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. The completeness of a fossil indicates whether or not the organism was fossilized in situ (e.g. partly decayed crinoids are readily broken by being transported, even over a short distance). Solution and other chemical action may reduce the tissues to a thin film of carbon; this process is called ‘carbonization’. Occasionally, rapid fires preserve flowers in three dimensions by carbonization and such fossils have been found that are 95 Ma old. The organism may be flattened by the compaction of sediments. If the organic content is preserved, the fossil is described as a ‘compression’; if it is not preserved the fossil is an ‘impression’. These terms do not necessarily imply that the material has been subjected to pressure. Porous structures (e.g. bones and shells) may be made more dense by the deposition of mineral matter by groundwater. 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, when it is not. Common replacement minerals include silica or iron sulphide, but there are many others. If the original cell walls survive with some of their organic material, but supported by minerals (i.e. the cytoplasm is replaced), the process is called ‘permineralization’. Because the organic material can be prepared on slides, such fossils yield more information than those produced by ‘petrifaction’, in which all the material has been converted to minerals, although the outlines of structures (e.g. organelles) can be studied on the polished surfaces. An organism may be fossilized in three dimensions as a nodule, formed if the decay process causes the nucleation of sediments around it. Organisms may also be preserved in almost perfect condition in amber, but it is difficult to obtain information from them. 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. See also death assemblage and taphonomy.

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.