Fossil

Fossil, evidence of ancient plant or animal life preserved in sediment or rock. Preservation of an organism in its entirety (i.e., unaltered hard and soft parts together) is exceptional. Entire mammoths have been preserved in Siberian permafrost. Unaltered hard parts are common in post-Mesozoic sediments but become increasingly scarce further back in geologic time. Petrification describes 2 ways in which the shape of hard parts of the organism may be preserved. In permineralization the pore spaces of the hard parts are filled by certain minerals (e.g., silica, pyrite, calcite) that infiltrate from the local groundwater. The resulting fossil is thus a mixture of mineral and organic matter. In many other cases substitution or replacement occurs, in which the hard parts are dissolved away but the form is retained by newly deposited minerals. Where this has happened very gradually, even microscopic detail may be preserved, but generally only the outward form remains. Often the skeletal materials are dissolved entirely, leaving either internal or external molds. The filling of a complete mold may also occur, forming a cast. The complete filling of a hollow shell interior may form a core or steinkern such as the corkscrewlike filling of a coiled snail shell. In the process of carbonization the tissues decompose, leaving only a thin residual carbon film that shows the outline of the organism's flattened form.