duck-billed dinosaurs (dinosaur) (hadrosaurs) found as fossils from the Late Cretaceous Period (99 million to 65 million years old) of North America and whose discovery led to the theory that these bipedal herbivores cared for their young.

In 1978 a Maiasaura nesting site was discovered in the Two Medicine Formation near Choteau, Montana, U.S. The remains of an adult Maiasaura were found in close association with a nest of juvenile dinosaurs, each about 1 metre (3.3 feet) long. Hatchlings that were too large (about 0.5 metre long) to fit into eggs, and nests with clutches of eggs, as well as many broken eggshells, were found nearby. The bones of the embryos, however, were not fully ossified, which means the young could not have walked immediately upon hatching and would have required some degree of parental care. Hundreds of skeletons preserved in one specific ashbed in Montana, as well as those preserved in nesting sites, suggest that Maiasaura was migratory. Such evidence also demonstrates that these dinosaurs were social animals that nested in groups; they probably returned to the same nesting site year after year. Studies of bone structure indicate that it would have taken about seven or eight years for Maiasaura to reach an adult size of eight metres.