Q How does a dead organism become a fossil?
A Most organisms do not become fossils after they die. This is because the conditions that promote fossilization are very rare. Also, because hard parts like shells and bones have a better chance of getting preserved than soft parts like skin and cartilage, preservation is not equal among all organisms. When fossilization does occur, however, the process can take many forms.
Some organisms can be preserved with their soft tissues almost completely unaltered; one famous example are the woolly mammoths that were found frozen in a lifelike state in the Siberian tundra in 1901. Permineralization, another method of fossilization, occurs when water-borne minerals like calcium carbonate flow through and precipitate out within porous tissues like bone and wood. Recrystalization, on the other hand, happens when the original mineral components of an organism alter into chemically different, though texturally similar crystals. If a fossil becomes filled with sediment and then dissolves away, it can leave behind an internal mold, or steinkern. Other fossils, like those of plants and graptolites (see picture), can be preserved as a thin film of usually dark carbon within a rock.
The activity of an organism can also get preserved in the rock record. These so-called trace fossils, or ichnofossils, can reflect what the organism was doing while it was alive, what it was eating, or what kind of an environment it lived in. Examples include preserved footprints, burrows, fecal matter (or coprolites), and root casts from an ancient plant.