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Maiasaura (from the Greek "μαία + σαύρα", meaning "caring mother lizard") is a large duck-billed dinosaur genus that lived in the area currently covered by the state of Montana in the Cretaceous Period, about 74 million years ago.

These get their name from the term good mother lizards. This comes from some various evidence that seems to indicate that they cared for their hatchlings.


Maiasaura was large, growing up to 9 metres (30 ft) and had the typical hadrosaurid flat beak and a thick nose. It had a small, spiky crest in front of its eyes. The crest may have been used in headbutting contests between males during the breeding season.

This dinosaur was an herbivore. It walked both on two or four legs and seemed to have no defense from predators, though its heavy muscular tail and its herd behaviour both could have been one. These herds were quite large and could have been made up of as much as 10,000 individuals.[1] Maisaura lived in an inland habitat.[2]


Maiasaura was discovered by Laurie Trexler and described by dinosaur paleontologist Jack Horner (paleontologic advisor for the Jurassic Park movies) and Robert Makela. He named the dinosaur after Marion Brandvold's discovery of a nest with remains of eggshells and babies too large to be hatchlings. These discoveries led to others, and the area became known as "Egg Mountain", in rocks of the Two Medicine Formation near Choteau in western Montana. This was the first proof of giant dinosaurs raising and feeding their young.[3] Over 200 specimens, in all age ranges, have been found.[4] The announcement of Maiasaura's discovery attracted renewed scientific interest to the Two Medicine Formation and many other new kinds of dinosaurs were discovered as a result of the increased attention.[5] Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.[6]


The closest phylogenetic relation of the genus Maiasaura seems to fall with Brachylophosaurus. To reflect on this, both are in the same tribe, and are thus differentiated from the other components of this subfamily: Aralosaurus, Hadrosaurus, and Gryposaurus.

Recently, along with Brachylophosaurus and Acristavus, Maiasaura was located in a new clade called Brachylophosaurini .[7]



Maiasaura lived in herds and it raised its young in nesting colonies. The nests in the colonies were packed close together, like those of modern seabirds, with the gap between the nests being around 23 ft; less than the length of the adult animal.[8] The nests were made of earth and had 30 to 40 eggs laid in a round or coiled pattern. The eggs were about the size of ostrich eggs.The eggs were incubated by the heat caused by rotting vegetation placed into the nest by the parents, rather than a parent sitting on the nest. Fossils of baby Maiasaura show that, when they hatched, their legs were not fully developed and thus they could not leave the nest. Fossils also show that their teeth were partly worn, which means that the adults brought food to the nest.The hatchlings grew from a size of 16 to 58 inches long in the span of their first year. At this point, or perhaps after another year, the animal left the nest. This high rate of growth may mean they were warm blooded. The hatchlings had different facial proportions from the adults, with larger eyes and a shorter snout.[9] These traits are associated with cuteness and are common in animals that depend on their parents when they are young.


In the Two Medicine Formation, Maiasaura lived alongside the tyrannosaur Daspletosaurus, the oviraptorosaur Chirostenotes, Troodon, the dromaeosaurs Bambiraptor and Saurornitholestes, the enantiornithe bird Avisaurus, the nodosaur Edmontonia, the ankylosaur Euoplocephalus, the ceratopsids Achelousaurus, Brachyceratops, Einiosaurus, and Styracosaurus ovatus, the hypsilophodont Orodromeus, and the hadrosaurids Hypacrosaurus stebingeri and Prosaurolophus.[10] Hypacrosaurus coexisted with Maiasaura for some time, as Hypacrosaurus remains have been found lower in the Two Medicine Formation than was earlier known. The discovery of Gryposaurus latidens in Maiasaura's range has shown that the border between theorized distinct faunas in the upper and middle is less distinct than once thought. There seems to be a major variation in ornithischian taxa after the appearance of Maiasaura within the Two Medicine Formation. The close examination of strata found along the Two Medicine River (which exposes the entire upper half of the Two Medicine Formation) indicates that the apparent variation was a real event rather than from preservational biases.[11]

  1. "Maiasaura," Dodson, et al.(1994); pages 116-117.
  2. "Judithian Climax," Lehman (2001); page 315.
  3. "Maiasaura," Dodson, et al.(1994); pages 116-117.
  4. Horner and Gorman (1988).
  5. "Introduction," Trexler (2001); pages 299-300.
  6. "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 304.
  7. Gates, T.A.; Horner, J.R.; Hanna, R.R.; and Nelson, C.R. (2011). «New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America». Journal of Vertebrate Paleontology 31 (4): pp. 798-811. doi:10.1080/02724634.2011.577854.
  8. Palmer (1999); page 148.
  9. "Maiasaura," Dodson, et al.(1994); pages 116-117.
  10. Weishampel, et al. (2004); pages 517–606.
  11. Faunal Turnover, Migration, and Evolution," Trexler (2001); page 306