The fossil must survive geologic events such as metamorphism and erosion. Even under ideal conditions, most life forms do not become fossils. Paleontologists have estimated that all known fossils species represent less than two percent of past life on earth. Of this two percent, ninety percent are marine invertebrates because the best place for preservation is on the sea floors. Sea floors tend to be relatively stable features with a fine-grained sediment supply and a geochemistry, which is kind to calcitic and chitinous shells.
Shells retain their original calcite or aragonite.
Unaltered remains are rare in older rocks but not unheard of. Shark teeth are usually not altered. This is the most common preservation type. Hard parts such a shell, bone, wood, chitin, and occasionally plants are altered chemically. Volatile organic compounds and water are driven off after burial as the sediment containing the fossil compacts.