Maiasaura (from , and , the feminine form of saurus, ) 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.

Discovered in 1978 during the dinosaur renaissance, the species description of Maiasaura first came in 1979 from paleontologists Jack Horner and Bob Makela following the discovery of a site located South of Choteau, Montana. They named the new genus and species Maiasaura peeblesorum, with the genus name Maiasaura, ("good mother lizard") referring to Makela and Horner's conclusion that this species took care of their offspring. Collected data at the site indicated that Maiasaura fed its young while they were in the nest, the first instance of parental and social behavior in dinosaurs.

Maiasaura, whose bones have been found by hundreds in the state of Montana, has been chosen as Montana's state fossil.

Description

thumb|left|Skull cast, [[Royal Ontario Museum]]

thumb|right|Size comparison with human

Maiasaura peeblesorum were large, attaining a maximum known length of about and an approximate body mass measured at up to . They had a large "duck-billed" mouth structure and rows of hundreds of teeth, typical of hadrosaurids. While hadrosaurids have very similar post-cranial body plans, the distinguishing characteristic of Maiasaura peeblesorum is a prominent short, solid crest-like structure situated between the eyes. This crest may have been used in headbutting contests between males during the breeding season.

Maiasaura were herbivorous, terrestrial dinosaurs. They were capable of walking both on two (bipedal) or four (quadrupedal) legs. Studies of the stress patterns of healed bones show that juveniles under four years old walked mainly bipedally, 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.

Discovery

thumb|left|Map of [[Saurolophinae|Brachylophosaurini (Saurolophinae) specimen discoveries in Alberta and Montana. The first remains of Maiasaura have been found south of Choteau, visible on the map.]]

For years until the 1960s and 1970s, anyone who has traveled through the area south of Choteau, Montana might have come across Maiasaura remains, whether such remains have been or have not been attributed to a dinosaurian origin.

The first people who are confirmed as having found Maiasaura remains laying on the "Egg Mountain" area (as it is called today), are two homestead families of Bynum, Montana: the Brandvolds and the Trexlers. Marion Kathryn Brandvold (1912–2014, née Nehring), had inherited the "rock shop", Trex Agate Shop, that had been founded in 1937 by her first husband, Clifford "Trex" Trexler (1908–1962). In the years that preceded 1978, she and her second husband, John Brandvold (1937–2020), had been finding small bones and they had been trying to put them together. But in 1978 paleontologist Bob Makela had been told by fellow Berkeley paleontologist Bill Clemens that the State of Montana had called him about a north-western Montana woman who had found bones which she wanted to identify. Makela informed his research partner Jack Horner and both went to the Trex Agate Shop in Bynum. There, Marion Brandvold showed them some dinosaur bones, which the paleontologists could identify, but also two tiny bones that Horner identified as baby hadrosaur bones. Brandvold said she had more at her place, so they followed her to the Brandvolds' house, where she'd been keeping the remains of at least four individuals in a coffee can. Two weeks later Brandvold took Makela and Horner to the site in Choteau where she have found the remains. There, bones were visible on the surface, not even needing to dig. During the same Summer of 1978, Makela and Horner excavated the spot and noticed that they were excavating a nest, where in the end they found a total of 15 baby dinosaurs. and in the Western Hemisphere. With time more eggs were discovered and the site of this discovery earned the name of "Egg Mountain", because of the abundance of hadrosaur eggs and eggshell pieces found in it.

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 David Trexler's wife Laurie Trexler in 1979. Trexler partially broke the skull as a consequence of her lack of appropriate excavating tools, but after informing Jack Horner and Robert Makela, both paleontologists managed to extract the whole remaining part of the fossil.

David Trexler, the second son of Clifford Trexler and Marion Brandvold, grew up in such a context that he ended up by becoming a paleontologist himself. In 1995 he founded in Bynum the Montana Dinosaur Center. That center plays an important role in connexion with the first identified remains of Maiasaura because during the years that spanned from 1998 to 2004, David Trexler's mother Marion Brandvold led a legal fight with the Yale and Princeton universities in order to recover the bones of the baby Maiasauras that she had found back in the 1970s. In 2004 she obtained a satisfactory issue and, since then, these historically important fossils are preserved at the Montana Dinosaur Center in Bynum.

Over 200 specimens, in all age ranges, have been found.

Classification

thumb|Cast of a juvenile skeleton

thumb|[[Life restorations of an adult and juvenile]]Maiasaura peeblesorum is in the tribe Brachylophosaurini along with these related taxa:

  • Acristavus gagslarsoni
  • Brachylophosaurus canadensis
  • Ornatops incantatus
  • Probrachylophosaurus bergei

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

Paleobiology

thumb|left|Reconstructed cast by [[Jack Horner (paleontologist)|Jack Horner of a Maiasaura emerging from its egg]]

Maiasaura lived in herds and it raised its young in nesting colonies. The nests in the colonies were packed closely together, like those of modern seabirds, with the gap between the nests being around ; less than the length of the adult animal. Fossilized M. peeblesorum eggs are black in color and have high, prominent ridges on the outer surface.

