Maiasaura peeblesorum was a large, herbivorous hadrosaurid, or “duck-billed,” dinosaur. This genus is famous because its name, which means “Good Mother Lizard,” reflects the first concrete evidence of complex social behavior and parental care in these extinct reptiles. Analyzing where and when this approximately 30-foot-long creature lived offers detailed insights into a specific, dynamic ancient ecosystem and dinosaur life cycles.
Geographic Location and Geologic Time
Maiasaura inhabited the western region of North America during the Late Cretaceous Epoch (Campanian stage), approximately 77 to 74 million years ago. Fossils are predominantly recovered from the Two Medicine Formation in Montana, with additional remains found in Alberta, Canada. This distribution placed the dinosaur on Laramidia, a narrow island continent stretching from modern-day Alaska to Mexico.
Laramidia was separated from Appalachia to the east by the Western Interior Seaway, a shallow sea that bisected North America. Maiasaura lived on the broad coastal plains and river floodplains bordering the western side of this massive inland sea. They preferred a low-lying, inland environment, suggesting habitats slightly further from the immediate coastline compared to some other hadrosaurs. The Two Medicine Formation sediments record these ancient river systems and volcanic ash layers that helped preserve the remains.
The Significance of the Egg Mountain Site
The most significant locality for Maiasaura is Egg Mountain in Teton County, Montana, discovered in the late 1970s. This site provided the world’s first definitive proof that some dinosaurs engaged in colonial nesting and cared for their young. It contained multiple nests clustered closely together, separated by about 23 feet, roughly the length of an adult Maiasaura.
The nests were bowl-shaped structures made of earth, containing the remains of eggs, hatchlings, and juveniles. Evidence showed the young remained in the nests after hatching, a crucial clue to parental care. The juveniles’ bones were unfused and spongy, indicating they were altricial and unable to walk or forage independently after birth.
Further analysis of the hatchlings’ small teeth showed wear patterns, suggesting the young were being fed plant matter by an adult while still confined to the nest. This evidence confirmed the hypothesis of extended post-hatching care, justifying the dinosaur’s “Good Mother” moniker. The discovery of individuals from all age groups—from eggs and embryos to fully grown adults—at Egg Mountain provided a complete picture of the species’ life history and social structure.
The Late Cretaceous Environment
The plains and floodplains of Laramidia where Maiasaura thrived were characterized by a warm, subtropical, and humid paleoclimate. Global sea levels were much higher than today, and the lack of polar ice caps contributed to this generally mild, moist world. Mean annual temperatures were significantly warmer than modern Montana, supporting a lush, diverse flora.
As a herbivore, Maiasaura relied on the abundant vegetation of its environment, which included conifers, ferns, cycads, and an increasing variety of flowering plants. The dinosaur possessed a broad, duck-like beak and specialized teeth suitable for grinding this tough plant material. Living in large herds, sometimes estimated to be thousands strong, was likely a defensive strategy against the large predators that shared its habitat.
The Maiasaura coexisted with other prominent dinosaur groups that roamed the Laramidian coastal plain. These contemporaries included armored dinosaurs, ceratopsians like Einiosaurus, and formidable predators such as the tyrannosaurid Daspletosaurus. This rich ecosystem, recorded in the Two Medicine Formation, provides a detailed snapshot of a complex community.