LESSON
PLANS
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What
is a Fossil?
Everything
we know about dinosaurs comes from the study of the fossilized
remains of the animals and the environment (plants and other
animals) in which they lived. Just what is a fossil and how
does a dinosaur bone turn into one? Fossil is from the Latin
word meaning "to dig", and it was originally applied
to any artifact or bone that was dug out of the ground. Gradually,
the word came to mean the preserved remains of a plant or
animal. Fossils are created over a long period of time. They
are most commonly found in limestone, sandstone and shale.
An organism
can also be fossilized by carbonization: the original tissues
turn to carbon but retain the form of the creature or plant;
tree trunks turned to coal have carbonized.
Fossils
can be classified into four types, according to the way they
were fossilized. True fossils, truly rare, preserve the actual
animal or animal parts. More common, cast fossils turn to
rock when minerals replace the organic remains – the
original material is often lost, but a near exact replica
remains. Sometimes the original material dissolves in ground
water, leaving only an impression or mould of the original
life-form. Trace fossils are not remains of the creatures
themselves, but the marks, structures or signs left behind
while the creature lived (e.g., nests, footprints). An insect
or plant preserved in amber or another form of hardened tree
sap is not a fossil.
Fossils
of any type are rare. Most have hard parts that hold their
shape long enough for the fossilization process to take its
course – a very long time. For a plant or animal to
become a fossil, it must be lucky enough to have been covered
by sediment, usually under water, and escape decay, destruction
by a scavenging animal and erosion.
But this
is only half the story, because millions of years later another
set of events must come to pass. The surrounding layers of
rock must slowly erode away until, with some luck, someone
comes along and recognizes the remains of a living creature.
Then a fossil has to be dug out of the ground very carefully
and documented properly (location, date, etc.). As you can
imagine, the chances of all these things occurring are very
small.
EXTENSION
ACTIVITY
LEVEL:
All grades
OBJECTIVE:
In this exercise students will better understand how mould
and cast fossils occur.
EXERCISE:
Students can make their own cast fossils using plaster of
Paris and objects such as shells, bone or even their own hand
or footprint.
MATERIALS:
Modelling clay, plaster of Paris, water, a bowl or bucket
to mix plaster of Paris in, shells, plastic or soft rubber
toy animals (such as crabs, or even toy dinosaurs), and vegetable
oil (optional).
PROCEDURE:
1. Use a piece of modelling clay approximately the size of
the object you wish to fossilize.
2. Flatten the clay out and press your object into it using
firm, even pressure. You might want to apply a small amount
of vegetable oil to the impression surface so that it does
not stick to the object.
3. Remove the object, leaving an impression. This is the mould.
4. Prepare some plaster of Paris according to the package
directions so that it is thick and smooth.
5. Spoon plaster into each impression thoroughly filling it
in.
Let the plaster dry completely (about 30 to 60 minutes), then
peel away the clay to reveal your cast fossil.
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