Maiasaura was a herbivorous (plant-eating) dinosaur that lived at the end of the Cretaceous period, approximately 80 to 65 million years ago. It was one of the last dinosaurs and became extinct in the Cretaceous-Tertiary mass extinction. An adult Maiasaura was about 30 feet (9.1 meters) long and weighed about 3 to 4 tons. Unlike other herbivorous dinosaurs, it had no obvious defenses against predators (such as horns or armor), but it did have a powerful muscular tail and lived in herds (herds may have contained up to 10,000 individuals) that would have provided some protection.

The name Maiasaura means “Good Mother Lizard”. This name was chosen because when the animal was first discovered, in 1978 in western Montana by John “Jack” R. Horner and Robert Makela, a complete nesting site was found, including convincing evidence that adult Maiasaura cared for of their offspring. Also, this was the first time such evidence had been found for a dinosaur. Evidence of breeding behavior included both fossilized nests and hatchlings:

* Maiasaura nests were mounds of mud, with a depression in the center, each containing about 30 to 40 eggs. It is thought likely that Maiasaura covered its eggs with layers of sand or mud to help insulate them, and that the heat from decaying vegetation would have helped incubate the eggs.

* Maiasaura hatchlings did not have fully developed legs and appear to have been unable to walk, however the hatchlings have worn teeth. This seems to indicate that the adults must have brought food to the nest.