Fossils are ancient biological remains or traces preserved in geological strata during geological history and subjected to petrification. Most fossils are made of calcium, with a few being made of silica, apatite, pyrite, etc. According to their preservation characteristics, fossils can be divided into five categories. The first category is solid fossils, which are mostly animal bones and shells as well as plant roots, stems, and leaves that have been petrified in strata, so they preserve the form, structure, and decorations of organisms. The second category is molded fossils, which are imprints of biological remains left in strata, including external molds and internal molds. External molds refer to the external forms of biological remains left in strata (including limestone, sandstone, and shale), while internal molds refer to the internal forms of biological remains (mostly the interiors of shells) left on the filling material. The third category is trace fossils, which can be divided into two types: one is traces left by animal activities, such as animal footprints and traces left by worms; the other is animals without skeletons or shells, such as jellyfish, whose bodies are fossilized in strata (mostly sandstone and shale). The fourth category is chemical fossils, which are fossils formed from molecules such as amino acids and fatty acids preserved under specific conditions.
The last category is special fossils, such as amber and fossils in permafrost. Ancient plants secreted sticky resin, which accidentally enveloped insects or other organisms. Due to the lack of external air and bacteria, the biological remains did not decompose. These resins later gradually turned into amber under high temperatures and pressures in strata, and the biological remains inside were not petrified but also called fossils. Amber fossils are often found in coal mines, and they are frequently discovered in Fushun Coal Mine in Northeast China. Biological remains in permafrost, which have not been petrified due to freezing, can also be called fossils. It can be seen that fossils come in a variety of forms and are not just petrified animal bones.