WHAT IS A FOSSIL AND HOW DO FOSSILS FORM?

DR. PAUL F. CIESIELSKI
DEPARTMENT OF GEOLOGICAL SCIENCES
UNIVERSITY OF FLORIDA

I. WHAT IS A FOSSIL?

Any evidence, direct or indirect, of the existence of organisms in prehistoric time.
derivation- the latin word "fossilis, meaning dug out or dug up.

II. TYPES OF FOSSILS

A. Unaltered soft parts (leaves, jellyfish, tentacles, etc.,) or hard parts (teeth, bones, shells)

1. Unaltered Soft Part Preservation

a. in peat bogs

a variety of fossils preserved by rapid burial in this anaerobic environment

b. in amber

amber is the pitch of conifer trees
Types of fossils found in amber

insects are commonly found in amber (see below)

This is a 95 million year old fossil insect in amber from the Raritan formation of New Jersey (This very bug may have bitten a dinosaur). This is an example of an organism preserved as UNALTERED ORIGINAL MATERIAL. From: The Amber Room: Amber of NJ. Click on the image to go to that web site.

frogs and salamanders, etc.

c. in tar pits

e.g. Rancho la Brea in Los Angeles
a variety of Pleistocene birds, mammals, and reptiles

d. mummification or desiccation

preservation from driving off water from tissues
mostly in arid regions

e. refrigeration- most found in Siberia and Alaska

frozen tissue
woolly mammoths
rhinoceroses

2. Unaltered Hard Part Preservation

a. calcium carbonate (CaCO₃) and aragonite

e.g. molluscs, gastropods, etc.

Solitary Coral. Well preserved, UNALTERED ORIGINAL SHELL MATERIAL. Corals are composed of aragonite, which tends to recrystallize to calcite, and usually only young fossils such as this (Pleistocene, from Miami) will be unaltered (G. Mead).

b. calcium phosphate (apatite)

e.g. sharks teeth and ray teeth

Above: Ray teeth and (top left, next to penny), Stingray spines and possible (3rd spine from left) fish spine. Below: Shark teeth. Both the Ray and Shark teeth and spines are UNALTERED MATERIAL, possibly somewhat PERMINERALIZED. All samples shown here were collected in Hogtown Creek, Gainesville (G. Mead). Ray teeth and (top left, next to penny), Stingray spines and possible (3rd spine from left) fish spine. Below: Shark teeth. Both the Ray and Shark teeth and spines are UNALTERED MATERIAL, possibly somewhat PERMINERALIZED. All samples shown here were collected in Hogtown Creek, Gainesville, Florida. (G. Mead).

Shark teeth. Both the Ray and Shark teeth and spines are UNALTERED MATERIAL, possibly somewhat PERMINERALIZED. All samples shown here were collected in Hogtown Creek, Gainesville, Florida. (G. Mead).

c. silica (SiO₂ * H₂0)

e.g. sponges, diatoms, radiolarians

d. organic walled

e.g. pollen, spores, dinoflagellates

B. Altered soft or hard parts

1. recrystallization- process by which the more unstable original skeletal material (e.g. aragonite and/or Mg calcite) is recrystallized to a more stable form of the mineral (e.g. calcite) or larger crystals of the same mineral.

During crystallization, the external form is unchanged but internal microstructures are obscured or destroyed.
The process usually involves the solution of the mineral and reprecipitation of a more stable form of the mineral.
This process is enhanced with time, burial pressure, and increasing temperature.
e.g. a brachiopod made of fibrous calcite recrystallized to non-fibrous interlocking calcite grains.

2. replacement- process of removal of the original skeletal material and simultaneous atom by atom secondary replacement of another mineral.

May involve ionic substitution of magnesium or iron for calcite.
The original microstructure is usually preserved!

calcite (CaCO₃) to silica (SiO₂)
silica (SiO₂) to calcite (CaCO₃)
calcite to iron sulfide (FeS_{2 or pyrite}), gypsum (CaSO₄* H₂O), dolomite [CaMg(CO₃)₂], silver (Ag) and copper (Cu) (to name a few)

Above: PYRITIZED sea urchin (left, above penny) and Brachiopod (right). G. Mead

A common and dramatic type of fossilization is PYRITIZATION, where the original material of the skeleton or sometimes soft tissue becomes REPLACED or coated with pyrite during fossilization. During decomposition of tissue, if all available oxygen is used up, some kinds of bacteria can reduce sulfate to sulfide to produce energy (instead of oxidizing organic carbon, the usual means of producing energy), which combines with iron to produce pyrite. This process produces some of the beautiful
fossils seen below.

