Maiasaura and the nesting grounds

Maiasaura peeblesorum, whose name means good mother lizard, was a duck-billed hadrosaur about 9 m long that lived in herds on the floodplains of Laramidia. In 1978 Jack Horner and Robert Makela found a nesting colony at Egg Mountain in the Two Medicine Formation of Montana, about 76 to 75 million years old. Nests, eggs, hatchlings and juveniles lay together, and the nests were about an adult length apart. Horner and Makela named the species in 1979.¹²

Hatchlings had underdeveloped limbs but worn teeth, which Horner took to mean they stayed in the nest while adults brought food. This was the first strong evidence for parental care in dinosaurs and helped change the picture of them as simple egg-layers. Not all agree on the details, and later work suggests some hadrosaurs grew quickly and left the nest sooner. Hadrosaurs themselves, with their batteries of hundreds of grinding teeth, were the dominant plant eaters of the Late Cretaceous across North America and Asia.²³⁴