Fossil remains of dinosaurs have been found in rock strata of every continent, indicating that they differed widely in structure, habitat, and diet. Their brain sizes varied, with some predators having brain-to-body ratios equivalent to those of some modern birds and animals. Many species built nests. Many theories regarding dinosaurs and their behavior are hotly debated by the experts. These include the debate over the grouping of birds with dinosaurs, the question of whether nonavian dinosaurs were cold-blooded (ectothermic) or warm-blooded (endothermic), the question of whether dinosaurs protected and nurtured their young in the nest after hatching or whether the young were mobile and self-sufficient at birth, and the reason for the disappearance of nonavian dinosaurs.
No complete fossil dinosaur has ever been discovered. Inferences must be made from fragments or pieces that have been compressed and distorted. Information about the diet has been gleaned from stomach contents and coprolites (fossilized dinosaur feces) and by comparing the teeth to those of living animals, for example, relating the large grinding teeth of hadrosaurs to those of living herbivores. Fossilized dinosaur footprints, such as the trackways found at Davenport Ranch in Texas, have been interpreted as evidence that some dinosaurs traveled in herds; bonebeds containing large numbers of certain dinosaurs, as have been found in Alberta, Canada, have also been seen as evidence of this. What is known about dinosaurs is that, far from being evolutionary failures, they dominated their habitats for much of their 160 million years of existence (the human species Homo sapiens has existed for approximately 150,000–200,000 years).
Although all dinosaurs were originally classified in a single order, it was later discovered that the group contained two distinct types distinguished by structural differences. The pelvis in the saurischian (lizard-hipped) dinosaurs resembles that of still-extant reptiles, but in the ornithischian (bird-hipped) dinosaurs the pubic bone of the pelvis has forward and backward extensions that resemble those found in birds. It was later determined, however, that the backward-tilting hips of ornithischian dinosaurs and birds were the result of convergent evolution and not inheritance. Many other shared characteristics have been noted between birds and saurischians, and it is now believed by many paleontologists that modern birds are in fact extant dinosaurs, descended from the theropods of the saurischian order.
The jaws and teeth of the two dinosaur orders also differ. The saurischian order, which includes both herbivores and carnivores, has teeth around the entire jaw or confined to the front of the mouth. Ornithischians have "cheek teeth" along the sides of the jaw, but never in the front; the bones at the front of the mouth sometimes developed into the horny beaks typical of modern turtles. All known ornithischians were herbivores.
Dinosaurs are further classified into some common groupings. In the saurischian dinosaurs, some were theropods [Gr., = beast feet], a group sharing hind feet with only three functional toes (e.g., the carnivorous bipeds Tyrannosaurus, Velociraptor, Deinonychus, and possibly the living birds); others were sauropods [Gr., = lizard feet] with small heads and long necks (e.g., the herbivorous quadrupeds Apatosaurus [Brontosaurus] and Diplodocus). Among the ornithischians, there were ornithopods (bird-footed dinosaurs), such as Iguanodon; thyreophorans (armored dinosaurs), such as Stegosaurus and Ankylosaurus; and ceratopsians (horned dinosaurs), such as Triceratops. The total number of dinosaur genera that existed is unknown; new species are discovered every year, but some species, on further examination, are found to be redundant with earlier finds. One estimate of the possible number of distinct genera exceeds 1,800.
Similarities of dinosaurs found on what are now different continents have given scientists clues to the breakup of the supercontinent Pangaea, which began about 170 million years ago. For example, the discovery of a 130-million-year-old African dinosaur similar to the North American Allosaurus suggests that the African plate was connected to the northern continents (Laurasia) longer than had been believed previously.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.