Maiasaura peeblesorum

Maiasaura peeblesorum is known from an extraordinary growth series rather than one isolated skeleton. Eggs, embryos, nestlings, juveniles and adults from Montana's Two Medicine Formation allow researchers to follow major changes from hatching to maturity. Closely spaced nests and bonebeds show repeated use of favourable nesting landscapes by many animals. Hatchlings remained in nests while their limb bones were still poorly developed, supporting prolonged attendance or provisioning by adults. The fossils do not preserve an adult actually feeding a chick, so the celebrated good-mother interpretation is strong behavioural evidence rather than a literal snapshot.

Its fossils occur between approximately 76.7 and 74 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Maiasaura had a broad toothless beak at the front of the jaws and dense dental batteries farther back for slicing and grinding plants. Unlike hollow-crested lambeosaurines, it had a low solid ridge above the eyes. Robust forelimbs could support the body during quadrupedal walking, while longer hind limbs and a stiffened tail also permitted bipedal movement, especially in smaller individuals. Growth-series bones show major changes in limb proportions and tissue as young animals developed into adults approaching nine metres long.

• Broad beak and extensive dental batteries for processing plants
• Low solid ridge above the eyes rather than a hollow head crest
• Strong forelimbs capable of supporting quadrupedal walking
• Longer hind limbs permitting bipedal movement
• Calcified tendons stiffening the back and tail
• Eggs, embryos and a nearly complete growth series preserved at nesting sites

In 1978, fossil collector Marion Brandvold showed John Horner and Robert Makela tiny hadrosaur bones from the Willow Creek Anticline near Choteau, Montana. Fieldwork revealed nests, eggs and juveniles at what became famous as the Egg Mountain region. Horner and Makela named Maiasaura peeblesorum in Nature in 1979. The genus name used the feminine Greek form saura, 'lizard', to express 'good mother lizard'; the species honours the Peebles family, on whose land important fossils were found. Continued Museum of the Rockies work uncovered colonies and bonebeds representing hundreds of individuals.

Discovery credit: Marion Brandvold, John R. Horner, Robert Makela.

Naming authors: John R. Horner, Robert Makela.

Maiasaura lived on seasonal uplands east of the rising Rocky Mountains. The Two Medicine Formation records river channels, floodplains, volcanic ash falls and a climate with pronounced wet and dry seasons. Conifers and flowering plants supplied browse. Ceratopsians, armoured dinosaurs, small theropods and the tyrannosaurid Daspletosaurus inhabited the broader formation, although they were not all preserved in the same moment or exact locality.

Nests occurred in colonies, and some nesting horizons were reused, indicating that many Maiasaura gathered at suitable breeding grounds. Very young animals had immature limb joints and worn nest floors, implying that they remained in the nest for a period after hatching. Plant fragments reported inside nests may represent food brought by adults, but direct feeding and the identity or sex of carers are not preserved. Large bonebeds indicate group living or at least mass aggregation; they do not by themselves reveal a permanent herd structure.

• Maiasaura was the first dinosaur genus given the feminine Greek ending -saura rather than -saurus.
• Its fossils preserve almost the entire growth sequence from embryos to adults.
• Nest spacing was roughly comparable to the length of an adult body, suggesting densely packed breeding colonies.
• Montana designated Maiasaura peeblesorum as its official state fossil in 1985.

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1. 01
   Nest of juveniles provides evidence of family structure among dinosaurs

   John R. Horner, Robert Makela · Nature 282 · 1979

   Open source
2. 02
   Maiasaura and the Siebel Dinosaur Complex

   Museum of the Rockies

   Open source