A seemingly innocent-looking femur belonging to a long-dead Tyrannosaurus rex revealed that the dinosaur was pregnant at the time of its death – a rare confirmation of the creature’s sex that could also explain when and how modern birds evolved to lay eggs.

The discovery, which is detailed in Tuesday’s edition of the journal Scientific Reports, “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, a paleontologist with both North Carolina State University and the North Carolina Museum of Natural Sciences, said in a statement.

According to the Washington Post, Schweitzer and her colleagues found a type of tissue in the femur known as medullary bone, which is only present in female birds that are currently carrying eggs or which have recently finished laying them. Since medullary bone needs to be laid down and mobilized quickly for the birds to shell their eggs, it is chemically distinct from other bone types, the researchers explained.

Like modern birds, theropod dinosaurs such as the T. rex also reproduced by laying eggs, and paleontologists have long hypothesized that female members of this dinosaur group would also have possessed medullary bone. In 2005, Schweitzer led a team that believed it found this bone type in the fossilized femur, and the new study appears to confirm that earlier discovery.

Research could lead to discovery of sex-specific dinosaur traits

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

Medullary bone contains a substance known as keratan sulfate, which is not present in any other type of bone. Researchers previously believed that none of the chemistry of a fossilized dinosaur bone would have survived a period of several million years, but tests were able to detect keratan sulfate in the bone and confirm that the T. rex tissue was indeed medullary bone.

“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,” Schweitzer said. However, due to the very nature of medullary bone, she cautions that finding more examples of it in the fossil record may be hard, especially since, as the Washington Post pointed out, the bones need to be broken in order for these tests to be conducted (the femur used in this study was already broken, they noted).

Nonetheless, as co-author and NC State paleontologist Lindsay Zanno explained, the research could provide valuable insight into dinosaurs. “It’s a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs,” Zanno said. “Dinosaurs weren’t shy about sexual signaling, all those bells and whistles, horns, crests, and frills, and yet we just haven’t had a reliable way to tell males from females. Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities. Now that we can show pregnant dinosaurs have a chemical fingerprint, we need a concerted effort to find more.”

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Image credit: Mark Hallett

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