How do you tell a girl T. rex from a boy T. rex?

Not very easily, but a team of paleontologists has identified a young lady from Montana's Hell Creek Formation because her fossilized remains contain a special bone tissue that forms in female birds when they are getting ready to lay eggs.

The research, reported today in the journal Science, marks the first time that scientists have ever sexed a Tyrannosaurus rex, and the technique could work with other species -- as long as the skeleton is that of a female during the egg-laying cycle.

"But it's a pretty rare event" to find such a fossil, cautioned North Carolina State University's Mary H. Schweitzer, the leader of the research team. "Not too many of them die in the middle of the laying season. They die when they stop producing eggs."

Schweitzer suggested that the research may be just as valuable as evidence in the still somewhat controversial debate over the link between modern birds and dinosaurs, which went extinct 65 million years ago. The tissue in the Hell Creek specimen closely resembles similar tissue in ratite flightless birds such as emus and ostriches.

The Hell Creek female, discovered beneath 1,000 cubic yards of sandstone in northeast Montana, is rapidly becoming one of the most famous dinosaur fossils ever found. In March, the Schweitzer team announced that a thighbone, or femur, from the specimen, a young adult about 18 years old when it died, contained soft tissue that had survived for 70 million years.

"It's an outstanding fossil," said team member John Horner, curator of paleontology at Montana State University's Museum of the Rockies, where the remains are being studied. "And the likelihood of finding another dinosaur with this kind of material is really low."

Schweitzer said the discovery reported today came during the team's examination of the same cross section of femur that eventually produced the soft tissue. Horner said a thin, spongy-looking layer of bone lining the femur's inner cavity was visible to the naked eye and had clearly been permeated with blood vessels when the creature was alive.

The team reasoned that the tissue was "medullary bone" similar to that formed by female birds today when ovulation begins the egg-laying cycle. Schweitzer said the bone is an "ephemeral feature," a reservoir of calcium deposited in the bone cavity and drawn upon to build eggshells. As the T. rex laid eggs, the medullary bone depleted and finally disappeared, as it does with modern birds, at the end of the cycle.

But while "the tissue is a good marker for [determining the sex of] ovulating females," Schweitzer said, its absence indicates nothing -- the specimen free of medullary tissue could be male or a non-ovulating female.

What the new skeleton may offer, however, is a benchmark for further T. rex research: "Now that we have an individual that we know is female, we can look for other characteristics in the skeleton that are different," Horner said.

Unfortunately, dinosaurs, including T. rex, do not show much evidence of "sexual dimorphism" -- physical differences between males and females. Many dinosaurs are strange-looking, such as stegosaurus or triceratops, but all the known fossils of these species are similar.

"If there was a difference, we would have already seen it," said Horner, a leading advocate of the theory that dinosaurs were odd-looking not because they wanted to show off for potential mates but simply so they could -- in an extremely hostile world -- easily identify other members of their own species.

"People have been wondering about sexing dinosaurs since the 1920s, and it's amazing how little we know," added Hans-Dieter Sues, associate director of research and collections at the National Museum of Natural History. "It's not like humans -- where you can tell by the shape of the pelvis. With dinosaurs, you don't get anything like that."

Also, Sues said, "the sample for most species is much too small." For most dinosaurs "you're lucky if you can get even a partial skeleton," he said. "And you can't find out very much from that."

The world's largest T. rex fossil, in Chicago's Field Museum, is nicknamed Sue after its discoverer, but its chances of being sexed by the medullary bone method may be limited because scientists reckon the dinosaur was 28 years old and suspect it may have suffered from arthritis and died at least in part because of old age.

Horner said researchers could try the medullary bone technique on other species, especially smaller raptors and the other predatory, meat-eating "therapods" that appear to be more closely akin to modern birds than other dinosaurs.

"Therapods also have big, hollow marrow cavities like what we saw in T. rex," Horner said. But while the leg bones in plant-eating "sauropods" are thicker with smaller cavities, they may also have medullary bone: "We find a lot of sauropods in nesting areas," Horner said. "Those are the first ones I'd look at."