What is a fossil
A fossil is any preserved remains, impression, or
trace of any once-living thing from a past
geological age. Examples include bones, shells,
exoskeletons, stone imprints of animals or
microbes, objects preserved in amber, hair,
petrified wood, oil, coal, and DNA remnants.
How are fossils formed?
Fossils are formed in a number of different ways
but most are formed when a plant or animal dies
in a watery environment and is buried in mud and
silt.
Phase 1 – Death The animal gets killed by a
predator whilst drinking at the edge of the river.
The skin, organs, and flesh are eaten by predators
and scavengers. The last of the soft tissue rots
away and the skeleton falls apart. The more
durable body parts like the bones and teeth
remain.
Phase 2 – Deposition Before the bones
disintegrate in the sun, the river rises and
deposits a layer of sand over the bones and teeth.
This burial eliminates oxygen and the action of
decomposing bacteria. Over time the sand and
mud layers build up, undergo compression, and
harden to form sedimentary rock.
Phase 3 – Mineralization Minerals in the bones
are dissolved away and replaced molecule by
molecule with minerals in the ground- it generally
takes up to 1 million years to completely turn a
bone to stone ie. before it becomes ‘petrified’.
Erosion of the land’s surface leads to the exposure
of fossils which are then found by
palaeontologists.
What information can fossils give?
Index Fossils
Index fossils are fossils that clearly indicate a specific
period of geological time. For a fossil group to be
useful as an index fossil it must: i) be found over a large
geographical distribution ii) have evolved fairly rapidly
When an index fossil is found in a layer of rock, it can
be used to establish the relative age of that layer of
rock.
Establishing Environment
Fossils are also good indicators of the type of
environment in which the rock layers formed. Rocks
that are found today in cold climates but containing
fossils of organisms that lived in shallow warm ocean
waters, such as corals, indicate a major change in
environment since the rock was formed. Also, marine
animals that live today at distinctive depths and in
particular areas, such as gastropods, are important to
work out the water depth that existed when the rock
containing that fossil was formed. This can help to
establish if an area has potential for bearing oil.
Establishing Lifestyle
In some cases, fossils are found preserved in their
actual "living position". This gives palaeontologists
much information about the plant or animal's way of
life and enables them to be compared to modern day
equivalents. However, the majority of fossils have been
reworked after death by ocean or river currents and
are no longer in their "living position".