Maiasaura

Maiasaura Temporal range: Late Cretaceous (Santonian – Campanian), 86.3–70.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted cast, Brussels Natural History Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Neornithischia
Clade:	†Ornithopoda
Family:	†Hadrosauridae
Subfamily:	†Saurolophinae
Tribe:	†Brachylophosaurini
Genus:	†Maiasaura Horner & Makela, 1979
Type species
†Maiasaura peeblesorum Horner & Makela, 1979

Maiasaura (from the Greek μαῖα, meaning "good mother" and σαύρα, the feminine form of saurus, meaning "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.^[1] in the Upper Cretaceous Period (mid to late Campanian), from 86.3 to 70.6 million years ago.^[2] Maiasaura peeblesorum is the state fossil of Montana.

The first remains of Maiasaura peeblesorum 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. The given genus name refers to the finding of Maiasaura peeblesorum 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 peeblesorum 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 peeblesorum have been dug up.

Reconstruction of Maiasaura peeblesorum

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.

Description

Skull cast, Royal Ontario Museum

Size comparison with human

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).^[3] 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,^[4] 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.^[5] 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.^[6] Hundreds of specimens have been found throughout all stages of life, allowing for M. peeblesorum to be used for understanding how hadrosaurids grew.^[7] Maiasaura peeblesorum lived in a terrestrial habitat.

Discovery

Reconstructed cast by Jack Horner of a Maiasaura emerging from its egg

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 refers to the Greek goddess Maia, the "Good Mother"; 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.^[8] 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.^[9]

Over 200 specimens, in all age ranges, have been found.^[10] 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.^[11] Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.^[12]

Classification

Cast of a juvenile skeleton

Life restorations of an adult and juvenile

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

• Probrachylophosaurus bergei^[13]
• Brachylophosaurus canadensis^[14]
• Acristavus gagslarsonoi^[15]
• Brachylophosaurus goodwini^[16]
• Ornatops^[17]

The following cladogram of hadrosaurid relationships was published in 2013 by Albert Prieto-Márquez et al.:^[18]

Saurolophinae

Brachylophosaurini Acristavus gagstarsoni Brachylophosaurus canadensis Maiasaura peeblesorum Shantungosaurus giganteus Edmontosaurus Edmontosaurus regalis Edmontosaurus annectens Saurolophini Kerberosaurus manakini Sabinas OTU Prosaurolophus maximus Saurolophus Saurolophus morrisi Saurolophus osborni Saurolophus angustirostris Kritosaurini Wulagasaurus dongi Kritosaurus navajovius ‘’Aquilarhinus Secernosaurus koerneri Willinakaqe salitralensis Gryposaurus Gryposaurus latidens Gryposaurus notabilis Gryposaurus monumentensis

Palaeobiology

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.^[19] 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.^[20] Fossilized M. peeblesorum eggs are black in color and have high, prominent ridges on the outer surface.^[20]

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.^[9]

Reconstruction of a nest with eggs

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.^[9] 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.

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.^[21][22]

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.^[23][24][25]

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.^[26][27]

Palaeoecology

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.^[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.^[28] 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.^[28] There seems to be a major diversification in ornithischian taxa after the appearance of Maiasaura within the Two Medicine Formation.^[28] 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.^[28] 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.^[29]

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]

