He Rex or She Rex? Experts Find Way to Tell Dino Gender

June 2, 2005

A Tyrannosaurus rex unearthed in Montana has undergone a sex change now that scientists have discovered a way to tell the gender of fossilized dinosaurs.

Named Bob—after its discoverer, paleontologist Bob Harmon—the remarkably well preserved Jurassic giant has turned out to be a female. "Perhaps we should now call it Bobette," said Mary Schweitzer, a paleontologist at North Carolina State University in Raleigh.

Schweitzer is part of a team that has devised a method to determine whether dinosaur fossils are from males or females.

Until now this had been an almost impossible task, because all obvious gender indicators vanish when soft tissues decay during fossilization.

The new method focuses on a specialized layer of bone similar to one found in living birds. Furthermore, the study team says, the discovery that dinosaurs can be sexed in this way adds to the evidence that birds are directly descended from dinosaurs.

The breakthrough research was carried out on Bob's remains, which were excavated by scientists from the Museum of the Rockies in Bozeman, Montana, at the nearby Hell Creek formation. The team's findings will be published tomorrow in the journal Science.

Researchers say the key to the dinosaur's gender was found in one of Bob's thighbones. In-depth analysis of fossil fragments showed the limb contains medullary bone, a substance previously known only in female birds.

Medullary bone is rich in calcium, providing female birds a ready source of material for making eggshells. So this particular T. rex, the researchers say, must have been an egg layer, too.

The research offers hope that other dinosaur specimens can be sexed for the first time.

"The fact that medullary bone has been discovered in a T. rex is interesting indeed," said Angela Milner, associate keeper of paleontology at the Natural History Museum in London.

Fossil Evidence

Milner, who wasn't involved in the study, says the thighbone fossil represents the best evidence yet for a female dinosaur.

"Unlike mammals, we don't see [gender] differentiation in the skeletons of reptiles," she added. "From the skeleton itself, it's impossible to tell whether an animal was a male or a female. Reptiles don't give birth to live young, so they don't have broad or narrow pelvises, for example."

Milner says experts have speculated that features such as different types of head crests in some hadrosaurs (also called duck-billed dinosaurs) may be gender- specific. "Some crests are taller and thinner than others, so it's been assumed that that's a sex difference. But there's absolutely no way to tell which one is which," she said.

Schweitzer says that—short of finding a whole dinosaur with bona fide eggs in its body cavity—the sexing method her team describes is the only surefire way of determining gender.

She said, "All other proposals put forward for gender—variation in head ornamentation, the presence or absence of chevrons [certain bones in the tails of reptiles], et cetera—are simply assumptions."

That said, medullary bone might not be present in female dinosaurs that died when they weren't breeding. The tissue's formation in the leg bones of female birds is triggered by increasing levels of hormones associated with egg production. This calcium supply is then drawn down as it's converted into eggshells.

But now that at least one T. rex has been confirmed a female, Schweitzer hopes to identify other, more durable features that are characteristic of the species's sex.

This dinosaur's thighbone tissues are also remarkably well preserved (read the earlier story). "Whether you're going to be able to detect [medullary bone] in a wider range of material remains to be seen," said Milner, the Natural History Museum paleontologist. "But at least people can go and try now."

Ostriches, Emus

The team's analysis showed that the flesh-eating predator's medullary bone is more like that of female ratites, such as ostriches and emus, than that of other living birds. Schweitzer speculates that this could be because, like T. rex, these birds are relatively large animals.

"Their bones are much larger, relative to the size of their eggs, so the calcium draw would be significantly less," she said. "Also, ratites retain more primitive traits than other birds, and so more likely share more characteristics with dinosaurs."

The researchers say their findings strengthen the link between dinosaurs and birds by suggesting both share similar egg- laying processes.

There has been heated debate over the years as to whether theropod dinosaurs such as T. rex gave rise to birds. While the majority of experts are now convinced this was so, there are still those who doubt this theory. For instance, some say today's birds are descended from Crocodylomorpha, an order of reptiles that includes modern crocodiles.

Yet there is no clear evidence that female crocodiles produce medullary bone. This may be because crocodiles don't have hollow limbs—as theropods and birds do—and because crocodile eggs are soft- shelled, so don't require the large amounts of calcium needed for hard-shelled bird eggs.

Theropod eggs were also hard- shelled. The first known examples of theropod eggs belonged to oviraptors (a type of theropod) that were discovered fossilized in the Gobi Desert. Milner says these animals lived around 90 million years ago.

Milner says the discovery that T. rex appears to have shared the same means of making egg calcium as modern birds is another piece in the dinosaur-bird ancestral jigsaw.

"It's one more piece of evidence that supports an idea that is almost universally accepted now," she added.

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