Maiasaura

Maiasaura peeblesorum were large, attaining a maximum known length of about 9 metres (30 ft) and an approximate body mass measured at up to 4 metric tons (4.4 short tons).^[7] 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,^[8] 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.^{[citation needed]}

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.^[9] 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.^[10] Hundreds of specimens have been found throughout all stages of life, allowing for M. peeblesorum to be used for understanding how hadrosaurids grew.^[11]

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),^[12] had been finding small bones and they had been trying to put them together.^[13] 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.^[14] 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.^[14][15] 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.^[14] That was their first excavated nest, but subsequently the site delivered more nests and juveniles, all of them of a till-then unknown species of hadrosaur. The next year in July 1979, during the second summer dig, a lady student named Fran Tannenbaum, under the direction of Jack Horner, discovered the first whole dinosaur egg ever found in North America^[16] 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.^[17][18]

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.^[14] It is the specimen that Makela and Horner had used the same year as the holotype of the new species, which they described and named as the type species Maiasaura peeblesorum. The generic name Maiasaura means 'good mother lizard' and the specific name, peeblesorum, honours the families of John and James Peebles, on whose land the finds were made.^[19][3]

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.^[20]

Over 200 specimens, in all age ranges, have been found.^[21] 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.^[22] Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.^[23]

The sites that close to Choteau contain Maiasaura remains of adults, juveniles, nests and eggs, have been preserved in the rock as if a rapid event had "frozen" the area. It is assumed that such event was the eruption of a volcano located among the nearby Elkhorn mountains. Nests, eggs and animals were covered by ash and once they fossilised they remained almost perfectly preserved until they were discovered in 1978.^[17][18]

In 1985 The Nature Conservancy purchased the site of Egg Mountain from the James and Marian Peebles family, the original owners of the land, leasing the property to the Peebles for cattle grazing and allowing curator Jack Horner and the Museum of the Rockies to run a paleontology field camp. In 2004, the Museum of the Rockies purchased the property and now the Museum gives "Egg Mountain" the name of "Museum of the Rockies' Beatrice R. Taylor Paleontology Research Site", honouring Beatrice Taylor, whose family donated the money for the purchase.^[24]

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

• Acristavus gagslarsoni^[25]
• Brachylophosaurus canadensis^[26]
• Ornatops incantatus^[27]
• Probrachylophosaurus bergei^[28]

The following cladogram of hadrosaurid relationships was published in 2013 by Albert Prieto-Márquez et al.:^[29]

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 7 metres (23 ft); less than the length of the adult animal.^[30] The nests were made of earth and contained 30 to 40 eggs laid in a circular or spiral pattern. The eggs were about the size of ostrich eggs and are oval shaped with one slightly more pointed end.^[31] Fossilized M. peeblesorum eggs are black in color and have high, prominent ridges on the outer surface.^[31]

The eggs were incubated by the heat resulting from rotting vegetation placed into the nest by the parents, rather than a parent sitting on the nest. Upon hatching, fossils of baby Maiasaura show that their legs were not fully developed and thus they were incapable of walking. Fossils also show that their teeth were partly worn, which means that the adults brought food to the nest.^[32]

The hatchlings grew from a size of 41 to 147 centimetres (16 to 58 in) 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 be evidence of warm bloodedness. The hatchlings had different facial proportions from the adults, with larger eyes and a shorter snout.^[32] These features are associated with cuteness, and commonly elicit care from parents in animals dependent on their parents for survival during the early stages of life. Based on metabolic growth rates and age estimations of juvenile specimens both in and outside the nests, it is suggested that infant Maiasaura were altricial and left the confines of their nests after 40-75 days (or roughly a little over 1-2 months old). These studies also were compared to the related lambeosaurine hadrosaur Hypacrosaurus, for which neonates are also known. These analyses suggest Hypacrosaurus was more precocial than Maiasaura, based on its ecological traits.^[33]

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.^[34][35]

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.^[2] Maiasaura lived alongside the troodontids Stenonychosaurus and Troodon and the basal ornithopod Orodromeus, as well as the dromaeosaurid Bambiraptor and the tyrannosaur Daspletosaurus.^[2] Another species of hadrosaurids, referable to the genus Hypacrosaurus, coexisted with Maiasaura for some time, as Hypacrosaurus remains have been found lower in the Two Medicine Formation than was earlier known.^[42] The discovery of an additional hadrosaurid, Gryposaurus latidens, in the same range as Maiasaura has shown that the border between hypothesized distinct faunas in the upper and middle is less distinct than once thought.^[42] There seems to be a major diversification in ornithischian taxa after the appearance of Maiasaura within the Two Medicine Formation.^[42] The thorough examination of strata found along the Two Medicine River (which exposes the entire upper half of the Two Medicine Formation) indicates that the apparent diversification was a real event rather than a result of preservational biases.^[42] While Maiasaura has historically been associated with the Two Medicine formation ceratopsid Einiosaurus in a single fauna, this is inaccurate, as Maiasaura is known exclusively from older strata.^[43]

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.^[1]

Within the first years of the discovery of Maiasaura, the importance of such a remarkable find had a durable impact on the public and media contributed to that:

As of 1983, dinosaur renaissance artist Doug Henderson started painting anatomically modern-view life restorations of Maiasaura for a children's book authored by Jack Horner, James Gorman and Jeri D. Walton. Finally published in 1985 the book was titled Maia: A Dinosaur Grows Up and, via its illustrations, showed to the public Maiasaura and other of its contemporary dinosaurs on horizontal-backbone posture, contrary to the till-then traditional, popular and incorrect posture seen in dinosaurs (bipedal or not) that permanently were dragging their tails on the ground.^[44]

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".^[45]

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.^[46]

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.^[47]

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.^[48]

• Timeline of hadrosaur research