Earth image Fossils and Geologic Time

Dr. Pamela Gore
Georgia Perimeter College


  1. Discuss what a fossil is.
  2. Discuss the ways in which fossils form.
  3. Distinguish between relative time and absolute time.
  4. Identify the eras of geologic time.
  5. Discuss how fossils are dated.
  6. Discuss the types of life forms that existed during the different eras.
  7. Discuss how fossils indicate ancient climates.
  8. Discuss how fossils indicate different sedimentary environments, and show evidence of changing sea level.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5f. Describe how fossils show evidence of the changing surface and climate of the Earth.
  • S7L5c. Trace evidence that the fossil record found in sedimentary rock provides evidence of the long history of changing life forms.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Thousands of layers of sedimentary rock confirm the long history of the changing surface of the Earth and the changing life forms whose remains are found in successive layers. The youngest layers are not always found on top, because of folding, breaking, and uplift of layers.
  • Many thousands of layers of sedimentary rock provide evidence for the long history of the earth and for the long history of changing life forms whose remains are found in the rocks. More recently deposited rock layers are more likely to contain fossils resembling existing species.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Fossils provide important evidence of how life and environmental conditions have changed.
  • Extinction of a species occurs when the environment changes and the adaptive characteristics of a species are insufficient to allow its survival. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; most of the species that have lived on the earth no longer exist.
  • Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.

I. What is a fossil?

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 burrowsócalled trace fossils).

Paleontology is the science that studies fossils.

II. How do fossils form?

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 of 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.

How are fossils preserved?
A. 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, such as skeletons of echinoderms and foraminifera

  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. 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. Silica, such as the skeletons of diatoms and radiolarians, and some types of sponges.

  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.
B. 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.)
C. 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.

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 that becomes cemented, and then the shell is dissolved away. Internal molds are sometimes called steinkerns.

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.)
D. 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). (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), or in peat bogs.

E. 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.)

III. What is the difference between relative dating and absolute dating?

The science that deals with determining the ages of rocks is called geochronology.

There are two basic methods of dating rocks:

  1. Relative dating
    Using fundamental principles of geology (Principle of Superposition, Fossil Succession, etc.) to determine which rocks are older and which are younger. "A is older than B".
  2. Absolute dating
    Quantifying the date in years. This is done primarily by radiometric dating (or analysis of the breakdown of radioactive elements in the rocks over time).

Relative Dating

It was recognized by Nicholaus Steno in the 1600's, that in a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top. This has come to be called the Principle of Superposition. You can visualize how this occurs if you imagine a stack of newspapers in the corner of a room. Every day you put another newspaper on the pile. After several weeks have passed, you have a considerable stack of newspapers, and the oldest ones will be on the bottom of the pile and the most recent ones will be on the top. This is fairly obvious, but it is a very important fact which is useful in relative dating, and is the first of three principles which have come to be known as Steno's Laws.

Note: Sedimentary rocks in many areas are flatlying and superposition is easy to note. But in mountain belts where the rocks are often complexly folded, careful observation is necessary to determine whether the rocks may have been overturned by folding. When you approach a sequence of beds which has been tectonically deformed, before you can determine which beds are younger and which are older, it is first necessary to determine the "up direction". This is done by examining the sedimentary structures for clues. Sedimentary structures such as graded beds, cross beds, mudcracks, flute marks, symmetrical (but not asymmetrical) ripples, stromatolites, burrows, tracks, and others can be used to establish the original orientation of the beds. (Fossils can also be used to establish up direction, if they are present in the rock in life position.) You should examine carefully the sedimentary structures in any dipping sedimentary sequence, because the rocks can be overturned by tectonic forces, and what initially appears to be younger because it is on top, may in fact turn out to be at the bottom of the section!

Sedimentary structures which may be used to interpret "up-direction".
The images on the left are right-side up. The images on the right are upside down.
Graded beds(top) , crossbedding and mudcracks(middle), stromatolites, symmetrical ripples, and burrows(bottom).

Note: For more information on sedimentary structures see this reference.

