http://www.nytimes.com/2005/06/03/science/03dinosaur.html

T. Rex Fossil's Surprise: She Was Ovulating
By JOHN NOBLE WILFORD
Published: June 3, 2005

For the second time in two months, a Tyrannosaurus rex recently excavated
in Montana has surprised scientists.

Among its rock-hard fossils, the scientists had already isolated soft
tissues, including blood vessels and cells lining them - a most improbable
discovery after 70 million years.

The same paleontologists may now have topped that. They are reporting today
that the same T. rex has yielded unusual bone tissue that shows that the
animal was an ovulating female. Until now, distinguishing the sex of
dinosaurs has been impossible without well-preserved pelvic bones.

Moreover, after careful testing, the scientists determined that the
estrogen-derived tissue was similar to substances now present only in
living birds that produce eggshells.

The discovery team concludes in a report in the journal Science that the
finding "solidifies the link between dinosaurs and birds" and "provides an
objective means of gender differentiation in dinosaurs."

Dr. Mary H. Schweitzer of North Carolina State University, the team leader,
and John R. Horner of the Museum of the Rockies at Montana State University
described the research on Tuesday in a teleconference arranged by the
American Association for the Advancement of Science, publisher of the
journal.

When she first examined the marrow cavities of the dinosaur leg bones, Dr.
Schweitzer said, "I knew right away there was special bone tissue, and it
had all the characteristics of medullary bone tissue."

Medullary tissue, previously associated just with female birds, is formed
by an increase in estrogen levels during a bird's egg-laying cycle and is
deposited on the interior walls of the leg bones. The tissue serves as a
reservoir of calcium for eggshells. After the last egg is laid, the tissue
is completely reabsorbed into the bird's body.

Dr. Schweitzer said the presence of such tissue in the T. rex indicated
that the reproductive physiologies of some dinosaurs might have been
similar those of to modern birds, in particular flightless ones like
ostriches and emus.

Other paleontologists agreed that the find, if verified in other specimens,
probably strengthened the hypothesis that birds are direct descendants of
certain carnivorous dinosaurs.

The research "looks interesting and important," said Dr. Mark A. Norell, a
dinosaur paleontologist at the American Museum of Natural History in
Manhattan. "But I'm not 100 percent sure and want to see the results
replicated in more than one animal. The big thing in science is
replication."

Dr. Norell is a leading proponent of the dinosaur-bird relationship and
said the support of that hypothesis was welcome but superfluous. The link,
he said, is "a done deal."

"It's been settled for years," he added.

Not, however, in the view of several prominent ornithologists.

Dr. Alan Feduccia, an ornithologist and evolutionary biologist at the
University of North Carolina and the author of a respected book on the
origin of birds, expressed skepticism about the new findings and nearly all
recent research claims in support of the dinosaur-bird link. Dr. Feduccia
said that he and most other ornithologists agreed that dinosaurs and birds
had a common reptilian ancestor at some early time, but that he had serious
doubts that birds are, in effect, living dinosaurs.

"I was skeptical of the soft tissue in T. rex to begin with," Dr. Feduccia
said.

He also expressed reservations about reports of feathered dinosaurs and
other lines of evidence advanced to support a direct dinosaur ancestry for
birds. He complained that the evidence was often weak and that the reports
in science journals were often premature.

Dr. Norell defended the research that favored the dinosaur-bird link.

"Every new piece of evidence - the feathers and feather structure, the
nesting behaviors, this soft-tissue stuff - all of these things are
congruent with the hypothesis," he said.

Mr. Horner, one of the most experienced dinosaur fossil hunters, was more
blunt about critics.

"I'd put them in the category of the flat-earth believers," he said.

Mr. Horner and Dr. Schweitzer said the identification of the tissue as
medullary bone appeared to be the only reasonable explanation. They said it
was chemically different from other bone material and infused with blood
vessels. In detailed laboratory tests, the scientists determined that the
tissue in the T. rex was virtually identical to that of the modern female
birds in form, location and distribution. Medullary bone has never been
identified in any other egg-laying species, including crocodiles, a distant
relative of dinosaurs.

Dr. Schweitzer and her colleagues agreed with the skeptics that it was now
necessary to examine the limb bones of other T. rex specimens and,
particularly, other predatory dinosaurs thought to be bird ancestors.

The presence of medullary bone, which is fragile and transitory, may be
hard to establish, scientists cautioned. Dr. Schweitzer's group recommended
looking for medullary traces in fossils of the oviraptor species that were
found carrying eggs. No tyrannosaur eggs have ever been found.

Dr. Schweitzer is also affiliated with the North Carolina Museum of Natural
History, in Raleigh, and Montana State. Another member of her team was
Jennifer L. Wittmayer, a researcher at North Carolina State.

The T. rex that they studied lived 70 million years ago on the subtropical
floodplain of what is now Montana. It died around the age of 18, apparently
toward the end of its egg-laying cycle.

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