
A fossil is any evidence of ancient life, this can mean anything from
bone fragments to leaf impressions.
Every fossil is unique because of the variation in living organisms
and those of fossilisation. The odds against fossilisation are millions
to one, although some animals and plants increase their chance of being
fossilised by living in certain places (eg. burrowing in sediment, living
in large groups or living in certain areas of the sea).
Even if they do become buried quickly after death the chemical and physical
conditions as the sediment turns to rock may be too harsh to preserve
fossils. If after all this fossils are preserved, the rock containing
them may remain many kilometres below the surface, not to be exposed
for millions of years to come.
It is for these reasons that fossils are relatively rare, but if the
conditions are right for preservation fossils can be preserved in large
numbers and in great detail.
