A Tyrannosaurus rex discovered by paleontologists may have been pregnant when she perished, researchers report. This discovery, made in Montana, could help answer questions about the ancient creatures, including a means of identifying fossils by their gender.

Paleontologists report the dinosaur lived roughly 68 million years before the present day.

The discovery could assist researchers in identifying gender differences between different specimens of certain meat-eating dinosaurs known as theropods. Medullary bone, the inner lining of bone inside the cavity which stores marrow, is specific to females in various bird species. A similar layer was discovered within the femur of the newly-discovered dinosaur. The layer is only found in birds during, and just prior to, egg laying. Biologists had theorized a similar structure may have been present in egg-laying dinosaurs.

"All the evidence we had at the time pointed to this tissue being medullary bone, 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," Mary Schweitzer of North Carolina State University said.

Keratan sulfate, a type of structural carbohydrate, is only found in the distinctive layer. Many researchers believed the substance would not last millions of years within fossils. However, investigators in the study were able to conclusively show the presence of the material in the ancient fossils.

Little is known about sex-related differences between dinosaurs, but searching for the chemical markers of medullary bone could positively identify specimens as females. However, carrying out this test means cutting into, or demineralizing, bones, ruining the samples. This could make researchers hesitant to carry out such studies on their own specimens. The femur used in this study was already broken, allowing the research to take place without wrecking the fossil.

Despite how little paleontologists know about identifying the gender of dinosaurs, the creatures signaled their sex to others through a number of physical characteristics. These included distinctive crests, horns, and frills, researchers stated.

Analysis of the ancient pregnant dinosaur and research into a new method of sex-typing bones was published in the journal Scientific Reports.

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