Maiasaura and Egg Mountain: Evidence of Dinosaur Parenting

Maiasaura is a duck-billed dinosaur from the Late Cretaceous of Montana whose discovery in 1978 transformed ideas about dinosaur family life. Jack Horner and Bob Makela found a nest containing the remains of juveniles near Choteau, together with evidence of a whole nesting colony. Named in 1979 as the “good mother lizard”, Maiasaura became the classic example of a dinosaur that cared for its young, and the nearby Egg Mountain site became one of the most important dinosaur nesting grounds in the world.

Background: what was known about dinosaur nests

By the 1970s dinosaur eggs were well known, above all from the Gobi Desert, where Roy Chapman Andrews’ expedition had found clutches in 1923. But baby dinosaurs were extremely rare, and almost nothing was known about how dinosaurs raised their offspring. Most scientists assumed that, like many modern reptiles, dinosaurs laid their eggs and left them.

Duck-billed dinosaurs, or hadrosaurs, had been known since the discovery of Hadrosaurus in New Jersey in 1858. They were among the most common large plant-eaters of Late Cretaceous North America, but their juveniles were poorly represented in museum collections.

What happened in 1978

Jack Horner, then a preparator at Princeton University, and his friend Bob Makela, a schoolteacher, were collecting in Montana in the summer of 1978. In the town of Bynum they visited a rock shop run by Marion Brandvold, who showed them small bones she had collected on land belonging to the Peebles family near Choteau.

Horner recognised them as bones of baby hadrosaurs, an extraordinary rarity. Brandvold showed the two men where she had found them, on a ridge in the rolling country known as the Willow Creek anticline. Excavating there, Horner and Makela uncovered a bowl-shaped nest structure containing the remains of around 15 juveniles, each roughly a metre long.

In 1979 Horner and Makela published the find in the journal Nature and named the new dinosaur Maiasaura peeblesorum. The genus name means “good mother lizard”; the species name honours the Peebles family, on whose land the nest lay.

The rocks of the site belong to the Two Medicine Formation, deposited on an upland plain east of the young Rocky Mountains. The Maiasaura layers date to roughly 77 to 76 million years ago.

Evidence of parental care

The key observation was the size of the juveniles. Hatchlings of Maiasaura were only about 40 centimetres long, yet the young in the nest were more than twice that length. They must have stayed in or near the nest for weeks or months after hatching.

Horner noted that the eggshell fragments in the nests were crushed into small pieces, as if trampled by young that remained there. He also argued that the leg bones of hatchlings were not fully formed at the joints, suggesting the young could not yet walk far and depended on adults to bring food. This interpretation, that the hatchlings were nest-bound, was later debated; some researchers argued that the babies were more mobile than Horner proposed.

Further excavation showed that the nests formed a colony. They were spaced about 7 metres apart, roughly the length of an adult, as in some modern bird colonies. The site appears to have been used repeatedly over many seasons. Nests are thought to have held clutches of perhaps 30 to 40 eggs, each about the size of a grapefruit.

Nearby, Horner’s team also found a vast bonebed in which the remains of thousands of Maiasaura, estimated at up to 10,000 individuals, were buried together, apparently in connection with a volcanic ash fall. This suggested that the animals lived in large herds.

Egg Mountain

In 1979 the team found a small hill nearby that contained many more eggs and nests. It became known as Egg Mountain. The eggs here were different from those of Maiasaura. At first Horner attributed many of them to a small plant-eater that he and David Weishampel named Orodromeus makelai in 1988, in honour of Bob Makela.

In 1996 Horner and Weishampel corrected this identification after an embryo inside one of the eggs was shown to belong to Troodon, a small, large-brained predatory dinosaur. Egg Mountain has since become one of the most important places for studying dinosaur nesting, egg structure and brooding behaviour, and the Museum of the Rockies in Bozeman continues to work there.

Key people

Jack Horner (born 1946) became one of the best-known palaeontologists in the United States. He later became curator of palaeontology at the Museum of the Rockies, received a MacArthur Fellowship in 1986 and served as a scientific adviser for the Jurassic Park films.

Bob Makela was Horner’s friend and field partner, and co-author of the 1979 paper. Marion Brandvold collected the first baby bones and led the palaeontologists to the site; her role is now widely acknowledged in accounts of the discovery.

Consequences and legacy

Maiasaura changed popular and scientific images of dinosaurs, adding social behaviour and parental care to the more active picture emerging from the Dinosaur Renaissance. It is now one of the best-studied dinosaurs, with hundreds of specimens covering every stage of growth.

A 2015 study by Holly Woodward and colleagues used bone tissue from many individuals to reconstruct the growth and survival of a Maiasaura population. It found that the animals grew very rapidly, probably reaching sexual maturity in about three years and full skeletal size in about eight, and that mortality was very high in the first year of life.

In 1985 Maiasaura was adopted as the state fossil of Montana. The same year, fragments of Maiasaura bone and eggshell were carried into orbit by the Montana scientist Loren Acton aboard the Space Shuttle Challenger on the Spacelab 2 mission, making it the first dinosaur in space.

Myths and facts

Myth: Maiasaura proved that all dinosaurs were caring parents. The evidence applies to Maiasaura and a few other species. Parental behaviour almost certainly varied between dinosaur groups, as it does among modern birds and reptiles, and some dinosaurs may have left their eggs to hatch unattended.

Myth: the Egg Mountain eggs belonged to Maiasaura. The Maiasaura nests lie nearby on the same anticline, but the eggs of Egg Mountain itself are mostly attributed to the small theropod Troodon, after an early misidentification was corrected in 1996.

Fact: the exact form of care is uncertain. Whether adults brought food to nest-bound babies, or simply guarded young that could feed themselves, is still discussed. What is widely accepted is that juveniles remained at the nesting site long after hatching, which implies some form of protection by adults.

Timeline

• 1978, summer: Marion Brandvold shows baby hadrosaur bones to Jack Horner and Bob Makela, who excavate the nest site near Choteau.
• 1979: Horner and Makela publish the find in Nature and name Maiasaura peeblesorum.
• 1979: The team discovers Egg Mountain on the Willow Creek anticline.
• 1985: Maiasaura becomes Montana’s state fossil; Maiasaura fossils fly on Spacelab 2.
• 1986: Horner receives a MacArthur Fellowship.
• 1988: Horner and Weishampel name Orodromeus makelai.
• 1996: Egg Mountain eggs are reattributed to Troodon after study of an embryo.
• 2015: A population study of Maiasaura growth and survival is published.

Why it matters

The Maiasaura nesting grounds offered the first strong evidence that dinosaurs could nest in colonies and look after their young. They turned dinosaurs from solitary reptiles into social animals with family lives, and they showed how much a single, carefully studied site can reveal about the biology of extinct animals.