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American Association for the Advancement of Science

A surprise inside a T. Rex fossil

When scientists looked inside the leg bone of a recently discovered Tyrannosaurus rex fossil, they found something they weren't expecting. Typically, only the hard parts of an animal, like the bones, are preserved as fossils. This T. rex fossil, however, contained some beautifully preserved soft tissue inside the bone, where the marrow once was.

Looking at the tissue up close, the scientists found that it was threaded with blood vessels and seemed to contain a variety of cells. In fact, after a special clean-up, the tissue became flexible and stretchy -- much different from your standard fossil bone.

Soft tissues can sometimes be preserved in fossils, but it's usually difficult to determine their original shape and composition in fossils more than a few million years old. These findings show that soft tissues can be clearly preserved for much longer than that. This T. rex fossil is roughly 70 million years old.

Mary Higby Schweitzer of North Carolina State University and the North Carolina State Museum of Natural Sciences in Raleigh, N.C. and Montana State University in Bozeman, Mont. and her colleagues describe their discovery in the 25 March 2005 issue of the journal Science.

Most animal fossils form after the body is buried in sediment like sand or mud. The soft tissues often rot away first. The bones, shells, or other hard tissues decay much more slowly. Over millions of years, as mineral-rich water seeps through the hard tissues, the minerals replace the original bone or shell materials, creating a fossil.

In this case, though, while some mineral growth did occur on the soft tissue, it didn't fully replace the original material. When Schweitzer and her colleagues dissolved the mineral deposits, they found that the tissue was still soft and elastic. It also looked very much like the tissue found inside modern-day ostrich bone, offering further evidence that the researchers were actually looking at some of T. rex's own blood vessels and cells.

The exquisite preservation of this tissue, which does not challenge the timing of dinosaur evolution, may open up avenues for studying dinosaur biology, especially if researchers can identify soft tissues in other dino-fossils as well.