Hunting for Fossils
How are fossil molds and casts made?
An LSU Museum of Natural Science presentation to
accompany the activity
What is a fossil?
• Fossil comes from the Latin fossilis
meaning ‘dug up’.
• They are defined as the remains,
molds, or traces of plants or animals
that lived a long time ago.
• Usually preserved in sedimentary
rocks such as sandstones, shale or
How are they formed?
• Phase 1- Death
– Something dies
• Phase 2- Deposition and Burial
– It may be washed into a river or lake, but is usually
quickly buried by sediments
• Phase 3- Replacement/Remineralization
– Minerals from the surrounding ground seep into
the organism slowly replacing the organic material
and virtually turning the bones into ‘rock’.
Preserved- Fossils that are unaltered and the original
organism stays relatively intact. Soft as well as
hard parts can be preserved. (Insect preserved in
hardened tree sap- amber)
Replacement- An organism’s hard parts are replaced with
minerals like calcite, silica, pyrite, or iron
Permineralization (petrification)- Fossil-type most people
recognize. After the original hard parts are buried,
water that seeps through the sediment passes
through the bone. The water often dissolves the
bone and replaces it with mineral deposits
ultimately changing it into stone.
Carbonization- Only carbon remains of the specimen-
other elements such as hydrogen and nitrogen are
Impression- These fossils are made by organisms that are
left in sediments.
Impression fossils are divided into 3
II. Mold and casts
III. Trace fossils
These are impressions
of thin organisms such
as feathers, leaves, or
fish that have fallen into
soft sediment before it
As the organism
decomposes it leaves a
carbon ‘film’ of the
organisms on the rock.
Molds and casts
When an organism dies it is covered with
sediment, decomposes slowly and when
completely decayed it leaves a cavity in the
rock that retains the exact shape and size of
When a cavity in rock is later filled with
sediments and these sediments take the
shape of the mold. It will look just like the
outside of the original organism. (Note:
Fossil casts are rare)
Two types of molds
Internal mold- imprints
of the inside of a fossil.
They are produced
when an organism such
as a shell, is filled with
sediment that becomes
cemented and then the
shell dissolves away.
External mold- imprints of the outside of a fossil. For
example, if the original fossil shell was convex, the
external mold will be concave.
Tracks or marks left by ancient organisms
that have been preserved in sedimentary
rocks. They show that the organisms were
in the area. They include:
– Tracks or footprints
– Trials or paths by a moving body
– Body imprints
Baylor, Byrd. (1980). If You are a Hunter of
Fossils. Illustrated by Peter Parnall. New
York: Charles Scribner's Sons. (ISBN 0-684-
Goldish, Meish. (1989). What is a Fossil?
Illustrated by Ivan Dieruf. Milwaukee:
Raintree Publishers. (ISBN 0-8172-3535-3).