Fossils

Types of fossils

  • Entire organism preserved
  • Original hard parts of invertebrates preserved
  • Skeleton preserved
  • Altered hard parts of organisms preserved
  • Traces of organisms preserved
  • Petrification

Entire organism preserved

  • In a continuous dry or cold region
  • All organic remains, even softer body parts, may remain unchanged for a long time under favourable conditions
  • Preserved from bacterial decay

Original hard parts preserved

  • Harder parts such as shells and spicules get fossilised

Skeleton preserved

  • Skeletons of vertebrates become preserved in their original shape and structure

Altered hard parts preserved

  • Original hard parts get altered
    • Due to carbonisation
    • Soft parts of a body leave behind a film of carbon, so the exact body outline obtained

Petrification

  • When the mineral-rich water penetrates through the pores made by decomposition of the organic matter gets deposited in these pores

Specific fossil types

  • Mineralisation
  • Internal mould
  • External mould
  • Cast
  • Tracks
  • Trails
  • Burrows
  • Borings
  • Coprolites
  • Gastroliths

Mineralisation

  • Certain structures of the fossil remains are replaced by specific minerals

Internal mould

  • The cavity is filled with mud or sediments or minerals before the shell is dissolved by percolating water

External mould

  • The shells of an organism are buried in sediment or mud which becomes hardened. Organic material is removed by percolating acidified water and a cavity is formed

Cast

  • When an animal or other organism dies, its flesh decays and its bones deteriorate. Minerals enter the cavity gradually

Tracks

  • Impression of feet passing animal on mud and wet sand covered by sediment before disturbed

Trails

  • Irregular markings of moving animals on sedimentary rocks

Burrows

  • Presence of burrows in sedimentary rocks

Borings

  • Holes made by animals for the sake of food or shelter in wood and rocks

Coprolites

  • Fossilised faeces of ancient animals
  • Provide information about food and feeding habits of fossil animals

Gastroliths

  • Hard, stony pieces found in the stomachs of ancient reptiles and fish.

Uses of fossils

  • Chronostratigraphy
  • Biostratigraphy
  • Index fossils and correlation of rocks
  • Paleogeography
  • Relative dating

Chronostratigraphy

  • Concerned with the age of strata (rock) and the time they represent in relation to other layers

Biostratigraphy

  • Fossil contents of beds are used in interpreting the historical sequence of their occurrence