Is it a she T-rex or a he T-rex?

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

The dinosaurs knew the difference, of course.

But scientists, with only fossilized bones to work from, have had little to go on as far as knowing which specimen was a male and which was a 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 department of pharmacy and biomedical sciences at the University of Portsmouth in England. "It has never previously been reported in any other class of animal than birds."

Dacke, who was not part of Schweitzer's research team, said the finding "certainly helps cement the notion that birds are directly descended from dinosaurs and also suggests that dinosaurs, like birds, developed similar reproductive strategies for providing an adequate supply of calcium for the eggshell."

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.

"We have 12 specimens of Tyrannosaurus rex here at this institution and we're about to find out if any more of them are female," said John R. Horner of the Museum of the Rockies in Bozeman, Mont.

But not every museum may want to check the sex of its specimens, since it requires cutting a long bone in half, said Horner, a co-author of the paper with Schweitzer.

And even then, finding medullary bone is a long shot, she said.