Laboratory 11
Fossil Preservation
Laboratory

Pamela J. W. Gore

Department of Geology, Georgia Perimeter College

Clarkston, GA 30021

Copyright © 1982-2006 Pamela J. W. Gore

WHAT ARE FOSSILS?

Fossils are the prehistoric remains or traces of life which have been preserved by natural causes in the Earth's crust. Fossils include both the remains of organisms (such as bones or shells), and the traces of organisms (such as tracks, trails, and burrows - called trace fossils). Many fossil species still have living representatives; in other words, fossils do not all represent extinct organisms.

 

HOW ARE ORGANISMS PRESERVED AS FOSSILS?

Most organisms that lived in the past left no record of their existence. Fossil preservation is a rare occurrence. To become preserved as a fossil, an organism must:

  1. Have preservable parts. Hard parts (bones, shells, teeth, wood) have a much better chance at being preserved than do soft parts (muscle, skin, internal organs).

  2. Be buried by sediment. Burial protects the organism from decay.

  3. Escape physical, chemical, and biological destruction after burial. The remains of organisms could be destroyed by burrowing (bioturbation), dissolution, metamorphism, or erosion.

 

Organisms do not all have an equal chance of being preserved. The organism must live in a suitable environment. In general, marine and transitional (shoreline) environments are more favorable for fossil preservation than are continental environments, because the rate of sediment deposition tends to be higher.

 

TYPES OF FOSSIL PRESERVATION

The remains of organisms may be fossilized in a variety of ways, including preservation of unaltered hard parts, chemical alteration of hard parts, imprints of hard parts in the sediment, markings in the sediment made by the activities of organisms, and the rare preservation of unaltered soft parts. Each of these types will be discussed below.

Some fossils are preserved in more than one way. For instance, an aragonitic coral may be replaced by silica, or recrystallized to calcite, but at the same time, it may also have its original pore spaces filled by permineralization. Similarly, a bone with original material also may have pore spaces filled by permineralization. As another example, fossil plants may be carbonized, but they may also leave external molds in the sediment.

 

I. PRESERVATION OF UNALTERED HARD PARTS (original material)

The shells of invertebrates and single-celled organisms, or vertebrate bones and teeth may be preserved unaltered. The different compositions of original material are detailed below.

    1. Hard parts made of calcite, such as echinoderms and foraminifera may be preserved unaltered.

    2. Aragonite shells of clams, snails, or scleractinian corals may be preserved unaltered in Cenozoic deposits, but they are generally dissolved or recrystallized in older deposits. This is because aragonite is more soluble than calcite, and because aragonite is metastable, and in time recrystallizes to calcite.


      3. Aragonitic mollusc shells in the Clinchfield Limestone,
      Georgia Coastal Plain, near Perry, GA

    3. Hard parts made of phosphate, such as the bones and teeth of vertebrates, conodonts, and the outer covering of trilobites. The shiny scales of fossil fish are phosphatic.


      4. Sharks teeth.
      First image is from display at the
      University of Georgia, Marine Extension Service,
      Skidaway Island, GA

    4. Hard parts made of silica, such as the skeletons of diatoms and radiolarians, and some types of sponges, may be preserved unaltered in some deposits.

    5. Organic hard parts, made of resistant materials such as chitin, cellulose, keratin, sporopollenin, or collagen are present in some groups of organisms. Many arthropods, including the insects, have chitinous skeletons (an organic material similar in composition to our fingernails). Plant hard parts (wood) are composed of cellulose.

  

II. CHEMICAL ALTERATION OF HARD PARTS

The hard parts of many fossil organisms have been chemically altered by the addition, removal, or rearrangement of chemical constituents.

    1. Permineralization is the filling of pores (tiny holes) in wood, shell, or bone by the deposition of minerals from solution. The added mineral matter makes the permineralized fossil much heavier than the original material. Petrified wood is a common example of permineralization.


      2. Petrified Logs from Petrified Forest.
      Photographed at the Smithsonian Institution
      National Museum of Natural History,
      Washington, D.C.

    2. Replacement is the molecule-by-molecule substitution of another mineral of different composition for the original material. The fine details of shell structures are generally preserved. Minerals which commonly replace hard parts are silica and pyrite. Look for fossils which should be calcareous (crinoids, molluscs, brachiopods, corals), but which don't fizz in acid.


      3. Pyritized brachiopods (Mucrospirifer) in limestone, Sylvania, Ohio.

    3. Recrystallization. Many modern shells are made of aragonite. Aragonite is a metastable form of calcium carbonate (CaCO3). With time, the aragonite will alter or recrystallize to calcite, a stable form of CaCO3. Paleozoic shells which fizz in acid are probably recrystallized from the original aragonite to calcite (except for echinoderms which are originally calcite).