See also

• Timeline of hadrosaur research

Footnotes

1. McFeeters, Bradley D.; Evans, David C.; Ryan, Michael J.; Maddin, Hillary C. (2021-03-01). "First occurrence of Maiasaura (Dinosauria, Hadrosauridae) from the Upper Cretaceous Oldman Formation of southern Alberta, Canada". Canadian Journal of Earth Sciences. 58 (3): 286–296. Bibcode:2021CaJES..58..286M. doi:10.1139/cjes-2019-0207. ISSN 0008-4077. S2CID 233851376.
2. Horner, J. R., Schmitt, J. G., Jackson, F., & Hanna, R. (2001). Bones and rocks of the Upper Cretaceous Two Medicine-Judith River clastic wedge complex, Montana. In Field trip guidebook, Society of Vertebrate Paleontology 61st Annual Meeting: Mesozoic and Cenozoic Paleontology in the Western Plains and Rocky Mountains. Museum of the Rockies Occasional Paper (Vol. 3, pp. 3-14).
3. Wosik, M.; Chiba, K.; Therrien, F.; Evans, D.C. (2020). "Testing Size–frequency Distributions As a Method of Ontogenetic Aging: A Life-history Assessment of Hadrosaurid Dinosaurs from the Dinosaur Park Formation of Alberta, Canada, with Implications for Hadrosaurid Paleoecology". Paleobiology. 46 (3): 379–404. Bibcode:2020Pbio...46..379W. doi:10.1017/pab.2020.2. S2CID 221666530.
4. Brett-Surman, M. K. (February 1979). "Phylogeny and palaeobiogeography of hadrosaurian dinosaurs". Nature. 277 (5697): 560–562. Bibcode:1979Natur.277..560B. doi:10.1038/277560a0. ISSN 0028-0836. S2CID 4332144.
5. Cubo, Jorge; Woodward, Holly; Wolff, Ewan; Horner, John R. (2015). "First Reported Cases of Biomechanically Adaptive Bone Modeling in Non-Avian Dinosaurs". PLOS ONE. 10 (7): e0131131. Bibcode:2015PLoSO..1031131C. doi:10.1371/journal.pone.0131131. PMC 4495995. PMID 26153689.
6. Varricchio, David J.; Horner, John R. (1993-05-01). "Hadrosaurid and lambeosaurid bone beds from the Upper Cretaceous Two Medicine Formation of Montana: taphonomic and biologic implications". Canadian Journal of Earth Sciences. 30 (5): 997–1006. Bibcode:1993CaJES..30..997V. doi:10.1139/e93-083. ISSN 0008-4077.
7. McFeeters, Bradley; Evans, David; Maddin, Hillary (2021). "Ontogeny and variation in the skull roof and braincase of Maiasaura peeblesorum from the Two Medicine Formation of Montana, U.S.A." Acta Palaeontologica Polonica. 66. doi:10.4202/app.00698.2019. ISSN 0567-7920. S2CID 239729209.
8. Horner, J.R.; Makela, R. (1979). "Nest of juveniles provides evidence of family structure among dinosaurs". Nature. 282 (5736): 296–298. Bibcode:1979Natur.282..296H. doi:10.1038/282296a0. S2CID 4370793.
9. "Maiasaura," Dodson, et al. (1994); pages 116-117.
10. Horner and Gorman (1988).
11. "Introduction," Trexler (2001); pages 299-300.
12. "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 304.
13. Freedman Fowler, Elizabeth A.; Horner, John R. (2015-11-11). "A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana". PLOS ONE. 10 (11): e0141304. Bibcode:2015PLoSO..1041304F. doi:10.1371/journal.pone.0141304. ISSN 1932-6203. PMC 4641681. PMID 26560175.
14. Prieto-Marquez, Albert (2005-03-11). "New information on the cranium ofBrachylophosaurus canadensis(Dinosauria, Hadrosauridae), with a revision of its phylogenetic position". Journal of Vertebrate Paleontology. 25 (1): 144–156. doi:10.1671/0272-4634(2005)025[0144:niotco]2.0.co;2. ISSN 0272-4634. S2CID 85767827.
15. Gates, Terry A.; Horner, John R.; Hanna, Rebecca R.; Nelson, C. Riley (July 2011). "New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America". Journal of Vertebrate Paleontology. 31 (4): 798–811. Bibcode:2011JVPal..31..798G. doi:10.1080/02724634.2011.577854. ISSN 0272-4634. S2CID 8878474.
16. Horner, John R. (1988-09-23). "A new hadrosaur (Reptilia, Ornithischia) from the Upper Cretaceous Judith River Formation of Montana". Journal of Vertebrate Paleontology. 8 (3): 314–321. Bibcode:1988JVPal...8..314H. doi:10.1080/02724634.1988.10011714. ISSN 0272-4634.
17. McDonald, Andrew T.; Wolfe, Douglas G.; Freedman Fowler, Elizabeth A.; Gates, Terry A. (2021-04-02). "A new brachylophosaurin (Dinosauria: Hadrosauridae) from the Upper Cretaceous Menefee Formation of New Mexico". PeerJ. 9: e11084. doi:10.7717/peerj.11084. ISSN 2167-8359. PMC 8020878. PMID 33859873.
18. Prieto-Márquez, A.; Wagner, J.R. (2013). "A new species of saurolophine hadrosaurid dinosaur from the Late Cretaceous of the Pacific coast of North America". Acta Palaeontologica Polonica. 58 (2): 255–268. doi:10.4202/app.2011.0049.
19. Palmer (1999); page 148.
20. Hirsch, Karl F.; Quinn, Betty (1990-12-20). "Eggs and eggshell fragments from the Upper Cretaceous Two Medicine Formation of Montana". Journal of Vertebrate Paleontology. 10 (4): 491–511. Bibcode:1990JVPal..10..491H. doi:10.1080/02724634.1990.10011832. ISSN 0272-4634.
21. "Largest dinosaur population growth study ever shows how Maiasaura lived and died: Decades of research on Montana's state fossil -- the 'good mother lizard' Maiasaura peeblesorum -- has resulted in the most detailed life history of any dinosaur known".
22. Woodward, Holly N.; Freedman Fowler, Elizabeth A.; Farlow, James O.; Horner, John R. (2015). "Maiasaura, a model organism for extinct vertebrate population biology: A large sample statistical assessment of growth dynamics and survivorship". Paleobiology. 41 (4): 503–527. Bibcode:2015Pbio...41..503W. doi:10.1017/pab.2015.19. S2CID 85902880.
23. Chin, Karen (1 September 2007). "The Paleobiological Implications of Herbivorous Dinosaur Coprolites from the Upper Cretaceous Two Medicine Formation of Montana: Why Eat Wood?". PALAIOS. 22 (5): 554–566. Bibcode:2007Palai..22..554C. doi:10.2110/palo.2006.p06-087r. JSTOR 27670451. S2CID 86197149. Retrieved 29 August 2020.
24. ""The Best of all Mothers" Maiasaura peeblesorum". bioweb.uwlax.edu/. University of Wisconsin-La Crosse. Retrieved 22 March 2021.
25. Chin, Karen; Gill, Bruce D. (1996). "Dinosaurs, Dung Beetles, and Conifers: Participants in a Cretaceous Food Web". PALAIOS. 11 (3): 280–285. doi:10.2307/3515235. ISSN 0883-1351.
26. "Using math to examine the sex differences in dinosaurs".
27. "Statistical analysis reveals differences between dinosaur sexes".
28. "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 306.
29. Sullivan, R. M.; Lucas, S. G. (2006). "The Kirtlandian land-vertebrate "age"–faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America". New Mexico Museum of Natural History and Science Bulletin. 35: 7–29.

References

• 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. ISBN 0-7853-0443-6.
• 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.
• Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 148. ISBN 1-84028-152-9.
• 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.