Eighth Edition
by Harold L. Levin

Chapter 6 - page 1

Life on Earth: What do Fossils Reveal?

Introduction

Fossils are the remains or traces of ancient 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).

Paleontology is the science that studies fossils.

Fossil Preservation

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 scavengers and 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 environments are more favorable for fossil preservation than are continental environments, because the rate of sediment deposition tends to be higher.

Preservation of Unaltered Hard Parts (original material)

The shells of invertebrates and single-celled organisms, or vertebrate bones and teeth may be preserved unaltered. These hard parts may have the following compositions:

1. Calcite (skeletons of echinoderms and foraminifera)

Echinoid from the Cretaceous of Mississippi.
 

Miocene foraminifera made of calcite.
2. Aragonite (shells of clams, snails, or scleractinian corals)

   Aragonite shells may be preserved unaltered in Cenozoic Era deposits, but aragonite is 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.


Aragonite clam shell.
Photo courtesy of Pamela Gore.

3. Phosphate (bones and teeth of vertebrates, conodonts, the outer covering of trilobites, and scales of fossil fish)

Conodont fossils.
4. Silica (skeletons of diatoms and radiolarians, and some types of sponges)

Diatoms. Their skeletons are made of silica.
5. Organic material (insects, spores, pollen, wood, fur, or hair)

   Organic hard parts are made of resistant materials such as chitin, cellulose, keratin, sporopollenin, or collagen. 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. The plant reproductive structures, spores and pollen, are made of sporopollenin.

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 bone or shell by the deposition of minerals from solution. The added mineral matter makes the permineralized fossil much heavier than the original material.
2. Replacement is the molecule-by-molecule substitution of another mineral of different composition for the original material. The fine details of shell structures and wood are generally preserved. Minerals which commonly replace hard parts are silica and pyrite.

Petrified Logs from Petrified Forest. Photographed at the Smithsonian Institution National Museum of Natural History, Washington, D.C. Photo courtesy of Pamela Gore.
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 preserves soft tissues of 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-bodied animals such as jellyfish or worms may also be preserved as carbonaceous films in black shales.
   Example: Cambrian Burgess Shale fauna.


Carbonized fossil of a seed fern.


Carbonized remains of a fish from the Green River Formation, Wyoming.

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. 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.
   
   Internal mold of a gastropod or snail.

2. 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.)

A mold (below) and cast (above) of a trilobite.

Preservation of Unaltered Soft Parts

In rare circumstances, the soft parts of an animal may be preserved. Two common methods of soft part preservation are freezing and desiccation (drying or mummification).

Ice Age wooly mammoths from the Pleistocene Epoch have been found frozen in Siberia and Alaska. Skin, hair, and soft tissue have been preserved in frozen soil.

Soft parts of organisms such as insects or small frogs are preserved if the organism becomes trapped in pine resin (later altering to amber).

Insect preserved in amber. Photo courtesy of Pamela Gore.

Larger mammals may become trapped in tar oozing from natural oil seeps, such as the LaBrea tar pits in Los Angeles, California.

Occasionally, human remains have been preserved in peat bogs in places like England, such as the 2000-year-old Lindow Man.

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.

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.

Trace fossils include tracks, trails, burrows, and borings.

Dinosaur tracks, Morrison Formation, Dinosaur Ridge near Denver, Colorado.
Photo courtesy of Pamela Gore.


Trails in red siltstone, Triassic of Virginia.
Photo courtesy of Pamela Gore.


Burrows, probably made by worms.

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Traces that reflect animal behavior.
A = crawling traces
B = resting traces
C = dwelling traces
D = grazing traces
E = feeding traces