    4. Carbonization (or distillation) preserves plants or animals as a thin carbon film, usually in fine-grained sediments (shales). Fine details of the organisms may be preserved. Plant fossils, such as ferns, in shale generally are preserved by carbonization. Soft-bodies animals may also be preserved as carbonaceous films in black shales. (Example: Cambrian Burgess Shale fauna.)


      5. Examples of carbonization.
      First image is carbonized leaf from the Eocene Green River Formation, Wyoming
      Second image is from the Cambrian Burgess Shale, Canadian Rockies

 

III. IMPRINTS OF HARD PARTS IN SEDIMENT

Many fossils are simply imprints with no shell material present at all. Hard parts are commonly destroyed by decay or dissolution after burial, but may leave a record of their former presence in the surrounding sediment.

  1. Impressions or molds are the imprints of an organism (or part of an organism) in the sediment. A shell buried in sandstone may be leached or dissolved by groundwater, leaving a mold of the shell in the surrounding sandstone. There are two types of molds:

    1. External molds are imprints of the outside of a shell in the rock. If the original shell was convex, the external mold will be concave.


      2. External mold of a bivalve in limestone from the North Carolina Coastal Plain.

    2. Internal molds are imprints of the inside of the shell in the rock. Look for such features as muscle scars which are present on the inside of bivalve shells. Internal molds are produced when a shell is filled with sediment which becomes cemented, and then the shell is dissolved away. Internal molds are sometimes called steinkerns.


      3. Internal mold of a bivale in limestone from the North Carolina coasatal plain.


Internal molds
First image is internal mold of Turritella gastropods, Eocene, Aquia Creek, Virginia
Second image is internal mold of pelecypod or bivalve

 

(An artificial external mold may be made by pressing the outside of a shell into modelling clay. An artificial internal mold may be produced by filling a shell with modelling clay.)

A cast may be produced if a mold is filled with sediment or mineral matter. A cast is a replica of the original. Casts are relatively uncommon. (A rubber mold of a fossil can be filled with modelling clay to produce a replica or artificial cast of the original object.)

IV. PRESERVATION OF UNALTERED SOFT PARTS

In rare circumstances, the soft parts of an animal may be preserved. The two most common methods of soft part preservation are freezing and desiccation (drying or mummification). (Example: Pleistocene wooly mammoths frozen in Siberia and Alaska.) Soft parts of organisms such as insects or small frogs may be preserved if the organism becomes trapped in pine resin (later altering to amber). Larger animals may become trapped in oily, tar-like asphalt (example: mammals preserved in the LaBrea tar pits in Los Angeles, California).

      
Insect preserved in amber.   Amber


Preserved wooly mammoth skin and hair
Smithsonian Institution, National Museum of Natural History

 

V. TRACE FOSSILS OR ICHNOFOSSILS

Trace fossils are markings in the sediment made by the activities of organisms. They result from the movement of organisms across the sediment surface, or the tunneling of organisms into the sediment, or the ingestion and excretion of sedimentary materials. The study of trace fossils is called ichnology. (See Lab 4, Sedimentary Structures for additional information.)

 Trace fossils provide geologists with much useful information about ancient water depths, paleocurrents, availability of food, and sediment deposition rates. In many cases, tracks of animals are the only record of their existence. For example, in many places, dinosaur tracks are much more abundant than dinosaur bones. During its lifetime, a single dinosaur makes millions of tracks, but leaves only one skeleton, which may or may not be preserved.

A. Tracks (footprints) are produced by the feet of a walking animal. A continuous series of tracks made by one animal is called a trackway. From a study of tracks, the geologist can determine the leg length and height of an animal, whether it walked on two legs (bipedal) or four (quadrupedal), its speed and whether it was running or walking, whether it was carnivorous (large claws), herbivorous (hooved), or aquatic (webbed feet).


Dinosaur tracks
Dakota Group, Dinosaur Ridge near Denver, Colorado

B. Trails are produced as a worm or arthropod crawls, or as the tail or belly of an animal drags the ground. Trails or crawling traces are usually linear and indicate movement in a particular direction. Some trails are more meandering in appearance and probably represent grazing traces as an invertebrate systematically combed an area of sediment for food. There are also resting traces produced by animals such as trilobites. Most trails are produced by organisms with bilateral (rather than radial) symmetry, with a well defined head (or anterior) and tail (or posterior) region.


