Borealopelta, an armoured dinosaur fossil featured in Dinosaur Cold Case, a documentary from The Nature of Things, had huge 51-centimetre-long shoulder spikes. Could that mean Borealopelta was a male, using its spikes for display and fighting, similar to animals today?

When it came to determining the sex of a fossil, scientists usually had to guess. That is, until paleontologist Mary Schweitzer found a definitive way of identifying the bones of a female dinosaur, after being dead for millions of years. She discovered medullary bone in the fossil fragments from a Tyrannosaurus rex.

Medullary bone is a special tissue that forms inside the bone cavities of modern birds, like chickens. The tissue is formed when a female is laying eggs and it acts as a calcium reservoir to form the eggshells. Schweitzer saw the same bone in a T.rex fossil and was able to determine that it was a female.

In the case of Borealopelta, the dinosaur’s body is so well preserved that researchers can’t see the skeleton, keeping the mystery of its sex a secret. For now.

Watch Schweitzer describe her discovery in the video above.

Face to face with a perfectly preserved dinosaur that looks like it was alive yesterday
100 million years ago, Alberta was a giant sea, surrounded by tropical forests
"Destroyer of shins" — a newly discovered dinosaur may have used its armour for more than defence

For more, watch Dinosaur Cold Case on The Nature of Things.

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