Minecraft wiki entry for Fossils, associated text: When caving, always be on the lookout for a bone block, as many times only 1 block will be exposed. Also, if you are in the correct Y coordinates (about Y = 40 - 49, a bit lower for land below sea level, and a bit higher for land above sea level), mine as much coal ore as possible, as it is possible that it is part of a fossil. This particularly goes for if you only see one coal ore, as more than one coal usually do not generate next to each other in fossils.