Trails
First image is Climactichnites, 505 million years old, Late Cambrian, New York
Second image is of trails in red siltstone, Triassic, Culpeper Basin, Virginia

C. Burrows are the excavations of an animal made into soft sediment. Burrows are probably used as feeding and/or dwelling structures. Continued burrowing or bioturbation of the sediment will destroy primary sedimentary structures, and result in a massive, homogeneous, structureless rock.


Burrows in Deep River Basin. Late Triassic, North Carolina


Skolithos worm burrows in quartzite. Cambrian Weverton Quartzite, Harpers Formation, or Antietam Formation.
Stream cobbles found in Henson Creek, Prince Georges County, Maryland.
Scale in centimeters and inches. Image courtesy of A. O'Neil.

D. Borings are holes made by an animal into shells, rock, wood, or hard sediment. Borings are usually circular. Some snails produce borings or drill holes into other molluscs, such as clams. Bivalves may bore into wood, rock, or other hard materials. Sponges also produce borings, often riddling shells with numerous tiny holes.


This bivalve shell has a circular, countersunk hole, drilled by a predatory gastropod.

E. Coprolites are the fossilized excrement of animals. Coprolites may contain fragments of undigested food, and thus provide valuable information about the feeding habits of fossil organisms. Coprolites smaller than 1 mm are called fecal pellets. Fecal pellets may be extraordinarily abundant in some environments, and are the dominant allochem in pelleted limestones. Some fecal pellets have a high phosphate content.


Phosphate-rich coprolite from the Triassic Durham Basin of North Carolina. Note rhombohedral fish scale embedded in coprolite near left end. Scale in millimeters.


Coprolites (Note: These specimens are rich in iron, and may be excrement-shaped ironstone concretions or pseudofossils, rather than true coprolites.)

F. Gastroliths are the highly polished stones from the gizzards of birds, or the stomachs of reptiles (including dinosaurs). Gastroliths or gizzard stones were probably used to grind food in the stomach of the animal.

G. Root marks are trace fossils produced by plants. They bifurcate or branch downward primarily into terrestrial sediments or soils (which may be red in color), and may be surrounded by green or gray zones where chemical reduction of iron has occurred. In some areas, roots may also be associated with caliche or sedimentary iron stones.


Root marks in red siltstone,
Late Triassic, Deep River Basin, North Carolina

 

HOW LIKELY IS IT FOR AN ORGANISM TO BECOME PRESERVED AS A FOSSIL?

There are about 1.5 million known species of living plants and animals. In all, there may be as many as 4.5 million living species. In contrast, there are only about 250,000 known fossil species. The fossil record covers many hundreds of millions of years, and the living flora and fauna represent only one "instant" in geologic time. Thus, you might expect the number of fossil species to far outnumber the number of living species, if fossil preservation were a relatively common event. The fact that the number of fossil species is so small suggests that the preservation of organisms as fossils is extremely rare. It has been estimated that fewer than 10% of the animal species living today are likely to be preserved as fossils.

 

WHY IS PRESERVATION SO RARE?

One reason that preservation is an uncommon event has to do with the environments which species inhabit. For example, the vast majority of living species are insects. (Of the 1.5 million known species, approximately 1 million are insects). Insects are rarely preserved as fossils because they generally live on dry land, and are unlikely to be buried by sediment after death. If an organism is not rapidly buried after death, chances are it will be rapidly broken down by scavengers and bacterial decay.

Some groups of organisms which inhabit soft sediment in marine environments, such as molluscs, are much more likely to be preserved as fossils. In fact, for some groups of animals (brachiopods and cephalopods, for example), there are more fossil species than living species. This is particularly true for organisms which were much more abundant in the past, and for groups which have suffered extinctions of many of their species.

Another reason that so few species are represented by fossils is that many organisms are soft-bodied and lack hard parts. In order for soft parts of organisms to be preserved, it is necessary to isolate them from oxygen almost immediately after death. This is most likely to occur when organisms are rapidly buried in fine-grained sediment in anoxic water, but this only happens in rare, isolated environments. There are a few spectacular examples of this rare type of preservation, including the Cambrian Burgess Shale of Canada. Soft-bodied forms are also preserved in the Late Precambrian Ediacara fauna of Australia. Feathers of the earliest birds, Archaeopteryx, are preserved in the Jurassic Solnhofen Limestone of Germany.

 

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This page created by Pamela J. W. Gore, Pamela.Gore@gpc.edu
Georgia Perimeter College

July 20, 1998
Modified December 23, 1998
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