Maiasaura

Maiasaura (from Greek: μαῖα, lit. 'good mother' and Greek: σαύρα, the feminine form of saurus, lit. 'reptile') is a large herbivorous saurolophine hadrosaurid ("duck-billed") dinosaur genus that lived in the area currently covered by the state of Montana and the Canadian province of Alberta, in the Upper Cretaceous (mid to late Campanian), from 86.3 to 70.6 million years ago. Maiasaura is the state fossil of Montana.

The first remains of Maiasaura were discovered in the Two Medicine Formation near Chouteau, Montana in 1978 by Bynum, Montana resident Laurie Trexler. This holotype specimen was later described by Horner and Makela in 1979 as the new genus and species Maiasaura peeblesorum. The given genus name refers to the finding of eggs, embryos, and juveniles in a nest-like structure by Marion Brandvold in 1978 relatively close to the holotype specimen. This discovery of fifteen juvenile dinosaurs in close proximity to an adult showed the first instance of parental and social behavior in dinosaurs. It allowed for interpretations such as that Maiasaura fed its young while they were in the nest. Further work in this area led to the discovery of more dinosaur eggs, leading to the area being named "Egg Mountain". Hundreds of bones of Maiasaura have been discovered. Maiasaura was about 9 metres (30 ft) long. Young animals walked on their hind legs, adults on all fours. Maiasaura was probably closely related to Brachylophosaurus.

Maiasaura peeblesorum were large, attaining a maximum known length of about 9 metres (30 ft) and a body mass is measured approximately up to 4 metric tons (4.4 short tons). They had a large "duck-billed" mouth structure and rows of hundreds of teeth, typical of hadrosaurids. Since hadrosaurids have very similar post-cranial body plans, the distinguishing characteristic of Maiasaura peeblesorum is a prominent short, solid crest-like structure situated between their eyes. This crest may have been used in headbutting contests between males during the breeding season.

Maiasaura were herbivorous. They were capable of walking both on two (bipedal) or four (quadrupedal) legs. Studies of the stress patterns of healed bones show that young juveniles under four years old walked mainly bipedal, switching to a mainly quadrupedal style of walking when they grew larger. Maiasaura, like most other hadrosaurs, possessed little in the way of obvious weaponry, though likely could defend themselves with kicks, stomps, or their muscular tails. It is likely that they primarily resorted to fleeing in the face of danger, using the vast sizes of their herds to be less likely to be targeted. Mass bone beds discovered in the Two Medicine Formation show that herds could be extremely large and comprise as many as 10,000 individuals. Hundreds of specimens have been found throughout all stages of life, allowing for M. peeblesorum to be used for understanding how hadrosaurids grew. Maiasaura peeblesorum lived in a terrestrial habitat.

A skull of Maiasaura, specimen PU 22405 (now in the collections of the Yale Peabody Museum of Natural History as YPM PU 22405 following the transfer of the Princeton University vertebrate paleontology collections), was discovered by Laurie Trexler in 1979 and described by dinosaur paleontologists Jack Horner and Robert Makela as the holotype of a new species. They named the type species Maiasaura peeblesorum. The generic name related to the Greek goddess Maia, the mother of Hermes; to emphasise this, they used the feminine form of saurus: saura. The specific name honours the families of John and James Peebles, on whose land the finds were made. The generic name refers to Marion Brandvold's discovery in 1978 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.

Over 200 specimens, in all age ranges, have been found. The announcement of the discovery of Maiasaura 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. Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.

Maiasaura peeblesorum is in the tribe Brachylophosaurini along with these related taxa:

The following cladogram of hadrosaurid relationships was published in 2013 by Albert Prieto-Márquez et al.: