Dinosaur gender: Is it he-rex or she-rex?
Published 9:00 pm Thursday, June 2, 2005
WASHINGTON – 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 new research, reported today in the journal Science, marks the first time scientists have 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 one, cautioned North Carolina State University’s Mary Schweitzer, the research team leader. “Not too many of them die in the middle of the laying season. They die when they stop producing eggs.”
Schweitzer suggested 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, found 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 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 egg shells. 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. A specimen free of medullary tissue could be either a male or a nonovulating female.
