Scientific American: Twenty years ago paleontologists discovered what looked like red blood cells and other apparent soft tissues in a slice of 67-million-year-old fossilized dinosaur bone. The finding contradicted a centuries-old assumption that when an animal dies, inert minerals from the surrounding environment replace all of the organic molecules. For that reason, many believed the alleged organic tissues must be biofilm formed by microbes after the animal died. To support their original interpretation, Mary Schweitzer and colleagues have been running a series of tests. In a recent paper published online in the journal Bone, they describe immunological and mass spectrometry evidence that the structures they isolated were indeed part of the once-living animals.