The formation of fossils is a very complex process. Not all organisms that once lived on Earth had the opportunity to form fossils. Most organisms either get eaten by other creatures or are decomposed by microorganisms after death, so the probability of fossil formation is very low. For this reason, people who have some knowledge of fossils and the history of life evolution will surely notice an obvious and paradoxical phenomenon: the most numerous and diverse animals on Earth, such as insects, spiders, scorpions, mites, arthropods, nematodes, various worms, and countless other animals lacking hard skeletons, do not have many fossil records. On the contrary, the most common groups in the fossil record account for only a small fraction of all biological species. Common fossils include molluscs, brachiopods, corals, echinoderms, vertebrates, and other animals with hard shells, skeletons, or teeth, which account for less than 20% of today's total animal species.
We are able to recognize organisms lacking hard shells or skeletons and difficult to fossilize mainly due to the fossil groups with exceptional burial. They are called exceptional burial because the fossils here preserve rich fine structures of organisms and even soft tissues, which is different from the common fossilized bones and shells. The formation of such fossils requires some very special environments, such as when the remains of organisms fall to the bottom of oxygen-depleted water after death, and often also require the help of microorganisms. Microorganisms produce a biofilm around organic matter through decomposition, and the chemical microenvironment within the biofilm can guide the deposition of minerals, thereby well protecting the parts that have not decayed and greatly increasing the chances of fossil formation.