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Name means: Good Mother Reptile


Species: peeblesorum

Range: Late Cretaceous (Campanian, 84-72 MYA) from Montana

Size estimate: 25-30 ft length, 3-4 tons

Discovery: John Horner and Robert Makela, 1979

Classification: dinosauria, ornithischia, ornithopoda, hadrosauria, saurolophidae


Scientists first discovered dinosaur eggs in 1922. Though they suspected at least some dinosaurs might take care of their babies, they couldn’t prove it. Over fifty years later, the discovery of fossil Maiasaura nests proved some of them were good parents. Instead of whole eggs, they found scattered fragments. The eggshells remained within the nest, so water had not broken the eggs. More likely the babies remained in the nest for a while, flopping around and breaking the shells. The leg bones of Maiasaura hatchlings had not turned fully to bone, so the babies could not walk when they first hatched. Fossils of berries also show that the adults were feeding their babies.


Maiasaura nested in colonies. Each nest lay about 30 feet apart so the adults could walk around without stepping on their eggs. Some colonies were located by the shores of ancient lakes. They would walk into the area before the wet season. When the lake rose, it would strand them on an island until the dry season started. This may have helped protect them from predators while they tended their nests. Maiasaura would not sit on their eggs. Instead, they filled their nests with weeds. As the plants began to rot, they would give off heat. Adults would turn the eggs so they would get heated evenly and could get oxygen. Modern alligators build their nests a similar way.


Bones from several different growth stages of Maiasaura help scientists understand how their skeletons developed as they aged. Such development is called “ontogeny.” This data also gives scientists an idea of how their species and closest relatives evolved. The oldest member of the group, Acristavus, greatly resembled Maiasaura and its sister species Brachylophosaurus. However, it lacked the low bump on its forehead found in the two later species. The next oldest member of the group, Probrachylophosaurus, showed a tiny bump on its forehead. It therefore seems that these animals were developing visual signals, possibly as the result of a change in social behavior.

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