|
|
|
© Michael
Collier
|
|
This is a fossil of a fish that is common
in the sedimentary rocks that were deposited in Cenozoic lakes
in Utah.
|
|
Sediments are home for many kinds of marine animals. Some animals live
on the surface of the sediment, and some burrow into it. Some fossil shells
are found mixed with the mud they lived in. Other fossil shells were moved
by strong currents and deposited along with sand or even gravel. If the
shells are buried by more sediment before they are worn away or dissolved,
they become fossilized.
|
|
|
© Michael
Collier
|
|
This fossil clam was found in the Green
River Formation near Utah's Roan Cliffs.
|
|
Sometimes a fossil consists of original shell material. This is common
in very young sediments that have not yet been turned into rock. Older
sediments usually have been buried deeply by later sediments and turned
into rock. Then it is more likely that the original shell has been dissolved
away by water seeping through the pore spaces in the sediment. The fossil
is left as an imprint of the original shell. An imprint like that is called
a mold. Sometimes the space that was occupied by the shell is now empty.
In other cases that space has been filled with later minerals that were
precipitated by the flowing pore water. That material, which has the shape
of the original shell, is called a cast.
Clams have shells that are in two parts, called valves. The valves are
hinged along one edge. They are left and right, like your hands when you
put them together along your little fingers. The clam can open its shell
to feed and close its shell for protection. Think about what you might
see when the clam shell is fossilized. Each valve has an outer surface
and an inner surface. Depending on which valve you are seeing, and whether
you are seeing the inside or the outside of it, and whether you are seeing
its cast or its mold, eight different views are possible! Paleontologists
have to be very careful to match up the fossils they see. Otherwise, they
might think they are seeing fossils of several different kinds of animal
rather than just one.
Back To Top
|
