 When laying eggs, female birds grow a special type of bone in their limbs.This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells.The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a "Tyrannosaurus rex" specimen's hind limb suggested that "T. rex" used similar reproductive strategies, and revealed that the specimen is female.