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Fossils
8th Grade Science
What is a Fossil?
 The remains, or traces, of
organisms that lived long
ago. A fossil can be a
bone, footprint, shell, etc.
I. Fossils in Rocks
 Fossils are usually found in
sedimentary rocks
 Most fossils were formed
underwater and were
buried very quickly.
 Take millions of years to
form.
 Soft parts decay.
 Only hard parts like
bone and shells are left.
 Sediments harden into
rock.
II. Molds and Casts
 Molds are a hollow impression of
a once living thing found in a
rock.
 Form when an organism is
buried by sediments and the
sediments change to rock.
 The organism decays and
leaves a cavity, or opening,
in the rock.
 The cavity is the mold.
 Casts occur when a mold is filled
with sediments or minerals that
harden or become a rock.
III. Imprints or Trace Fossils
 Usually made from the soft
parts of an organism.
 There are many fish and
leaf imprints.
 Footprints, nests,
burrows and tracks, are
also imprints.
 These can tell us about the
animal’s speed or stride,
whether they walked on 2
or 4 legs, if they were lone
stalkers or roamed in herds.
IV. Petrification or Permineralization
 Occurs when plant or
animal tissue is replaced
by minerals.
 Carbonization or
coalification is when the
tissue is replaced by
carbon only.
 Mineral replacement is
when tissue is replaced
with other types of
minerals.
V. True Form or Whole Organisms
 True Form fossils are fossils of the
actual animal, plant, or body
part. There are three substances
that contribute to the formation
of True Form Fossils.
 Fossils in Ice
 Freezing preserves things by
preventing decay.
 Animals such as mammoths
have been found totally
preserved in ice.
 Fossils in Tar
 Tar pits were often covered
with water, so when animals
came to drink they became
stuck in the tar.
 Example: Saber tooth tigers.
Whole Organisms Continued…
 Fossils in Amber
 Hardened tree sap is
called amber.
 Insects become
trapped and are later
found preserved.