Pregnant T. Rex Might be the Key to Dinosaur Sex-Typing

A pregnant Tyrannosaurus rex may reveal the secrets to identifying gender differences in meat-eating dinosaurs.

A pregnant Tyrannosaurus rex may reveal the secrets to identifying gender differences in meat-eating dinosaurs. Image: © Mark Hallett 2016 | North Carolina Museum of Natural Sciences.

Mary Schweitzer, a paleontologist working at the North Carolina Museum of Sciences and North Carolina State University, has been able to confirm the gender of a Tyrannosaurus rex who lived some 68 million years ago. She was able to do this because the dinosaur in question was pregnant when she died and was subsequently fossilized.

How did she figure that out? Well, back in 2005 she found what looked like medullary bone in samples from that specimen. Since then, she’s done a number of tests to make sure that it wasn’t a case of a disease mimicking that kind of bone, but tests confirmed it.

Medullary bone is unique to birds and theropod dinosaurs, two-legged, upright creatures like T. rex. This material is chemically unique from other kinds of bone, and is part of the process by which birds create eggshells for their young. It’s only present just before, during, and after egg laying, so its presence is a good indicator of a bird being pregnant.

The discovery is amazing for several reasons. For one, it gives us a way to potentially sex dinosaurs, something that paleontologist have struggled with for a long time. With their tendency toward frills, horns, and other signals, it’s likely that dinosaurs were big on sexual dimorphism, but we haven’t been able to figure out who had the horns in the relationship, so to speak.

Of course, researchers need to find other specimens that contain medullary bone to use it as a determinant, but in complete enough finds, researchers could determine what physical features belonged to each sex.

And, as a bonus, it’s been learned that at least some of the original chemical composition of fossils remains from their life. This isn’t something that was expected to happen, but with this new knowledge, and with newer tests, it should be possible to reveal secrets about dinosaur development based on the remaining chemical composition of their bones.


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