Egg Mountain: the first evidence dinosaurs cared for their young
A Montana rock-shop find becomes 'the good mother lizard'
Quick facts
- Found
- 1977 (Marion Brandvold), identified and excavated 1978 (Jack Horner's team)
- Fossils date to
- Late Cretaceous, roughly 76 million years ago
- Where
- Two Medicine Formation, Montana; site named Egg Mountain
- Scale
- 14 dinosaur nests in one area
- Key evidence
- Unossified hatchling bones; could not walk immediately, implying parental care
What happened
In 1977, Marion Brandvold, who ran a small rock shop in Bynum, Montana, found the remains of juvenile dinosaurs and showed them the next year to paleontologist Jack Horner. Horner and his team, working in the Two Medicine Formation, went on to uncover 14 dinosaur nests in a single area, a site that became known as Egg Mountain. Alongside eggs and broken eggshells, they found an adult skeleton in close association with a nest of young dinosaurs about a metre long, and hatchlings too large to have just emerged from an egg. A peer-reviewed study of the specimens found their bones were not fully developed, or ossified, meaning the young could not have walked immediately after hatching and would have needed care from an adult. The species was named Maiasaura, meaning good mother lizard.
Why it matters
The National Park Service states plainly that this find supplied the first strong evidence that any dinosaur fed and cared for its young, and the first evidence of complex dinosaur behavior at all. Before Egg Mountain, dinosaurs were widely pictured as simple, instinct-driven reptiles that abandoned their eggs; this discovery is the reason that picture changed.
How we know
The U.S. National Park Service account gives the 1977 initial find by Marion Brandvold, its handoff to Jack Horner in 1978, the 14 nests, the Egg Mountain name, and states directly that it was the first strong evidence of dinosaur parental care and complex behavior. A peer-reviewed paper on Maiasaura, hosted on the NIH's PubMed Central, independently confirms the 1978 collection date, the Two Medicine Formation location, and the researchers involved, and cites the original studies establishing the parental-care interpretation. That interpretation, while widely accepted and reflected in the animal's name, is not treated as beyond all challenge by every specialist, so it is presented here as the strongly evidenced conclusion the sources describe, not an absolute certainty.
Sources
- U.S. National Park Service. Egg Mountain, Montana: the discovery of Maiasaura nesting behavior (U.S. National Park Service) (2014) · Reputable sourcenps.gov · The domain "nps.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Various (peer-reviewed). Perinatal specimens of Maiasaura from the Upper Cretaceous of Montana: insights into the early ontogeny of saurolophine hadrosaurid dinosaurs (PeerJ, 2018, via PubMed Central) (2018) · Peer-reviewed (author-declared)ncbi.nlm.nih.gov · Cited as a "journal" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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