Below: Exogyra, a Cretaceous to early Cenozoic oyster. The top two are coated with pyrite. The bottom two Exogyras were collected along Interstate 10 in western Texas, and are ORIGINAL, UNALTERED SHELL (G. Mead).

3. permineralization- process by which porous spaces in the shell, wood, or bone are filled with minerals. Permineralization may occur to fossils which were previously unaltered, replaced or recrystallized. The minerals filling the void spaces are carried in solution within groundwater moving through the sediment/ sedimentary rocks the fossils are buried in. Permineralized or petrified word is most common in sediments also containing volcanic ash because the ash provides a source of silica to permineralize the wood. The wood below is from the Chinle Formation (Triassic) which had a significant amount of volcanic ash originally deposited with the wood in floodplain deposits.

most dinosaur bones and fossil wood is permineralized

Petrified wood (permineralized) from the Upper Triassic Chile Formation of Ghost Ranch, N.M. (P. Ciesielski). Silica from weathered volcanic ash has filled the pore spaces of the wood.

Petrified Wood, purchased near Petrified Forest National Park, in Arizona. Preserved by PERMINERALIZATION. Note that you can see individual wood cells and rings very easily (G. Mead).

4. carbonization- process by which the soft tissue is preserved as a carbon film by volatilization of the hydrogen, oxygen, and nitrogen.

a. carbon films usually have a black, light brown, or dark brown color

b. most carbon films are leaves, crustaceans, or fish

Carbon film of fossil leaf from the Eocene lake deposits of Colorado (P. Ciesielski)

IMPRESSION and possible CARBON FILM. IMPRESSIONS in Green River Formation rocks from Colorado, collected by Dr. P.F. Ciesielski.

C. Molds and casts

1. mold- a natural cavity left by the outside or interior surface of a fossil which has been dissolved.

external mold- impression of the outside of the shell showing only the external detail of the fossil. External molds are negative in relief.

External mold of a gastropod (marine snail) from a Cretaceous concretion in Antarctica (P. Ciesielski)
internal mold- a filling of the interior of the shell. Forms by sediment filling the interior of the shell followed by dissolution of the shell. An internal mold will show internal features of the shell and has positive relief.

Internal mold of a pelecypod (bivalve) from the Cretaceous of Antarctica (P. Ciesielski)

Bottom: Straight-coned Ammonites. All 3 of them have lost their outer shell, and what you see is primarily the filling of the chambers (INTERNAL MOLDS) and the septae (shell divisions) between the chambers. The large one on the top has ORIGINAL SHELL MATERIAL( internal mold) still in the septae, while the ones on the bottom have been REPLACED by pyrite.

Coiled ammonite, collected in Texas. Top: off of I-10 in west Texas, at the same locality as the Exogyra samples shown above. Interestingly, I found the top piece but couldn't collect the rest because I didn't have the proper tools. I happened to be back a year later, with tools, and collected the rest at that point. Note that the rate of erosion in this area seems to be about 1 mm per year, as you can see by the amount of rock missing from between the two pieces. The preservation here is primarily as an EXTERNAL MOLD (G. Mead).

2. cast- a replica of the original form. Formed by dissolving of the shell and filling of the former interior and shell area with sediment or minerals (filling of external mold). A cast is the same size as the original shell and has positive relief.

D. Trace Fossils

Tracks
Trails
Burrows

This image shows a cast of a Dinosaur footprint from Dinosaur State Park, in Connecticut, along with a couple of illustrative cards from the same park. This kind of fossil, evidence of the activity or behavior rather than the actual body, is known as a TRACE FOSSIL (G. Mead).

E. Other items produced by organisms

Coprolites- solid excretory products of animals
Gastroliths- stomach stones

III. FACTORS ENHANCING PRESERVATION

rapid burial
deposition in an anoxic (oxygen poor) environment
encasement in sediment which inhibits dissolution

IV. FACTORS INHIBITING PRESERVATION

a. mechanical processes of weathering and transportation

b. chemical attack

oxidation
dissolution

c. microbial activity

d. scavenging animals

V. WHERE FOSSILS CAN BEST BE FOUND

a. Areas of erosion

badlands topography
hillsides
tallus piles at the base of cliffs

b. Man made exposures

road-cuts
limestone quarries
hydraulic mining sites

c. Areas of animal activity

ant hills
animal burrows