Steno's second law is the Principle of Original Horizontality, which states that sediments are deposited in flat, horizontal layers. We can recognize this easily if we consider a sedimentary environment such as the sea floor or the bottom of a lake. Any storm or flood bringing sediment to these environments will deposit it in a flat layer on the bottom because of the sedimentary particles settling under the influence of gravity. As a result, a flat, horizontal layer of sediment will be deposited.

Steno's third law is the Principle of Original Lateral Continuity. If we consider again the sediment being deposited on the seafloor, the sediment will not only be deposited in a flat layer, it will be a layer that extends for a considerable distance in all directions. In other words, the layer is laterally continuous.

William Smith, an English surveyor and author of the first geologic map, (late 1700's) discovered that certain rock units could be identified by the assemblages of fossils they contained. This knowledge led to the "Principle of Biologic Succession" (or "Principle of Fossil Succession"), which states that fossils occur in a consistent vertical order in sedimentary rocks all over the world. Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.

Fossil species appear and disappear throughout the stratigraphic record. The Geologic Time Scale is based on these appearances and disappearances. Each of the Eras ends with a mass extinction event in which many species went extinct. For example, at the end of the Paleozoic Era, more than 90% of all marine species disappeared. Period boundaries coincide with smaller extinction events, followed by appearances of new species, evolving from the surviving taxa.

Absolute Dating or Radiometric Dating

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

Radioactive parent elements decay to stable daughter elements.

Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

The oldest materials in the solar system (moon rocks and meteorites) are about 4.5 to 4.6 billion years old. Zircon grains as old as 4.4 billion years old have been found on the Earth, in sandstones deposited in ancient rivers in Australia. The scientifically-accepted age of the Earth is 4.5 to 4.6 billion years old.

Note to teachers: More detailed explanation of radiometric dating requires the use of chemistry, in particular such concepts as atomic number, atomic mass, isotopes, and recognition of chemical symbols for radioactive elements and their stable daughter products. Since Earth Science is now being taught in the 6th grade, and the basics of chemistry are not taught until 8th grade Physical Science, the chemical explanations cannot be covered as part of this course.
For more details see: Reference 1, Reference 2.

IV. What are the Eras of Geologic Time?

The geologic time scale has been determined bit-by-bit over the years through relative dating, correlation, examination of fossils, and radiometric dating.

Eon = Largest division of time scale.

The Eons, in order from oldest to youngest, are Archean, Proterozoic, and Phanerozoic. The Archean and the Proterozoic are sometimes grouped into the a unit called the Precambrian. The Precambrian consists of approximately 87% of geologic time. This is the time before abundant life with hard parts such as shells.

The term Phanerozoic means "visible life", and it refers to the time when many animals had hard parts such as shells and bones, which are readily preserved as fossils.

Era = A major division of geologic time, divisible into geologic periods.

The Phanerozoic Eon is divided into three eras. In order from oldest to youngest, the eras are as follows:

  1. Paleozoic Era- "ancient life" (such as trilobites)
  2. Mesozoic Era - "middle life" (such as dinosaurs)
  3. Cenozoic Era - "recent life" (such as diverse mammals)

Geologic Time Scale

Eon Era Period Date
(Millions of years
before present)
Neogene   End of Cenozoic = 0 m.y.
Cretaceous   End of Mesozoic = 65.5 m.y.
Permian   End of Paleozoic = 251 m.y.
Part of the
      End of Precambrian = 542 m.y.
Part of the

V. How are fossils dated?

Fossils can be used to recognize the approximate age of a unit and its place in the stratigraphic column. They can also be used to correlate strata from place to place.

The geologic range of a fossil species is defined as the interval between its first occurrence and last occurrence in the geologic record. The geologic range of a species is determined by recording the occurrence of the fossils in numerous stratigraphic sequences from hundreds of locations. Ranges are well known for some species, and poorly known for others.

Appearances and disappearances of fossils may indicate:

Index fossils or guide fossils are fossils which are useful in identifying the age of rock units, and in correlation of rock units from place to place . To be an index fossil, a fossil must be:

Fossils allow us to date where a rock unit lies within the geologic column. In order to put a numerical date (or absolute date) on a fossil or a rock, we must use other scientific techniques.

