Geology Reference
In-Depth Information
Against All Odds: The Fossilization of
Lifeforms
In order to answer questions about past organisms and their evolutionary relationship
to modern plants and animals, scientists examine fossils preserved in the geologic re-
cord.
Fossils
may be actual remains, such as bones, or traces of past behaviors, such as
footprints or tracks.
Bones, teeth, and shell: Body fossils
Body fossils
are preserved body parts. Body fossils can be preserved in different ways. In
some cases the body parts remain unchanged and are simply preserved as they existed.
These are called
unaltered body fossils.
Unaltered body fossils are rare and may result from freezing or mummification. In both
cases the skin and soft tissue, as well as the hard body parts (such as bone), can be pre-
served. Another method of unaltered body fossil preservation is entrapment in a thick
fluid, such as tar or tree sap (which hardens into amber).
More commonly, fossils in the geologic record are
altered
body fossils. In this case, the
remains may be changed through a chemical process, preserving the shape of the body
part but not its original composition.
Altered body fossils are preserved in three basic ways:
Replacement:
Replacement
fossilization occurs when remains of an organism (usu-
ally the hard parts, such as shell or bone) are buried in sediments, dissolved, and
replaced by new minerals.
Permineralization:
Preservation by
permineralization
occurs when minerals seep
into the open spaces of buried remains, such as bones or wood, but leave some of
the original organic material in place.
Carbonization:
Some organic materials, such as leaves or insects, can be pre-
served as
carbon film.
This preservation results when all that remains of the ori-
ginal organism is a thin film of carbon preserving its shape.
