The next stage: burial

If the hard parts survive, the next stage in fossilization is for them to be buried in sediment at the bottom of a water column: for only sedimentary rocks contain fossils. (Geologists distinguish three main rock types: igneous rocks, formed from volcanic action; sedimentary rocks, formed from sediments; and metamorphic rocks, formed deep in the Earth's crust by the metamorphosis of other rock types - when sedimentary rocks undergo metamorphosis, any fossils are lost.)

Animals that normally live within sediments are more likely to be buried in sediment before being destroyed, and species that live on the surface of the sediment (i.e. on the sea bottom) are more likely to be fossilized than are species that swim in the water column. The image opposite for example is a relatively common fossil: that of fossilized coral. Terrestrial species are least likely of all to be fossilized. The further a species lives from sediments, the less likely it is to be fossilized.