Tyrannosaur pregnant?

Washington Post has peeked as experts sexed a T. rex:

The team from North Carolina State University and the North Carolina Museum of Natural Sciences published its findings Tuesday in the journal Scientific Reports.

“This analysis allows us to determine the gender of this fossil, and gives us a window into the evolution of egg-laying in modern birds,” lead author Mary Schweitzer said in a statement.

This particular fossil belongs to a T. rex that roamed what is now Montana millions of years ago, during the Cretaceous Period, and it has a kind of tissue — medullary bone — found only in female birds that are carrying eggs or have just finished laying them, according to the research paper.

Medullary bone is unlike other bone types. It contains keratan sulfate and is only present during a brief window; once eggs are laid, you can’t find it anymore.

The researchers initially thought the chemistry of the bone couldn’t have survived. But this time, they used a different method to test the femur sample for keratan sulfate. They then ran the same tests on ostrich and medullary bones, and after comparing the results, concluded the T. rex indeed had medullary bone tissue.

What may be even more exciting for researchers is what this finding means for identifying the sex of dinosaurs. As study co-author Lindsay Zanno of the North Carolina Museum of Natural Sciences said, “We just haven’t had a reliable way to tell males from females.”

“It’s a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs,” Zanno said in a release.

(More on the discovery at the NatGeo Phenomena Laelaps blog.)