Thousands of complete or par tial skeletons have been found on nearly every continent, strewn through strata representing nearly 140 million years of earth history. They have made it possi ble to infer quite well what the animals looked like. The same cannot be said about how they lived and what their be havior was. Nevertheless, over the past five years dinosaur skeletons and dino saur eggs have been found at two sites in Montana in associations that yield clues about the social behavior of the three types of dinosaur represented. The broad setting for the tale is the North American continent of 80 million years ago. The continent was bisected by a shallow sea that has been named the Western Interior Cretaceous Seaway. The newly forming Rocky Mountains, together with a few isolated volcanic systems, made up the highland topog raphy of" West America" and the Ap palachian Mountains formed the high lands of" East America." A broad coastal plain extended from the eastern edge of the Rockies to the western shore of the sea; it was the place where sediments eroded from the moun tains were deposited. In what is now Montana and southern Alberta the plain was as much as 400 kilometers wide, although the distance fluctuated with changes in sea level. Along the coast and for some distance inland extensive riv ers, river deltas, swamps and marshes were common, bearing dense vegetation that was probably quite similar to what is found on the southern coast of Loui siana today. Bald cypresses, redwoods and numerous broad-leaved trees grew in the wet subtropical climate. Inhabiting this environment were fishes, amphibians, aquatic turtles, croc odiles and small primitive mammals. Dinosaurs were also present, represent ed mainly by hadrosaurs (duck-billed dinosaurs), ceratopsians (horned dino saurs) and numerous small and large carnosaurs. Since dinosaurs are known to have lived there, it is remarkable that