A fossilized femur belonging to a pregnant Tyrannosaurus rex provides the first evidence of a gender-specific dinosaur fossil. It is believed that this specimen may also shed light on the evolution of egg laying in modern birds.

Theropod dinosaurs - a group of meat-eating dinosaurs, which includes modern birds and other toothy relatives such as the T. rex - laid eggs to reproduce. In the latest study from North Carolina State University and the North Carolina Museum of Natural Sciences, researchers identified the presence of a medullary bone in the fossilized T. rex femur.

The medullary bone is a gender-specific reproductive tissue found only in female birds during egg laying. This medullary bone was originally found by lead author Mary Schweitzer in 2005 in Montana. It was located in the femur of a 68 million year old T. rex fossil, MOR 1125. However, it was not until recently that researchers were able to confirm their findings.

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

The medullary bone is chemically distinct from other bones, as it contains keratan sulfate. Previously, researchers did not think that a dinosaur bone could preserve its original chemistry for millions of years.

The latest study, however, disproves that alleged theory. Researchers conducted a series of tests on the T. rex specimen and compared their findings to known medullary tissue from ostrich and chicken 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 added.

However, finding more evidence of fossilized medullary bones might be difficult, as it is only present during egg laying and has to be laid down and mobilized quickly in order for birds to shell their eggs.

While the femur of MOR1125 was already broken when researchers examined it, Schweitzer noted that most paleontologists would not be inclined to cut open and compromise fossils in order to search for a rare medullary bone. However, CT scans may help researchers tackle this obstacle.

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

Their findings were recently published in the journal Scientific Reports.