13.7 fossils

what are fossils + what do they do

remains of organism from many years ago found in rocks, they tell us about the evolution of species

gradual replacement by minerals fossilisation ?

hard material (bones teeth shells) take long time to decay and so are eventually replaced by minerals as they decay forming rock like substance

surrounding sediment also turns to rock but fossil stays distinct inside rock

cast and impression fossilisation?

when clay surrounding organism hardens ,organism decays,leaving a cast of itself. animal burrow and rootlet traces can be preserved as casts

footprints can be pressed into soft materials leaving an impression when material hardens

from preservation in places without decay

microbes involed in decay need right conditions

if not present , no decay will occur and organism will be perfectly preserved (tollund man in peat bog )

why is there a lack of evidence to support any hypothesis on beginning of life

many soft bodied organisms completely decayed

fossils may of been destroyed by geological activity