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hunting for fossils

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
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 limestone.
types of fossils
Types of fossils
  • Plant remains
  • Animal remains
  • Coal, oil, gas (aka fossil fuels)
  • Trace fossils (including borings, footprints, molds, casts, coprolites)
how are they formed
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’.
fossil types
Fossil types

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 removed.

Impression- These fossils are made by organisms that are left in sediments.


Impression fossils are divided into 3 categories:

  • Imprints
  • Mold and casts
  • Trace fossils

These are impressions of thin organisms such as feathers, leaves, or fish that have fallen into soft sediment before it hardened.

As the organism decomposes it leaves a carbon ‘film’ of the organisms on the rock.

molds and casts
Molds and casts
  • Mold
    • 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 the organism.
  • Cast
    • 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
Two types of molds

External mold- imprints of the outside of a fossil. For example, if the original fossil shell was convex, the external mold will be concave.

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.

trace fossils
Trace fossils

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
  • Burrows
  • Coprolites
  • Gastroliths
  • Body imprints
reference materials
Reference materials

Baylor, Byrd. (1980). If You are a Hunter of Fossils. Illustrated by Peter Parnall. New York: Charles Scribner's Sons. (ISBN 0-684-16419-1).

Goldish, Meish. (1989). What is a Fossil? Illustrated by Ivan Dieruf. Milwaukee: Raintree Publishers. (ISBN 0-8172-3535-3).