When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex fossil, her discovery raised an obvious question -- how the tissue could have survived so long?
The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes.
The result is like a radioactive clock that ticks away as unstable isotopes decay into stable ones.
Only hard parts, like bones and teeth, can become fossils.
But for some people, the discovery raised a different question.