Studies led by Holly Woodward, Jack Horner, Freedman Fowler et al. have given insight into the life history of Maiasaura, resulting in what is perhaps the most detailed life history of any dinosaur known, and to which all others can be compared. From a sample of fifty individual Maiasaura tibiae, it was found that Maiasaurs had a mortality rate of about 89.9% in their first year of life. If the animals survived their second year, their mortality rate would drop to 12.7%. The animals would spend their next six years maturing and growing. Sexual maturity was found to occur in their third year, while skeletal maturity was attained at eight years of age. In their eighth year and beyond, the mortality rate for Maiasaura would spike back to around 44.4%. The studies that followed also found that Maiasaurs were primarily bipedal as juveniles, and switched to a more quadrupedal stance as they aged. It was also found that Maiasaura also included rotting wood in its diet, as well that its environment had a long, dry season prone to drought. The results of the study were published in the journal Palaeobiology on September 3, 2015.

Diet

A paper from 2007 showed that Maiasaura had a diet consisting of fibrous plants, wood, rotting wood, tree bark, leaves, branches, ferns, angiosperms and possibly grasses. This would imply that Maiasaura was both a browser and a grazer. Analysis of its dental wear patterns show that juvenile M. peeblesorum exhibited more crush wear than adults, which displayed more shear wear than juveniles. This suggests that adults fed more on tough, fibrous vegetation while juveniles fed more on hard, brittle objects like nuts, seeds, or berries.

Sexual dimorphism

Studies of Maiasaura by Saitta et al., suggest that one sex was roughly 45% larger than the other according to the mathematical analysis known as size statistics. However, it cannot be ascertained at this time whether the larger sex was male or female.

Palaeoecology

thumb|Illustration of a herd of Maiasaura walking along a creekbed, as found in the semi-arid [[Two Medicine Formation fossil bed. This region was characterized by volcanic ash layers and conifer, fern and horsetail vegetation.]]

Maiasaura is a characteristic fossil of the middle portion (lithofacies 4) of the Two Medicine Formation, dated from about 86.3 to 70.6 million years ago.

In the Oldman Formation of Alberta, Maiasaura lived alongside the ceratopsians Albertaceratops, Anchiceratops, Chasmosaurus, Coronosaurus, and Wendiceratops, as well as the dromaeosaurids Dromaeosaurus, Saurornitholestes, and Hesperonychus, the tyrannosaurid Daspletosaurus, the orodromine thescelosaurid Albertadromeus, the pachycephalosaurs Foraminacephale and Hanssuesia, the ornithomimid Struthiomimus, the other hadrosaurids Brachylophosaurus, Corythosaurus, and Parasaurolophus, and the ankylosaurid Scolosaurus.

In 1985 as well, the CBS television documentary Dinosaur! showed Jack Horner talking to the camera about the "Egg Mountain" site and its related discoveries. Dinosaur! was seminal too showing to the CBS audiences the CT scan image of a Maiasaura embryo found inside an egg that had been previously collected at "Egg Mountain".

In 1987, the television documentary film Digging Dinosaurs, produced by the television channel WHYY Philadelphia with fund by the William Penn Foundation, showed a fossil dig team led by Bob Makela and Jack Horner, all excavating at a Maiasaura fossilised nesting ground in northern Montana. In the film, Horner talks to the camera about his conclusions on how the juveniles hatched and grew, first in the nests, then in company of the adults when joining the herd. He also mentions his interest in the opportunity that this species, found in hundreds of individuals, brings to studies on individual variation within a species of dinosaur.

Horner reappeared on television, this time with extended screen time about the Maiasaura discoveries, in another programme, "The Great Dinosaur Hunt", part of The Infinite Voyage series of documentaries. The Great Dinosaur Hunt first aired on January 4, 1989.

In 1991 two short documentaries produced by Earthtalk Studios: A Giant Leap for Dinosaurs and Dinosaur Hunters, both directed by Daniel J. Smith, showed Jack Horner at Camp Makela (the scientists' camp that is on site at "Egg Mountain") talking with children and adolescents about the cutting edge of dinosaur research.

See also

  • Timeline of hadrosaur research

Notes

References

Bibliography

  • Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. p. 116-117. .
  • Horner, Jack and Gorman, James. (1988). Digging Dinosaurs: The Search that Unraveled the Mystery of Baby Dinosaurs, Workman Publishing Co.
  • Lehman, T. M., 2001, Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.
  • Trexler, D., 2001, Two Medicine Formation, Montana: geology and fauna: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 298–309.