Fossils and sedimentary rocks are not generally datable radiometrically, unless you have a unique rock type or mineral that formed through precipitation in the depositional basin, and had datable elements, such as potassium. An example would be a rock with the mineral glauconite in it. It contains potassium, which you can date, and it forms at the time of deposition.

Some rocks in sedimentary sequences are datable. A good example of a datable rock layer in a sedimentary sequence would be a layer of volcanic ash. The volcanic ash would be relatively easy to date radiometrically (but not with C-14). Perhaps potassium-argon dating would be used. Another example of a datable layer would be a lava flow.

You know that the fossils in the rocks ABOVE the ash bed are younger than the ash, and the fossils in the rocks BELOW the ash bed are older than the ash. That is what we call relative dating using the Principal of Superposition.

Ideally you need several ash beds (or lava flows) in a sequence, so that you can BRACKET the age of the fossils. (Older than X million years, but younger than Y million years. The X and the Y would be the dates obtained from two ash beds, one above the fossil and one below the fossil.)

So you need to look for the particular fossil species in rocks in MANY localities around the world and find the dates of any ash beds, lava flows, or glauconite-bearing rocks that might be associated with the fossil. This often requires collaboration from colleagues in distant areas, or it requires lots of travel and painstaking collection of fossils in the field.

You have to establish the geologic range of the fossil, correlate it from place to place, and then you have to find a locality where there is a datable bed stratigraphically at or near the first appearance of the fossil, and a datable bed stratigraphically at or near the last appearance of the fossil.

Over time, enough data are assembled that it can be stated that the age of a certain fossil ranges from X million years to Y million years. Some fossil species have very long ranges, and aren't very useful for dating. Other fossil species have short geologic ranges (in other words, they evolved rapidly), and will be more useful. So the geologic range of a particular fossil species is determined.

Once you get some dates on the fossil (X to Y million years), you can correlate it with fossils of the same species in other areas. Everywhere you find THAT PARTICULAR SPECIES you would know the date was X to Y million years.

Sometimes there are not datable beds for particular species, so the geologic range of each species is carefully determined in terms of location in the stratigraphic section in many localities. Groups of species that occur together, with similar first and last appearance datums might be called an "assemblage". Stratigraphic "zones" (or biozones) are determined based on particular species or assemblages of species. And then these stratigraphic zones can be correlated or traced from locality to locality. Hopefully at some point they can be dated radiometrically. But in the meantime, they are named for a characteristic species. Groups of zones are used to establish larger and larger intervals, which are placed in their relative vertical order, and correlated from place to place.

In this way we get geologic stages, series, and systems, which give their names to geologic epochs and periods (the TIME during which the fossil species lived). So you can use these large biostratigraphic units and trace them from place to place. And if you are lucky, some of them will get their boundaries pinpointed radiometrically.

So basically, you trace the limits of each fossil species in the stratigraphic section over a large area (such as the Appalachians), or globally. The you determine which system, series, or stage it belongs to. All based on fossil ranges. We hardly ever do radiometric dates. We just tie in to the biostratigraphic framework.

If you can date some of the systems, series, or stages (by finding an ash bed or a lava flow or a glauconite unit), then you can put a date on the fossils, or a date on the system, series, or stage. At this point in time, a number of dates have been pinpointed (from the occasional datable bed) and "golden spikes" have been driven into the rock for system or series boundaries. Once you get that date, it can be extended to other correlative outcrops and fossils.

VI. What types of life forms existed during the different eons and eras?

  1. Archean Eon of the Precambrian
    The earliest evidence of life is present in Archean sedimentary rocks. Archean fossil evidence for life consists of stromatolites, cyanobacteria or blue green algae, algal filament fossils, and spheroidal bacterial structures. Oldest direct evidence of life is in rocks 3.5 billion years old.

  2. Proterozoic Eon of the Precambrian
    Larger cells with nuclei and organelles are found in Proterozoic rocks, by 1.6 to 1.4 billion years ago. Fossils of soft-bodied multicellular animals appeared in the Late Proterozoic about 630 million years ago (0.63 billion years ago). They are preserved as impressions in sandstone. Some resemble imprints of jellyfish in beach sand, but of a more primitive type than the jellyfish you find washed up on beaches today. There are also trace fossils, such as trails and burrows, and small fossils with hard parts or shells (like the tubes of tube-dwelling worms, and structures resembling sponge spicules.

