Fossils are defined as any evidence of prehistoric life. There are two types of fossils: Body Fossils include remains of skeletal bones, shell, carapace, test and teeth. Trace Fossils are clues the organism existed such as foot prints, tracks, burrows and coprolites (fossil dung).

The 5 conditions to become a fossil are as followed:

1. Death – if not dead the process is very painful!

2. Quick burial – to prevent scavenging and disassociation.

3. Low energy environment – must stay still to maintain shape and allow minerals and sediment infiltrate and or replace organic matterial.

4. Anoxic conditions – oxygen aids in decay, so it is not welcomed. 

5. Luck – most of the life forms on this Earth do not become fossils.

PROF. TOM HOBBS’ METHODS OF FOSSIL PRESERVATION

Note: Professor Tom Hobbs was my mentor and frankly the man who taught me Geology! 

1. Original Remains (unaltered preservation): the actual remains.

Soft parts – tissue, hair, feathers, cartilage and fluids.

• Amber – Jurassic Park tree resin (not sap) with insects

• Freezing in ice – glaciers – woolly mammoths

• Tar pits – La Brea, Los Angeles trapped animals

• Mummification (desiccation) – heat of the Ethiopia Desert or freeze dried by sublimation or burial caves of Greenland

• Anoxic (without oxygen) burial – peat bogs/moors

Amber from tree resin with insect inside

Hard parts – internal and external skeletal remains.

• Bones composed of Calcium Phosphate – Ca3(PO4)2

• Shells composed of Calcite (CaCO3), Aragonite (CaCO3) is mother of pearl and Silica (SiO2).

• Carapace composed Chitin, Protein (composed of the elements Carbon, Hydrogen, Oxygen and Nitrogen).

Carapace:

Fossil shrimp

2. Mold and Cast, Imprints:

Imprint – a shallow impression of usually a soft bodied organism.

Insect imprint

Mold – a cavity in rock of any organism, usually an exoskeleton.

Trilobite cast (left) and mold (right) in limestone 

Cast – replica of the original formed from minerals, clay, any filler.

Ammonite internal cast

3. Carbonization:

Compacting leaves or worms in shale; desiccating volatile liquids or gases (hydrogen, oxygen and nitrogen) materials leaving carbon imprinted.

The most common fossils are leaves, worms and conodonts.

Carbonized fern imprints

4. Replacement:

Occurs when the original skeleton is of a mineral (shell – calcite (CaCO3)) dissolve and is replaced by another mineral usually by pyrite (FeS2) or chalcedony (SiO2).

One of the most prized form of replacement is Pyritization. This is often seen in shelled invertebrates such as Brachiopods and occasionally Trilobites.

Pyritized brachiopod, original shell was aragonite

5. Recrystallization:

Occurs when hard parts revert to more stable minerals or a skeleton made of micro crystalline minerals recrystallizes during diagenesis to form larger grains.  This can be seen most commonly in calcite shells or crinoids.

Recrystalized ammonite

6. Permineralization:

Occurs when a porous skeleton is filled in with a mineral. Bones composed of Calcium Phosphate – Ca3(PO4)2 are saturated with chalcedony (SiO2). This is similar to a sponge filled with water to form ice).

Another explanation is a process of fossilization in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue.

Permineralized Mesozoic dinosaur bone

7. Petrification:

Occurs when soft organic material of an organism is replaced molecule by molecule with a mineral retaining the organic structure – wood pulp, most common minerals: silica, goethite, calcite, pyrite, and gypsum.

Petrified wood

8. Ichnofossils (trace fossils): marks left by organisms:

• Footprints, tracks and trails: These fossils can indicate speed and anatomy.

• Bioturbation – the mottled bedding in mud-usually made by worm burrows).

• Tooth marks: tooth marks on bone suggest predation. Tyrannosauus rex tooth marks have been found on Triceratops bones. The size of the bite, plus the fact that the only large carnivore in the environment is Tyrannosaurus rex, makes it a prime suspect.

• Gastroliths more commonly called gizzard stone: Like many modern birds some dinosaurs swallowed stones to help grind their food. Gastroliths are usually smooth, rounded and polished similar to river rocks.

Insect tracks in sandstone

Crustaean burrows (fill)

9. Coprolites: fecal fossils which reveal:

• Food sources of the environment as well as feeding patterns – herbivore, carnivore, omnivore.

• Size of organism and organic structures, degree of digestive track development.

• Age – correlation to pollen or other species included in the specimen.

• Note: Technically coprolites are trace fossil, but my late mentor Prof. Hobbs thought they were worth their own number.

Ammonite coprolite in limestone

10. Chemical fossils

Bio-signatures: are chemicals found in rocks and fossil fuels (petroleum, coal and natural gas) that provide an organic signature for ancient life. Molecular fossils and isotope ratios represent two types of chemical fossils.

Click here to see my Fossil Specimen Gallery.