According to a new study the discovery of a Tyrannosaurus Rex (T-Rex) fossil which contains reproductive tissue could help scientists identify sexes in carnivorous dinosaurs.

Researchers behind the study say that a fossilized T-Rex femur contains estrogen-dependent reproductive tissue. The tissue is present only during the period or while laying eggs, so the fossil has been confirmed as female.

Pregnant T-Rex Fossil Could Help Scientists

Researchers determine sex of T-Rex fossil

The study was the result of a collaboration between scientists at North Carolina State University and the North Carolina Museum of Natural Sciences. Its findings were released Tuesday.

The dinosaur fossil dates from 68 million years ago. Researchers did not believe that the original molecular components of the bone would survive in the fossil, until study lead author Mary Schweitzer, study lead author and paleontologist with North Carolina State and the museum, found the tissue in 2005.

Schweitzer found the reproductive tissue, known as medullary bone, which is historically only present in female birds. The tissue must be used quickly so that birds can shell their eggs, and is different other types of bone.

Chemical tests necessary to confirm hunch

Paleontologists had been curious as to whether the Theropoda dinosaur group, which includes birds, the T-Rex and other meat-eaters, might have medullary bone. The T-Rex also laid eggs to reproduce.

The search was complicated by the fact that, under a microscope, some bone diseases like osteoporosis can look like medullary tissue. Therefore it was necessary to perform a chemical analysis.

Researchers carried out a series of tests including using monoclonal antibodies to look for keratan sulfate, which is present in medullary bone but not in other bone types. They were then able to confirm that the tissue was medullary bone.

“All the evidence we had at the time pointed to this tissue being medullary bone,” Schweitzer said in a statement. “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.”

Further research could use CT scans to find medullary bone

Lindsay Zanno, a paleontologist at the university and museum and study author, said that the findings show how pregnant dinosaurs contain unique chemicals. However scientists are still largely unaware of how to differentiate the sex of dinosaurs.

“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,” Zanno said. “Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities.”

Schweitzer believes that finding more medullary bone in other fossils will be a tall order, as it is rare. In this particular case the T-Rex femur was also already broken when it was dug up, but most paleontologists would be unwilling to break open or tamper with fossils to look for medullary bone.

However Zanno discovered that CT scans of fossils could help find more medullary bone, providing hope for further research.

The study, which is titled “Chemistry supports the identification of gender-specific reproductive tissue in Tyrannosaurus rex,” can be found online, published by Scientific Reports. Scientists from Des Moines University and Niigata University in Niigata, Japan also contributed to the study.

Other research into T-Rex recently found a fossil which helps scientists fill in the evolutionary history of one of the most famous predators.