“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.

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Theropod dinosaurs like the T. rex evolved into modern-day birds.

Schweitzer, a paleontologist at N.C. State, led a team that believed it found the medullary bone back in 2005. In preparing the bone for further study, Schweitzer also uncovered what she and some other paleontologists believe to be blood vessels — soft tissues somehow preserved for millions of years. That finding has led to her work being co-opted by young-Earth creationists, who claim such tissues prove that dinosaur bones are much younger than scientists think. Schweitzer does not agree with that theory.

The work remains controversial even a decade later. But this latest paper, which describes new tests conducted on the hotly-contested femur, may at least confirm the presence of medullary bone at long last.

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"All the evidence we had at the time pointed to this tissue being medullary bone," Schweitzer said in the statement. "But there are some bone diseases that occur in birds, like osteopetrosis, that can mimic the appearance of medullary bone under the microscope. So to be sure, we needed to do chemical analysis of the tissue."

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.

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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."

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

“Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities," she added. "Now that we can show pregnant dinosaurs have a chemical fingerprint, we need a concerted effort to find more.”

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That won't be easy, especially given that medullary bone tissue is present only for a brief time.

And not everyone will be so keen to break up their dinosaur fossils to conduct such tests. The femur bone used in this research was already broken.