Old softy: Tyrannosaurus fossil yields flexible tissue.

Scientists analyzing fragments of a Tyrannosaurus rex's leg bone have recovered pliable material containing structures that appear to be cells and blood vessels.

Paleontologists usually find only a creature's hard body parts, such as bones, teeth, or shells, preserved as fossils. In the rare instances when internal organs, muscles, skin, and other soft body parts turn up, the original tissue has been replaced by minerals that create hard replicas, says Mary H. Schweitzer, a paleontologist at North Carolina State University in Raleigh. Sometimes, a soft tissue's shape is recorded by sediments that surround it.

Now, the first report of flexible material from a fossil describes an extraction from the femur, or upper leg bone, of a T. rex that lived about 68 million years ago in what is now Montana.

The researchers dissolved minerals from the fossil by soaking it in a series of slightly alkaline solutions. After a week, much of the remaining material was surprisingly soft and pliable, say the researchers. Many parts of the remains were translucent and fibrous, and they retained their elasticity after repeated cycles of dehydration and rehydration. Schweitzer and her colleagues report their findings in the March 25 Science.

Some stretchy, translucent samples were high in carbon and appeared to be part of a network of blood vessels, says Schweitzer. Similar demineralization experiments on modern-day ostrich bones--with an added step required to digest the collagen strengthening those bones--yielded blood vessels of a similar size and texture, she reports.

The researchers squeezed round, microscopic structures out of the presumed T. rex blood vessels. Those small spheres, which ranged from dark red to deep brown, may be red blood cells, says Schweitzer.

Also, some fibrous remains contained small, elongated features that look like cells called osteocytes, which are found in mature bones.

Finally, a spongy material extracted from the bones triggered a chemical response in tests to detect proteins commonly found in bones of modern chickens and cows.

Using the extraction technique, Schweitzer and her colleagues subsequently recovered what appear to be blood vessels and osteocytes from two other well-preserved specimens of T. rex. They've also obtained osteocytes from an 80-million-year-old hadrosaur, a plant-eating dinosaur.

The state of preservation noted in the new study is "improbable but obviously not impossible," says Cameron J. Tsujita, a paleontologist at the University of Western Ontario in London. The osteocytes are "dead ringers" for those present in living vertebrates, he notes. As for the purported blood cells, he says that he "can't really think of what else they could be"

This is a "totally novel discovery," says Derek E.G. Briggs, a paleontologist at Yale University. The tissue preservation that Schweitzer's team has described may be more commonplace than scientists might expect, he says.

Additional analyses of the T. rex fossils, as well as of material from other specimens, could provide insight into the early stages of fossilization, says Briggs. Such information may also reveal new details of dinosaur physiology and metabolism.