence suggests that the hollow crests of the lambeosaurines may have functioned as resonance chambers used for a wide range of vocalizations.

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, proving that dinosaurs did indeed attack and eat each other. While cannibalistic behavior among theropods is no surprise, this too was confirmed by tooth marks from Madagascar in 2003.

There seem to have been no burrowing and few climbing dinosaur species. This is somewhat surprising when compared to the later mammalian radiation in the Cenozoic, which included many species of these types. As to how the animals moved, biomechanics has provided significant insight. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have demonstrated how fast dinosaurs could run, whether diplodocids could create sonic booms via whip-like tail snapping, whether giant theropods had to slow down when rushing for food to avoid fatal injuries, and if sauropods could float.