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