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FOSSILS
Types and how they form
Image source: http://www.nature.nps.gov/geology/photodb/photos/flfo_4879.JPG
1. What are fossils?
Fossils are evidence of past life; they are preserved remains and traces of animals and
plants.
2. Where are they found?
Fossils are usually found in sedimentary rocks.
3. How do fossils form?
The living thing must usually
 Be buried quickly (stops rapid decay)
 Have hard parts (skeleton or shell)
Other things that promote fossilization are:
 Low oxygen water (stops decay)
 Extreme temperatures
 Salty environment
 Rapid mineralisation (hardening of the living tissue)
4. How are fossils useful?
Fossils called index fossils allow rock layers to be correlated. This means that rock
layers in different areas can be matched up and be said to be of the same age. Index
fossils must
 Be spread over a wide area
 Be of a species that only existed for a short geological time
Fossils also show that life has evolved over time.
Fossil evidence can be used to show that certain countries were once joined (provides
evidence for continental drift/plate tectonics).
Fossil type
How is it formed?
Petrified
Minerals slowly replace the hard
parts of a living thing.
Eventually, the parts of the
living thing break up and leave
only the hardened minerals
behind.
Description (add sketch if
possible)
Looks a lot like the
original organism except
that it is hard rock e.g.
might look like a tree
trunk.
Carbonised
A dead organism trapped in a
A black imprint of the
rock can be ‘cooked’ by heat and living thing.
pressure. The organism breaks
down leaving only carbon
behind.
Mould
A decomposed organism trapped A hollow shaped like a
in rock leaves a space behind
living thing.
that is like the imprint of the
organism.
Cast
When minerals seep into a
mould, they harden and create a
fossil that looks like a rock
version of the original creature.
Organism made of rock.
Unaltered
Very rarely, complete organism
can be found preserved, for
example in tar or sap.
Looks exactly like the
organism.
trace
Markings left by an animal
(footprint, trail, or
burrow)