Maiasaura (from the Greek “μαία” and the feminine form of Latin saurus, meaning “good mother reptile” or “good mother lizard” ) is a large herbivorous hadrosaurid (“duck-billed”) dinosaur genus that lived in the area currently covered by the state of Montana in the Upper Cretaceous Period (mid to late Campanian), about 76.7 million years ago.
The first fossils of Maiasaura were discovered in 1978. In 1979, the genus was named. The name refers to the find of nests with eggs, embryos and young animals, in a nesting colony. These showed that Maiasaura fed its young while they were in the nest, the first time such evidence was obtained for a dinosaur. Hundreds of bones of Maiasaura have been dug up. Evidence suggest the “Maiasaura” was covered rapidly by a global flood. This preserved the nesting site and fossils.
Maiasaura was about nine metres long. Young animals walked on their hind legs, adults on all fours. Maiasaura was probably closely related to Brachylophosaurus.
Maiasaura were large, attaining a maximum known length of about 9 metres (30 ft). They had a flat beak typical of hadrosaurids, and thick noses. They had a small, spiky crest in front of the eyes. This crest may have been used in headbutting contests between males during the breeding season.
Maiasaura were herbivorous. They were capable of walking both on two (bipedal) or four (quadrupedal) legs. Studies of the stress patterns of healed bones show that young juveniles under four years old walked mainly bipedal, switching to a mainly quadrupedal style of walking when they grew larger. They appeared to have no defense against predators, except, perhaps, its heavy muscular tail and their herd behavior. Herds were extremely large and could have comprised as many as 10,000 individuals. Maiasaura lived in an inland habitat.
Maiasaura lived in herds and it raised its young in nesting colonies. The nests in the colonies were packed closely together, like those of modern seabirds, with the gap between the nests being around 7 metres (23 ft); less than the length of the adult animal. The nests were made of earth and contained 30 to 40 eggs laid in a circular or spiral pattern. The eggs were about the size of ostrich eggs.
It appears that young Maiasaura grew quickly. To some researchers, this suggests that they were warm-blooded. The nests that Horner and Makela found also throw light on the social organization of these hadrosaurs. The number and proximity of the nests indicate that females nested in large groups. Some scientists believe that Maiasaura were strongly social animals that lived in herds of many thousands.
Maiasaura is a characteristic fossil of the middle portion (lithofacies 4) of the Two Medicine Formation, dated to about 76.4 million years ago. Maiasaura lived alongside the troodontid Troodon and the hypsilophodont Orodromeus, as well as the dromaeosaurid Bambiraptor and the tyrannosaur Daspletosaurus.