He rex, she rex: Montana fossil helps scientists discover the difference

Associated Press Writer

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.

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 today'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 since medullary bone is present only during the egg-laying cycle. 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.

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.

First it has to be an ovulating female, then it has to die before it has finished laying eggs, then it has to be fossilized and finally that fossil has to be found by humans, she said.

The skeleton in which the medullary bone was found is the same one from which the researchers were able, for the first time, to recover soft tissue from a dinosaur. They reported in March the discovery of tissues including what may be cells and blood vessels from the skeleton.

Known as ''MOR 1125'' for Museum of the Rockies specimen 1125, the fossil is between 68 million and 67 million years old, Horner said.

The researchers said the medullary bone in the dinosaurs was more like that found in ostriches and emus than that of smaller birds.

Schweitzer also noted that the egg-laying process differs between birds and animals like crocodiles, which do not produce medullary bone.

In crocodiles, she said, the eggs all get hard shells at one time. In birds, the eggs develop hard shells one at a time and the medullary bone serves as a storehouse for the calcium needed during the egg-laying process.

The research was funded by North Carolina State University and grants from the National Science Foundation and Nathan Myhrvold.

On the Net:

Science: http://www.sciencemag.org

Museum of the Rockies: http://www.montana.edu/wwwmor/