Preservation of fossils
Consider what is going to be preserved and where:
 Invertebrates – soft tissues and sometimes shell or skeleton
Chitin e.g. graptolites and insects
Calcium carbonate – calcite (molluscs) and aragonite
(echinoderms)
Silica – sponges and radiolarians
 Plants – cellulose and carbon
 Vertebrates – Bone (calcium phosphates and collagen – fibrous
protein) and teeth (calcium phosphates)
Methods of preservation
 Complete – Mammoths – Siberia
 Amber (tree resin) – flies etc. Soft inner tissues missing
 Mummification – dried out in a hot climate (Sloth in New Mexico) – soft
parts and skeleton are preserved
 Peat or tar – bones remain unchanged because antiseptic properties
arrest the process of decay. E.g. giant deer in Irish peat bogs, tar pits
in the USA.
 Carbonisation – material is carbonised under anaerobic conditions or
heat at deep burial. The relative carbon content is increased by
liberation of volatile constituents. Outline of organisms preserved (chitin
and cellulose alter to carbon in graptolites and plants).
 Aragonite to calcite – calcite is the more stable polymorph. Modern
invertebrate shells are aragonite but it changes to calcite with time.
 Petrification – literally ‘turning to stone’
Impregnation or
Replacement of hard parts by minerals in solution, percolating through
permeable remains:
a) infilling the interstices (gaps)
b) substitution
Silica and calcium carbonate are most common, also pyrite.
Loss of hard parts – acidified water percolating through permeable rock will
dissolve calcium carbonate shells leaving internal and external moulds.
Trace fossils – tracks and trails (moulds and casts)
burrows
coprolites (give details of diet and gut)
bores