Introduction
It is important to understand the state of preservation of a specimen (either fossil or Holocene) to appreciate
how it came to be part of the fossil record. This activity reviews the concept of fossilization. Underlined terms
are those discussed in the class lectures. Examine the boxes labeled with different types of preservation for 20
minutes (parts I-III), then do part IV on the back of this sheet. 10 points total.
Types of Fossil Preservation:
Fossil remains are subject to the same chemical and physical processes as those that turn the enclosing sediments
into sedimentary rocks. Though most shells were originally composed of calcium carbonate and bones of
calcium phosphate, it is common to find that the original material has been altered in structure (recrystallization),
replaced by other minerals (replacement), had its pore spaces filled in by minerals (permineralization), or
completely dissolved away by solutions percolating through the rock, leaving only impressions and cavities
(molds or casts) where the hard skeleton had once been. In other cases, plant or animal remains decayed partially
during burial, leaving only black residue or a film of carbon or coaly material. Preservation of this type is
carbonization. Here is a summary of types of fossil preservation:
I. Preservation of unaltered soft parts
- by far the rarest types of preservation - soft parts as well as hard parts preserved because bacterial decay was in
some way prevented. Two examples are the following:
a) Remains of mammoths and rhinoceroses that were frozen in the tundra of Russia, or even humans preserved
in the peat moss of Scandinavia, or in the glaciers of Europe and South America.
b) Natural mummies formed by dry desert air. Examples: Desiccated and permineralized soft tissues of
dinosaurs (If you visit New York City you can see these at the American Museum of Natural History.)
c) Smaller organisms or parts of organisms (e.g., seeds, insects) originally caught in tree sap can be preserved in
amber. Fresh tree sap hardens, then over time polymerization and oxidation harden it further. Sap has some
antibacterial properties.Amber is good protection from oxygen but drying occurs.
II. Preservation of unaltered hard parts:
This type of preservation is common, particularly if the hard parts are made of chemically resistant material such
as silica or calcium phosphate, even in fossils half a billion years old. It is less common if the hard parts are more
reactive such as with CaCO3 (particularly aragonite), in which case only younger fossils are unaltered. Three
examples are the following:
a) Silica: Protozoans such as Radiolaria and Diatoms, and sponge spicules
b) Phosphatic parts: shells of certain brachiopods, skeletons of arthropods (crab-like and insect-like animals),
and bones of vertebrates (fish, dinosaurs, and mammals)
c) Calcium carbonate (CaCO3) parts: corals, mollusks (seashells, clams and snails)
III. Preservation of altered hard parts:
a) Permineralized: shells, bones and plants that are somewhat porous are made denser by deposition of mineral
substances by groundwater in the pore spaces. The microstructure of the organism may be finely preserved, as in
some examples of petrified wood, where the cellulose was replaced by minerals.
b) Replaced: replacement of the original minerals by one or more minerals of different composition.
c) Recrystallized: original mineral has been transformed into larger crystals of the same composition.
IV. Test of Your Knowledge: Unknown specimens
Examine the specimens with various types of preservation in the small cubic boxes. Draw at least one specimen
from each type of preservation and use each specimen only once. Under each drawing, write down the specific
criteria you used to arrive at your conclusions.
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