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Types of fossils Paleontologists find many
kinds of fossils.
 Water
often seeps into the pores of buried plant or animal material. Minerals
in the water may slowly replace the original parts of the dead creature
or plant. This is called a mineralized fossil. The mineralized fossil
shown here is a poplar leaf that's 25 million years old.
 Often
even the hard parts of a creature or plant rot away after being buried.
If the sediment around them has already turned to stone, however, this
leaves a hole, called a mold, in the rock. The mold may later fill up
with sediment. When the sediment hardens, it forms a cast fossil like
the trilobite shown here.
 Some
ancient insects are preserved well enough for us to study them in
detail. This insect became trapped in the sticky sap of a pine tree
hundreds of thousands of years ago. The sap hardened around the insect,
forming the piece of amber in which we can see the insect today.
 The
fossilized signs of animals, such as footprints, droppings, and tracks,
are called trace fossils. Footprints are impressions, formed when the
sediment containing the print turned to stone. Impressions may also form
when the soft parts of a plant or animal decay before the sediment
around them hardens to rock.
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