Seriously, what IS a fossil? Fossils are the preserved remains, impressions, or traces of ancient living things — plants, animals, and other organisms — that lived millions (or even billions) of years ago. Here’s a simple breakdown: Body fossils: Actual parts of the organism that were preserved, such as bones, teeth, shells, or leaves. Over time, the original material often gets replaced by minerals — turning it to stone. Trace fossils: Evidence of an organism’s activity, like footprints, burrow