It should not come as a surprise that Deinonychus was a social animal, because many animals today are gregarious and form groups. Fossil evidence documents similar herding behaviour in a variety of dinosaurs. The mass assemblage in Bernissart, Belgium, for example, held at least three groups of Iguanodon. Group association and activity is also indicated by the dozens of Coelophysis skeletons of all ages recovered in New Mexico, U.S. The many specimens of Allosaurus at the Cleveland-Lloyd Quarry in Utah, U.S., may denote a herd of animals attracted to the site for the common purpose of scavenging. In the last two decades, several assemblages of ceratopsians and duckbills containing thousands of individuals have been found. Even Tyrannosaurus rex is now known from sites where a group has been preserved together.

These rare occurrences of multiple skeletal remains have repeatedly been reinforced by dinosaur footprints as evidence of herding. Trackways were first noted by Roland T. Bird in the early 1940s along the Paluxy riverbed in central Texas, U.S., where numerous washbasin-size depressions proved to be a series of giant sauropod footsteps preserved in limestone of the Early Cretaceous Period (145 million to 100.5 million years ago). Because the tracks are nearly parallel and all progress in the same direction, Bird concluded that “all were headed toward a common objective” and suggested that the sauropod trackmakers “passed in a single herd.” Large trackway sites also exist in the eastern and western United States, Canada, Australia, England, Argentina, South Africa, and China, among other places. These sites, dating from the Late Triassic Period (235 million to 201.3 million years ago) to the latest Cretaceous (66 million years ago), document herding as common behaviour among a variety of dinosaur types.

Some dinosaur trackways record hundreds, perhaps even thousands, of animals, possibly indicating mass migrations. The existence of so many trackways suggests the presence of great populations of sauropods, prosauropods, ornithopods, and probably most other kinds of dinosaurs. The majority must have been herbivores, and many of them were huge, weighing several tons or more. The impact of such large herds on the plant life of the time must have been great, suggesting constant migration in search of food.

Nesting sites discovered in the late 20th century also establish herding among dinosaurs. Nests and eggs numbering from dozens to thousands are preserved at sites that were possibly used for thousands of years by the same evolving populations of dinosaurs.

Growth and life span

Much attention has been devoted to dinosaurs as living animals—moving, eating, growing, reproducing biological machines. But how fast did they grow? How long did they live? How did they reproduce? The evidence concerning growth and life expectancy is sparse but growing. In the 1990s histological studies of fossilized bone by Armand de Ricqlès in Paris and R.E.H. Reid in Ireland showed that dinosaur skeletons grew quite rapidly. The time required for full growth has not been quantified for most dinosaurs, but de Ricqlès and his colleagues have shown that duckbills (hadrosaurs) such as Hypacrosaurus and Maiasaura reached adult size in seven or eight years and that the giant sauropods reached nearly full size in as little as 12 years. How long dinosaurs lived after reaching adult size is difficult to determine, but it is thought that the majority of known skeletons are not fully grown, because their bone ends and arches are very often not fused; in mature individuals these features would be fused.

Reproduction

The idea that dinosaurs, like most living reptiles and birds, built nests and laid eggs had been widely debated even before the 1920s, when a team of scientists from the American Museum of Natural History, New York, made an expedition to Mongolia. Their discovery of dinosaur eggs in the Gobi Desert proved conclusively that at least one kind of dinosaur had been an egg layer and nest builder. These eggs were at first attributed to Protoceratops, but they are now known to have been those of Oviraptor. In 1978 John R. Horner and his field crews from Princeton University discovered dinosaur nests in western Montana. A few other finds, mostly of eggshell fragments from a number of sites, established oviparity as the only known mode of reproduction. In recent years an increasing number of dinosaur eggshells have been found and identified with the dinosaurs that laid them, and embryos have been found inside some eggs.

The almost complete absence of juvenile dinosaur remains was puzzling until the 1980s. Horner, having moved to Montana State University, demonstrated that most paleontologists simply had not been exploring the right territory. After a series of intensive searches for the remains of immature dinosaurs, he succeeded beyond all expectations. The first such bones were unearthed near Choteau, Montana, and thereafter Horner and his crews discovered hundreds of nests, eggs, and newly hatched dinosaurs (mostly duckbills). Horner observed that previous explorations had usually concentrated on lowland areas, where sediments were commonly deposited and where most fossil remains were preserved. He recognized that such regions were not likely to produce dinosaur nests and young because they would have been hazardous places for nesting and raising the hatchlings. Upland regions would have been safer, but they were subject to erosion rather than deposition and were therefore less likely to preserve nests and eggs. However, it was exactly in such upland areas, close to the young and still-rising Rocky Mountains, that Horner made his discoveries.

Egg Mountain, as the area was named, produced some of the most important clues to dinosaurian habits yet found. For example, the sites show that a number of different dinosaur species made annual treks to this same nesting ground (though perhaps not all at the same time). Because of the succession of similar nests and eggs lying one on top of the other, it is thought that particular species returned to the same site year after year to lay their clutches. As Horner concluded, “site fidelity” was an instinctive part of dinosaurian reproductive strategy. This was confirmed more recently with the discovery of sauropod nests and eggs spread over many square kilometres in Patagonia, Argentina.