Unambiguous indicators of gender in dinosaurs are usually lost during fossilization along with other aspects of soft tissue anatomy. We report the presence of endosteally derived bone tissues lining the interior marrow cavities of portions of Tyrannosaurus rex (MOR 1125) hindlimb elements, and hypothesize that these tissues are homologous to specialized avian tissues known as medullary bone. Because medullary bone is unique to female birds, its discovery in extinct dinosaurs solidifies the link between dinosaurs and birds, suggests similar reproductive strategies, and provides an objective means of gender differentiation in dinosaurs.From the North Carolina State University press release:
Paleontologists at North Carolina State University have determined that a 68 million year-old Tyrannosaurus Rex fossil from Montana is that of a young female, and that she was producing eggs when she died.In a case of a literal “lucky break,” the scientists discovered unusual bone tissue lining the hollow cavity of the T. rex’s broken leg bone. Schweitzer believes that the unusual tissue inside the T. rex bone is actually medullary bone: a thin layer of highly vascular bone that is found in present-day female birds only during ovulation. This estrogen-linked reproductive bone tissue is laid down inside the hollow leg bones of the birds and persists until the last egg is laid, at which time it is completely resorbed into the bird’s body. Its formation is triggered by an increase in estrogen levels, and the temporary tissue provides the calcium necessary to form eggshells. Medullary bone is only found in present-day female birds; no other egg-laying species – including crocodiles, the other living dinosaur relative – produces this tissue naturally.
Art © Frank Cho. Shanna © Marvel Comics.
Because the dinosaur tissues didn’t look exactly like pictures published of medullary bone in living birds like chicken and quail, Schweitzer’s team compared the tissue from the femur of the T. rex to that taken from leg bones of more primitive ratites, or flightless birds, such as ostriches and emus. These birds share more features with dinosaurs than other present-day birds. They selected an ostrich and an emu in different stages of their laying cycles, when medullary bone is present.
“The discovery of medullary bone in the T. rex is important because it allows us to objectively sex a dinosaur,” says Schweitzer. “It also adds to the robust support linking birds and dinosaurs and shows that their reproductive physiologies may have been similar. Hopefully we’ll be able to identify features within dinosaurs that will help us determine the gender of our other fossils, and lead to more information about their herd structure or family groups.”
Read the complete press release HERE.