How do you tell a male dinosaur from a (possibly) female dinosaur such as Sue, The Field Museum's famous Tyrannosaurus rex? (Sue may not be female—more on that in a minute.) 

For decades, paleontologists have sought more accurate ways of identifying the sex of dinosaurs. Characteristics like size and, in some species, fancy ornamentation—horns and crests, for instance—can be sex indicators, but not always beyond the shadow of a doubt. In fact, Sue was so named in honor of Sue Hendrickson, the paleontologist who discovered the dinosaur's remarkably intact remains—not because we know for sure she was female. The Field Museum says the T. rex's sex is actually unknown. 

Our ability to ID the sex of dinosaurs like Sue should improve in the future, thanks to the 68-million-year-old femur of a pregnant Tyrannosaurus rex. Using this bone, researchers have found a new molecular tool to more reliably determine from fossil remains whether a dinosaur was male or female. The team recently published their findings in Scientific Reports.

The key lies in the medullary bone, a reproductive tissue only found today in female birds around the time of egg laying that is chemically distinct from other types of bone. Meat-eating theropod dinosaurs like T. rex, which are related to modern birds, also laid eggs, raising the possibility that pregnant female theropods may have produced medullary bone during their reproductive cycle.

“We know that there’s sexual signaling among almost all organisms on the planet that reproduce sexually,” lead author Mary Schweitzer, a paleontologist at North Carolina State University and the North Carolina Museum of Natural Sciences, told mental_floss. “You’ve got to have a way to tell males and females, so it’s not surprising that dinosaurs would have this.”

About a decade ago, Schweitzer found what she believed was medullary bone in a T. rex fossil that had been discovered in Montana. But she needed to rule out other possibilities, such as symptoms of diseases that can mimic the appearance of medullary bone under a microscope.

A chemical analysis was the only way to be sure. Schweitzer and her team knew that medullary bone contains a substance called keratan sulfate that is not present in other kinds of bone. If they identified keratan sulfate in the Montana fossil sample, it would confirm the presence of medullary bone. 

But could evidence of keratan sulfate molecules persist in a fossil for tens of millions of years? As it turned out, yes. 

Schweitzer and her colleagues then compared the tissue found in the T. rex to medullary tissue in ostrich and chicken bones. They used a chemical stain that turns a vivid blue when it reacts with the keratan sulfate molecules found in medullary bone. Both the bird samples and the T. rex sample reacted similarly, indicating the presence of medullary bone.

Schweitzer said the research points to new ways that molecular biology might be used to resolve some of the conflicts and ambiguities in the fossil record. The hope now is that this chemical method can be extended to help unravel other mysteries, like determining dinosaur age and studying population structure.

“It’s one more piece of evidence that original molecules persist in fossils, and we can begin to use this molecular information to ask questions that we previously thought we could only ask of living organisms,” said Schweitzer.