Page 33 - The Restless Earth Fossils

32 Fossils

A collector can section a completely mineralized piece of
wood with a rock saw and see even microscopic cellular detail.
Bones have a similar porous structure that makes them perfect for
permineralization. The phosphate compounds that make up bone
may change composition somewhat by incorporating chlorine
and/or fluorine atoms rather than being completely replaced.
Iron sulfides may replace calcium carbonate shells or other
organic materials buried in low-oxygen, ocean-bottom mud, in a
process aided by bacteria, to produce fool’s gold, or pyrite. Such
fossils truly look like creatures dipped in gold.

casts, Molds, and other variations on
the theMe

Not all fossils become exact replicas of their living model. In
the tree example, the sand and mud around the trunk may have
hardened to stone, but all the organic material of the tree ulti-
mately decayed completely. This left a hollow cylinder, or mold,
that displayed the exact shape and outer detail of the tree trunk.
If this natural mold then filled with minerals or more mud before
breaking apart or eroding away, a cast of the tree would remain
behind. The cast would have the shape and outer texture of the
tree but inside it could be pure agate, a form of silicon dioxide.
This kind of petrified fossil is also called a pseudomorph, or
“false form.”
Shells also make good fossils because they are hard and resis-
tant to change. Animals create their shells out of a form of cal-
cium carbonate called aragonite. Aragonite often recrystallizes
into the more stable form of calcium carbonate called calcite. At
other times, shells end up as casts or molds or both.
Some popular fossils called trilobites are the casts, molds, and
petrified remains of arthropods whose shells were composed of
chitin, like modern crabs and insects. A person finding a mold of
a trilobite shell can easily make a cast of the animal by pressing
some clay into the mold. Because trilobites and other arthropods
grow larger by shedding old shells and growing new ones after
their bodies take up water and expand, each animal has the

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