fossilization

fossilization The process by which a fossil is formed. It is unusual for organisms to be preserved complete and unaltered; generally, the soft parts decay and the hard parts undergo various degrees of change. Solution and other chemical action may reduce the tissues to a thin film of carbon; this process is called ‘carbonization’. The organism may be flattened by the compaction of sediments to form compressions. Porous structures, e.g. bones and shells, may be made more dense by the deposition of mineral matter by groundwater; this process is called ‘permineralization’ or ‘petrifaction’. The internal physical structures of some shells may be changed as a result of solution and reprecipitation; in this process (‘recrystallization’) the original structure may be blurred or lost. Many shells which were originally composed of aragonite are recrystallized into the more stable mineral calcite. The solution of an original shell and the simultaneous deposition of another mineral material constitutes ‘replacement’; this may occur molecule by molecule, in which case the microstructure is preserved, or en masse, where it is not. Common replacement minerals include silica or iron sulphide, but there are many others. The impression of skeletal remains in surrounding sediments constitutes a ‘mould’. Where the external structures are preserved it is called an ‘external mould’ and where the internal features are preserved it is called an ‘internal mould’ or ‘steinkern’. Filling of a mould cavity by mineral matter may produce a ‘natural cast’. Tracks, trails, burrows, and other evidence of organic activity may also be preserved. These are called ‘ichnofossils’ or trace fossils.