The dinosaur known from nesting sites containing eggs and embryos, and nicknamed the “good mother lizard,” is Maiasaura. The name comes from Greek words meaning “good mother lizard,” reflecting evidence that adults cared for their young.
Paleontologists Jack Horner and Robert Makela described Maiasaura in 1979 after discoveries near Choteau, Montana. The site, later called Egg Mountain, contained nests, eggs, embryos, hatchlings, juveniles, and adults. Nests held roughly 30 to 40 eggs, and the close spacing of nests suggests that Maiasaura nested in colonies or herds.
The evidence indicates that hatchlings remained in the nest after hatching. Their legs were not fully developed, and worn teeth suggest that adults brought food to them. The eggs were probably warmed by decomposing vegetation rather than by an adult sitting directly on the nest. Maiasaura was a duck-billed hadrosaur that lived in western North America during the Late Cretaceous, about 86 to 70 million years ago. It was herbivorous, not a meat-eating “raptor,” and its parental care is inferred from fossils rather than observed directly.