T. Rex Specimen’s Leg Bones Show Gender, Link to Birds

Scientists studying the mighty T. rex might have found a way to tell a she rex from a he rex.

With only fossilized bones to work from, scientists have had little to go on to tell which specimens were male and which were female.

Now, a team led by Mary H. Schweitzer of North Carolina State University reports finding a layer of medullary bone inside the leg bones of a Tyrannosaurus rex discovered in Montana. Medullary bone is a calcium-rich layer that develops in the long bones of birds during the egg-laying process. It provides a ready supply of calcium to form eggshells.

The presence inside this T. rex’s legs indicates that she was a female, Schweitzer said. The finding will enable researchers to determine the sex of at least some dinosaurs. It also adds weight to the widespread belief that today’s birds descended from dinosaurs.

Her findings are reported in Friday’s issue of the journal Science.

“The discovery of medullary bone in a specimen of T. rex is hugely exciting,” said Chris Dacke of the University of Portsmouth in England, who was not part of Schweitzer’s research team. “It has never previously been reported in any other class of animal than birds.”

The discovery won’t enable paleontologists to determine the sex of all dinosaurs, because 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.

Not every museum may want to check the sex of its specimens, because it requires cutting a long bone in half.

The fossil, known as MOR 1125, for Museum of the Rockies specimen 1125, is between 67 million and 68 million years old.