Dinosaur's Sex No Longer a Mystery

MICHELE NORRIS, host:

The largest, most complete skeleton of a Tyrannosaurus Rex is named Sue. Another big one found in Montana has been named Bob. That suggests that scientists can tell T. rex from T. Regina, but there's no reliable way to do that. The names belong to the people who found the fossils. Now a team of scientists believes it has a technique to identify at least some female dinosaurs. NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE reporting:

Dinosaurs may not be able to lie about their age--scientists can date their bones pretty handily--but they are coy on the topic of sex. They did lay eggs, and recently, science has found a dinosaur skeleton with eggs inside it, so at least one female has been identified. But now a team of scientists has stumbled on a new idea for sexing dinosaurs. It started with a fluke, actually. Paleontologist Jack Horner of Montana State University was running an excavation in a Montana wildlife preserve. His team had found the thigh bone of a Tyrannosaurus Rex, and Horner had to make a tough decision.

Mr. JACK HORNER (Paleontologist, Montana State University): It was so far out in the country that we needed to helicopter it out, and we actually had to split the specimen in two pieces.

JOYCE: The pieces eventually went to North Carolina State University. Biologists there took a look inside the bone. Much like a bird's leg bones, it had an internal cavity. But there was something in the cavity they had never seen before in a dinosaur fossil. It was medullary bone. It's a kind of bone only known in present-day birds. And according to biologist Mary Schweitzer, it has a specific purpose.

Ms. MARY SCHWEITZER (Biologist): The function of medullary bone is to provide calcium for shelling eggs.

JOYCE: Female birds create a medullary layer as a reservoir of calcium to make eggshells. When the shells are done, the layer disappears. Writing in the journal Science, the team says the Montana T. rex was a female and was making eggs when it, or she, died.

So far, the technique has only been used on one specimen, but the team says if it holds up, it could provide a way to identify at least female dinosaurs, but only members of the theropod group such as T. rex that are believed to be related to modern birds. And the lack of medullary bone doesn't mean you've got a male dinosaur. It could just be a female that was not making eggs when it died. But Schweitzer says the technique helps tease out details of dinosaur biology and the winding path of evolution.

Ms. SCHWEITZER: In addition to demonstrating gender, it's another nail in the coffin that unites dinosaurs and birds, that links them much more closely than any other of their living relatives.

JOYCE: The medullary bone looks most like the bone in the legs of ostriches and emus. Crocodiles, however, don't appear to use medullary bone at all when they make eggs. Paleontologists say determining things like gender is part of a trend. They're teasing more information out of bones that explains the actual biology and behavior of dinosaurs. For example, scientists are comparing dinosaur skeletons with those of elephants and ostriches to figure out how dinosaurs walked and ran. At Florida State University, paleobiologist Greg Erickson figured out how fast T. rex matured by studying its bones.

Mr. GREG ERICKSON (Paleobiologist, Florida State University): In the past, our field has really focused in on who's related to who among dinosaurs, but there hasn't been that much interest in what these animals were really doing for a living. So you're seeing an influx of people coming into our field with a lot more biological training, and that's really changing the focus of our research programs.

JOYCE: And that's provoking scientists to think of new ways of looking at old bones. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.