These and other decomposers are sealed off, preventing decomposition. After time the sedimentary layers become rock. The most common fossils found in sedimentary rocks are from the hard parts of organisms, including shells, bones, teeth, and woody stems. Sometimes minerals replace the original remains, often molecule by molecule. Such replacement preserves the microscopic structure of the organism.
Fossils can form in other ways.
For instance, insect fossils have been found trapped in hardened resin. Such fossils show details as small as the insect's tiny leg hairs. In another instance, woolly mammoths that were frozen in Arctic ice have been dug up with their skin, bones, and muscles perfectly preserved. Fossils are not always the body parts of an organism. An imprint is a type of fossil in which a film of carbon remains after the other elements of an organism have decayed.