One of the frustrating aspects that paleontologists face when studying dinosaur fossils is the odd lack of sexual dimorphism in the ancient creatures — being able to tell the difference between male and female individuals based on their physical features. However, one method of telling whether an individual specimen is a female or not has been uncovered, with the confirmation of a Tyrannosaurus rex fossil that came from a pregnant female.

"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," says the study’s lead researcher, North Carolina State University evolutionary biologist Mary Schweitzer.

The T. rex fossil in question was excavated from Montana’s Hell Creek formation in 2000, and had previously been the subject of a study regarding what appeared to be the presence of a medullary bone in the fossil’s femur. The medullary cavity is the interior part of the bone that typically holds the marrow, but in some egg-laying animals, such as birds, extra bone tissue build up in the cavity, to help them regulate their calcium levels just before they lay their eggs.

Questions about the accuracy of the assessment came up from critics: what if the medullary structures were simply an abnormality, such as osteopetrosis, a disease that causes an increase in bone density. The researchers turned to chemistry to verify their find: medullary bones in birds contain an organic compound called keratan sulfate, where bones afflicted with osteopetrosis do not. The tests came back positive: the structures did indeed contain keratan sulfate, meaning the dinosaur was pregnant.

"It’s a dirty secret, but we know next to nothing about sex-linked traits in extinct 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," explains Lindsay Zanno, NCSU paleontologist and study co-author. "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 [medullary bone]."