Maiasaura

The Birth of the “Merciful Mother”: The 1979 Miracle in Montana

The story begins in the Two Medicine Formation (approx. 76.7 million years ago) in Montana, USA.
Dr. Horner and his team discovered a crater-like depression about 2 meters in diameter along with fossils of a new species of duck-billed dinosaur.

Children Left in the Nest

It was undoubtedly a dinosaur “nest.” Inside were eggshells and baby bones, and fossils of juveniles at different growth stages were found nearby.
This overturned the conventional wisdom of “lay and leave (laissez-faire)” behavior, proving that “parents stayed at the nest and looked after their children.”

In honor of this discovery, the dinosaur was named “Good Mother Lizard” (Maiasaura) in Greek.

Group Life on “Egg Mountain” and Herds of 10,000

The discovery site was later named “Egg Mountain,” revealing that these dinosaurs possessed a high level of sociality.

Massive Breeding Colonies

Countless nests were arranged in an orderly fashion, spaced about 7 meters apart (the length of an adult parent).
They returned to the same place every year, forming huge “nesting colonies” like penguins to breed cooperatively.

A Bonebed of 10,000 Individuals

Additionally, a “bonebed” containing an estimated 10,000 individuals that died at once was found nearby.
Believed to be traces of a herd wiped out by a disaster, this tells us that they migrated and lived in massive herds numbering in the tens of thousands, much like modern wildebeest.

They migrated and lived in massive herds

Did They Not Feed Their Young? Scientific Objections to the Parenting Theory

While “child-rearing (specifically feeding)” is the origin of the “Good Mother” name, recent years have seen a more cautious view regarding the evidence.

Basis for the Feeding Theory: Worn Teeth

Initially, because the teeth of babies in the nest—too young to walk—showed “signs of wear (abrasion),” it was speculated that “parents brought food and fed them.”

Counterarguments from Recent Research

However, recent research has raised the following questions:

Pre-hatching Abrasion

Tooth wear has been found even in embryos inside eggs, so this is not evidence of feeding (it implies possibilities like teeth grinding).

Trampled Shells

The fact that parents did not remove shells from the nest is considered unnatural for bird-like parenting.

Carcass Theory

There is a possibility that the discovered juveniles were “carcasses” that died for some reason and were eaten by scavenging insects.

While it is unclear if they fed them by mouth, circumstantial evidence makes it almost certain that they protected the nest collectively and guarded the children as a herd up to a certain point.

Astonishing Growth Speed and Characteristics of Duck-billed Dinosaurs

Fossils from all growth stages, from eggs to adults, have been found at Egg Mountain.

Explosive Growth

Although less than 50 cm at hatching, they exceeded 3 meters in one year and reached adult size (9 meters, 3 tons) in 7 to 8 years.
This growth rate is far faster than modern reptiles, serving as evidence that dinosaurs possessed high metabolisms like warm-blooded animals.

Plant-Eating Specialists

They possessed hundreds of teeth known as a “dental battery,” which allowed them to grind down and eat tough vegetation.