Aside from these dinosaur fossils, medullary bones are only found in pregnant female birds.
The medullary bone forms in the inner cavity of certain bones when birds are pregnant, serving as a calcium storage depot that the birds depend on in order to not have to drain calcium from elsewhere in the body while forming eggs. It disappears once the mother has finished laying eggs.
This discovery is yet another piece of evidence in the dinosaur-bird link that scientists have speculated since the 19th century.
Located in the T rex‘s femur through a chemical analysis, the medullary tissue was found to related to that uniquely found in ostriches and chickens.
Dr. Mary Schweitzer, the project’s lead paleontologist, said, “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. The discovery of medullary bone is just one more piece of evidence that blurs the line between birds and other theropod [carnivorous two-legged] dinosaurs like T rex.”