Dinosaurs A to Z
Hadrosauridae

Maiasaura peeblesorum MY-uh-SOR-uh

"Good mother lizard"

Maiasaura silhouette
You 1.8 m (5.9 ft) tall
Maiasaura 9 m (29.5 ft) long
as long as 6 people standing fingertip to fingertip
silhouette · Will Toosey (CC-BY) via PhyloPic
Length
9 m (29.5 ft) — About 9 m long, ~2.5 tonnes — known from nesting colonies.
Period
Late Cretaceous (76–74 Mya)
Place
North America · Montana
Food
Herbivore — Tough plants chewed up first by parents, then probably regurgitated to feed hatchlings.
Clade
Hadrosauridae Dinosauria Ornithischia Cerapoda Ornithopoda Iguanodontia Duck-billed dinosaurs with tooth batteries.

The "good mother lizard" that nested in colonies and proved dinosaurs cared for their young.

Maiasaura reconstruction by Nobu Tamura (http://spinops.blogspot.com)
Reconstruction · Nobu Tamura (http://spinops.blogspot.com) (CC-BY)

Maiasaura got its name — ‘good mother lizard’ — from a discovery that changed how we think about dinosaurs. Jack Horner and Bob Makela found a nesting colony in Montana in the late 1970s, with dozens of nests laid out in rows. Each nest contained eggs and bones of babies too big to be newborns but with unworn teeth, meaning the parents were bringing food back to them.

More about Maiasaura

Discovery

Named by Jack Horner and Bob Makela · 1979 · A site south of Choteau, Montana, in an area later named "Egg Mountain"

In 1978 Marion Brandvold showed Horner and Makela tiny bones at her rock shop in Bynum, Montana, which Horner identified as baby hadrosaur bones. Following her lead to the fossil-rich site south of Choteau, they found nests and juveniles of a then-unknown hadrosaurA 'duck-billed' dinosaur with batteries of grinding teeth; often crested. and described it the next year.

The name is commonly translated as "good mother lizard." It uses the feminine form of saurus ("lizard"), chosen to reflect the conclusion that this dinosaur cared for its offspring.

Jack Horner. A paleontologist who, with Bob Makela, named and described Maiasaura in 1979 and interpreted the nesting site as the first evidence of parental and social behavior in dinosaurs.

How do we know Maiasaura actually cared for its young?
Fossils of hatchlings show their legs were not fully developed, so they were likely incapable of walking, and their teeth were already partly worn. The worn teeth suggest adults brought food to the nest, which is read as the first instance of parental and social behavior in dinosaurs.
Was Maiasaura warm-blooded?
Hatchlings grew rapidly during their first year before leaving the nest. The text notes this high growth rate may be evidence of warm-bloodedness, though it is framed as a possibility rather than a settled fact.

What we know

  • First dinosaur with strong evidence of parental care — found in a nesting colony.
  • Nests laid out in rows, each one body-length apart.
  • Babies grew from 45 cm to ~1.5 m in one year — extremely fast growth.
  • Lived in vast herds — bonebeds preserve thousands of individuals together.

What we guess

  • How long the young stayed at the nest — months, based on tooth wear.
  • What the parents fed them — chewed plant material is the leading guess.
  • Whether the nesting colony was annual or just a one-time event.