A pregnant Tyrannosaurus rex died around 68 million years ago, and may just be the key to gender differences between theropod or meat-eating dinosaur species. It could also give some insight into the evolution of egg laying in modern birds.
Researchers from North Carolina State University and the North Carolina Museum of Natural Sciences have confirmed the presence of medullary bone — a gender-specific reproductive tissue — in a fossilized T. rex femur.
Paleontologist, Bob Harmon from the Museum of the Rockies in Montana, discovered the T. rex in Hell Creek Formation back in 2000. While exploring the dinosaur territory he sat down, and then felt a fossil behind his back. Harmon then shared his news with his colleagues, where they then spent the next three years excavating the enormous specimen, according to live science.
Medullary bone is only found during the period before or during egg laying, and is chemically different from other bone types. According to an NC State University statement:
“This is because medullary bone has to be laid down and mobilized quickly in order for birds to shell their eggs. Theropod dinosaurs, the broader dinosaurian group that includes modern birds and other toothy relatives such as T. rex, also laid eggs in order to reproduce, and paleontologists have hypothesized that they may have had medullary bone as well.”
Mary Schweitzer, an NC State paleontologist with a joint appointment at the NC Museum of Natural Sciences and the lead author of the paper published in journal, Scientific Reports, made the discovery of what she believed to be medullary bone in the femur of the 68 million year old T. rex fossil in 2005. Schweitzer explains:
“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.”
A substance called keratan sulfate is only present in medullary bone, however, it was believed that the original chemistry of dinosaur bone could not survive millions of years.
But this has now been put into question; while Schweitzer and her colleagues were conducting a number of different tests on the T. rex sample, one of which tested for keratan sulfate using monoclonal antibodies. When they compared their results to tests performed on known medullary tissue from ostrich and chicken bone, it was confirmed that the tissue from the T. rex was 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.”
Schweitzer recognizes while most paleontologists would not cut open or demineralize their fossils to search for rare medullary bone, the femur of MOR 1125 (the sample name) was already broken when it arrived to Schweitzer. Lindsay Zanno, co-author from N.C. Museum of Natural Sciences paleontologist with a joint appointment at NC State, has shown by a CT scan of the fossils it could help in narrowing down the search.
Zanno explained in a statement:
“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.
“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.”