Maiasaura

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Maiasaura
Temporal range:
Ma
↓
Mounted cast,
Brussels Natural History Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: †Ornithischia
Clade: †Ornithopoda
Family: †Hadrosauridae
Subfamily: †Saurolophinae
Tribe: †Brachylophosaurini
Genus: †Maiasaura
Horner & Makela, 1979
Type species
†Maiasaura peeblesorum
Horner & Makela, 1979

Maiasaura (from the

Period (mid to late Campanian), about 76.7 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).

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

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 habitats.

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

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
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

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

angiosperms and possibly grasses. This would imply that Maiasaura was both a browser and a grazer.[23][24]

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 gender was male or female.[25][26]

Palaeoecology

horsetail
vegetation.

Maiasaura is a characteristic fossil of the middle portion (lithofacies 4) of the Two Medicine Formation, dated to about 76.4 million years ago.

ornithischian taxa after the appearance of Maiasaura within the Two Medicine Formation.[27] 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.[27] 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.[28]

In the

ankylosaurid Scolosaurus.[1]

See also

Footnotes

  1. ^
    S2CID 233851376
    .
  2. ^ a b c 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. S2CID 221666530
    .
  4. S2CID 4332144
    .
  5. PMID 26153689
    .
  6. ISSN 0008-4077
    .
  7. S2CID 239729209
    .
  8. S2CID 4370793
    .
  9. ^ a b c "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. PMID 26560175
    .
  14. S2CID 85767827
    .
  15. S2CID 8878474
    .
  16. ISSN 0272-4634
    .
  17. PMID 33859873
    .
  18. doi:10.4202/app.2011.0049
    .
  19. ^ Palmer (1999); page 148.
  20. ^
    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. S2CID 85902880
    .
  23. 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. ^ "Using math to examine the sex differences in dinosaurs".
  26. ^ "Statistical analysis reveals differences between dinosaur sexes".
  27. ^ a b c d "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 306.
  28. ^ 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

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