The science team that previously announced the discovery of surviving soft tissue in the fossilized thighbone of a Tyrannosaurus rex has determined the dinosaur was a young female and she had been producing eggs when she died.
The team detected microscopic evidence of a specialized reproductive tissue, known as medullary bone, that is found in modern birds.
The first such discovery in any dinosaur, it strengthens the link between dinosaurs and living birds by suggesting they shared similar egg-laying processes, the scientists said.
It also provides one possible way around a longstanding obstacle for paleontologists: Fossilization destroys any visible sign of whether a dinosaur was male or female.
Key to gender
"The discovery of medullary bone in T. rex is important because is allows us to figure out the gender of a dinosaur," said North Carolina State University paleontologist Mary Higby Schweitzer, who reports her team's findings in today's issue of the journal Science.
"This is very exciting," said Peter Makovicky, curator of dinosaurs at the Field Museum of Natural History in Chicago. "This is only the first salvo in what could really be important work.
"The real strength of the discovery will come with more samples. If you have one animal, it's suggestive, but not definitive. You get two or three animals that show this type of bone, and you're on your way to showing gender."
In birds, a thin layer of temporary bone tissue is produced by estrogen to provide a source of calcium for eggshells. The tissue forms inside the hollow bones of birds until the last egg is laid, after which it is completely reabsorbed by the body.
Discovery of similar tissue in a 68-million-year-old fossil provides evidence of a connection between the extinct giants and today's birds, Schweitzer said.
Because the dinosaur tissues didn't look exactly like the pictures published of medullary bone in living birds such as chicken and quail, Schweitzer's team compared the tissue from the femur of the T. rex with tissues from leg bones of primitive flightless birds, such as the ostrich and emu.
Schweitzer viewed the tissues with both light and electron microscopes and found the dinosaur tissues were virtually identical to those of the modern birds in form, location and distribution.
"We think this adds to the robust support linking birds and dinosaurs and shows their reproductive physiologies may have been similar," Schweitzer said.
"Hopefully we'll be able to identify features within dinosaurs that will help us determine the gender of our other fossils and lead to more information about their herd structure or family groups."
Resemblance to birds
T. rex apparently reproduced more like a bird than a crocodile, the two living organisms believed to be most closely related to dinosaurs.
Medullary bone is found today only in female birds; no other egg-laying species - including crocodiles - produces the tissue.
Field Museum officials said they had been in contact with the Schweitzer team about determining the sex of Sue, the museum's famous T. rex fossil.
"The problem is, if you have mounted specimens, like Sue, any medullary bone tissue would be in large marrow cavities - shinbones and thighbones - all of which are out there, complete, with no broken edges that you can test," Mackovicky said.
Found in Montana
Famed dinosaur hunter John Horner, of Montana State University's Museum of the Rockies and a member of the research team, said the fossil was a smallish T. rex found beneath 1,000 cubic yards of sandstone in northeastern Montana at the base of the celebrated Hell Creek Formation.
"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 Horner.
Finding females among fossils will be a long shot, Schweitzer said: "First it has to be an ovulating female, then it has to die before finishing laying all its eggs, then it has to be fossilized, then it has to be found by a pesky human."
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