  3. Paleozoic Era
    First abundant life in the seas with hard parts
    • Trilobites
    • Tabulate Corals
    • Rugose Corals
    • Abundant brachiopods
    • Bryozoans
    • Crinoids
    First land plants (first spore-bearing then seed-bearing), first trees
    First insects
    First fish. Fish become very diverse.
    First amphibians. Some amphibians are very large.
    First reptiles
    Coal swamps with large trees and many types of ferns.
    MASS EXTINCTION EVENT AT THE END OF THE PALEOZOIC affecting trilobites, tabulate and rugose corals, and many but not all brachiopods and crinoid (among other groups).

    Paleozoic fossils. Trilobites. Genus Phacops.

    Paleozoic fossils. Crinoid.

    Paleozoic fossils. Brachiopods.

  4. Mesozoic Era
    Age of dinosaurs. Dinosaurs evolve from reptiles and go extinct at end of era.
    Flying reptiles.
    Large marine reptiles (mosasaurs, plesiosaurs, etc.)
    First mammals. Mammals and dinosaurs evolve around the same time. Late Triassic. Mammals remain small.
    First birds
    First flowering plants. Angiosperms.
    Modern corals appear
    Ammonites common and used as index fossils.
    Modern types of crabs appear.
    MASS EXTINCTION EVENT AT THE END OF THE MESOZOIC affecting dinosaurs, marine reptiles, large land mammals, ammonites, and marine planktonic organisms (among other groups).

    Mesozoic fossils. Dinosaurs - Allosaurus and Stegosaurus
    Denver Museum of Nature and Science

    Mesozoic fossil. Flower.

    Mesozoic fossil. Mosasaur from Greene County, Alabama.

  5. Cenozoic Era
    Age of Mammals. Mammals began to diversify (flying mammals = bats, swimming mammals = whales), and become larger.
    Many types of grazing mammals.
    Mastodons and Wooly Mammoths.
    Grasslands expand.
    Lots of small non-woody plants with seeds. Huge flightless birds
    Modern types of fish, shellfish, insects, mammals, reptiles, amphibians, plants, etc.
    Humans appear.

Cenozoic fossil. Insect.

Cenozoic fossils. Fish. Wyoming.

Evolution of the horse during the Cenozoic Era. Kentucky International Horse Park. Left to right: Pliohippus (10 mya), Merychippus (25 mya), Mesohippus (40 mya), Hyracotherium (55 mya).

Cenozoic fossil. Pecten shell, a type of scallop.

VII. How do fossils indicate ancient climates?

Fossils can be used to interpret paleoclimates or ancient climates, for example:

VIII. How do fossils indicate different sedimentary environments, and show evidence of changing sea level?

Environmental limitations control the distribution of modern plants and animals. For example, it is relatively easy to distinguish between marine fossils (molluscs, corals, brachiopods, trilobites, barnacles, fish, animals with flippers) and non-marine fossils (plants, animals with walking legs, such as insects or mammals). In addition, tracks made by animals with walking legs (such as dinosaur tracks) indicate a non-marine environment.

When marine fossils are found in areas that are now dry land, it is possible to conclude that the area was once covered by the sea. Detailed examination of the fossils and associated sedimentary rocks may show a detailed history of sealevel rises and falls over time.

You can plot on a map the locations of non-marine (terrestrial) deposits using locations of land-dwelling organisms such as dinosaurs or mastodons, fossilized tracks of land animals, and fossils of land plants, to outline the basics of the paleogeography or ancient geography.

Modern coral reefs occur in the tropics, within 30o north and south of the equator. Ancient coral reefs likely had similar distributions.

Mixtures of marine and non-marine fossils may indicate a stream entering the sea, or a delta.

The migration and dispersal patterns of land animals can indicate the existence of " land bridges" or former connections between now-separated areas.

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 22, 2005
Additional information added May 29, 2005
Image links updated June 29, 2009