A Tyrannosaurus rex recently excavated in Montana, U.S. has surprised scientists. A layer of medullary bone found inside the leg bones of the animal - a most improbable discovery after 70 million years - showed it was an ovulating female.

　　The finding provides a way to tell at least some dinosaurs' sexes. Until now, distinguishing the sex of dinosaurs has been impossible without well-preserved pelvic bones.

　　Medullary bone is a calcium-rich layer that develops in the long bones of birds during the egg-laying process. It provides a supply of calcium to form eggshells.

　　Moreover, after careful testing, the scientists led by Mary Schweitzer of North Carolina State University determined that the estrogen-derived tissue was similar to substances now present only in living birds that produce eggshells.

　　This discovery won't enable paleontologists to determine the sex of all dinosaurs, however, since medullary bone is present only during the egg-laying cycle. But when it is present, it at least enables them to say that particular example is female.