How to Administer a Pregnancy Test to a 68-Million-Year-Old T. Rex

Tyrannosaurus rex was one of the most intimidating predators to ever roam the Earth, which makes it difficult to imagine one tackling a nurturing role like motherhood.

But in a study released this week in Scientific Reports, paleontologists announced that they have discovered that a T. rex that lived 68 million years ago in what is now Montana was pregnant when she died.

The tipoff? Her fossilized femur contains traces of medullary bone, which is a calcium-rich bone tissue that is only produced during the reproductive cycle of female birds and their egg-laying ancestors—including theropod dinosaurs like T. rex.

In other words, the presence of preserved medullary bone is the paleontological equivalent of a plus sign on a pregnancy test. Indeed, this particular specimen, known as MOR 1125 or "B-rex," is not the first dinosaur to be found knocked up, though she is the first pregnant T. rex on record. Paleontologists have also identified medullary bone in Allosaurus and Tenontosaurus fossils, suggesting that this diagnostic feature is widespread among dinosaurs of all shapes and sizes.

Not only does this research provide a captivating glimpse into the reproductive life of this iconic Cretaceous carnivore, it also enables paleontologists to definitively classify MOR 1125 as a female. Determining the sex of dinosaurs can be a major challenge for scientists, given that distinct sexual characteristics are rarely preserved by the fossilization process, so the new find establishes a useful benchmark for future research into dinosaur reproduction.

"It's a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs," said paleontologist Lindsay Zanno, a co-author of the study, in a statement.

"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," she continued. "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."