The Fossil Record

I. The Record of Life on Earth

The organisms which inhabited the Earth are sometimes preserved in the rock as fossils.

The study of fossils is called paleontology.

In addition, the presence of organisms can be recorded through marks they leave in sediment as a result of their activities:

Geologists call these features trace fossils or ichnofossils.
The study of trace fossils is called ichnology.

A. What types of life exist (or have existed)?

We group organisms based on their similarities into taxonomic groups or taxa.


There are six kingdoms of organisms:

  1. Animalia (animals)
  2. Plantae (plants)
  3. Monera (bacteria and blue-green algae)
  4. Fungi (mushrooms, fungus)
  5. Protoctista - formerly Protista (single-celled organisms)
  6. Eubacteria (normal type environments bacteria and cyanobacteria)
  7. Archaebacteria (extreme type bacteria methane, sulfur bacteria)

All organisms are composed of cells.

There is a fundamental difference between organisms based on the type of cells:

  1. Cells with a nucleus (or nuclei) - Eukaryotic cells
  2. Cells without a nucleus - Prokaryotic cells - Kingdom Archaebacteria and Eubacteria only. Even though these are generally single celled organisms, recently there have been found to be predators which are bacteria: Bdellovibrio

Revisions of this classification scheme have been proposed, based on evolutionary relationships determined through the study of molecular structures and sequences. In one revision recently proposed, organisms would be grouped into three superkingdoms or domains:

  1. Bacteria (bacteria)
  2. Archaea (sometimes called archaebacteria (perhaps incorrectly) - as different from the bacteria as the eukaryotes are from the prokaryotes)
  3. Eucarya (animals, plants, fungi, and protists)

The Archaea have been recently discovered, and live under extreme conditions (such as in high temperatures, or in naturally acidic waters) where life was not thought be able to exist. Such severe conditions are considered to be like those encountered on the Earth during its earliest stages.

How do organisms get preserved as fossils?

Most fossils are the hard parts of organisms:

Soft parts are rarely preserved:

To be preserved, an organism must be in an environment where it is protected from oxidation and bacterial decay.

  1. Must be rapidly buried by sediment
  2. Must be shielded from oxygen (anaerobic or anoxic conditions)

Some rocks are made up almost entirely of an accumulation of organic remains or shells or other hard parts of organisms (coquina, fossiliferous limestone, chalk, diatomite, coal).

C. In which environments are fossils most readily preserved?

Environments covered by water.
Particularly environments with a high sedimentation rate or anoxic waters.

Examples: swamps, deep lakes, tar pits, oxygen-minimum zone in the oceans.

What are the major animal groups?

Animals can be grouped into the invertebrates (animals without backbones) and the vertebrates.

There are more than 20 invertebrate Phyla, but the chief ones that are preserved as fossils include:

  1. Phylum Porifera - the sponges
  2. Phylum Cnidaria (formerly Coelenterata) - the corals and jellyfish
  3. Phylum Bryozoa - the colonial moss animals
  4. Phylum Brachiopoda - the brachiopods or lamp shells
  5. Phylum Arthropoda - the insects, crabs, shrimp, lobsters, trilobites and eurypterids
  6. Phylum Mollusca - the clams, snails, octopus, squid, nautilus, and ammonites
  7. Phylum Echinodermata - the starfish, sand dollars, sea urchins, crinoids, and blastoids

The vertebrates belong to Phylum Chordata (referring to the nerve chord that extends down the center of the spine).

What can these fossils tell us about the past?

The study of the interaction of ancient organisms with their environment is called paleoecology. In large part, paleoecology depends on comparisons of ancient organisms with living organisms. We use modern analogs to help us interpret something about the way in which the fossils lived and related to their environment.

Fundamental concepts of ecology and paleoecology: Ecosystem - the organisms and their environment - the entire system of physical, chemical, and biological factors influencing organisms

Habitat - the environment in which the organism lives

Community - the association of several species of organisms in a particular habitat (the living part of the ecosystem)

Niche - the way in which the organism lives - its role or lifestyle.

Where do organisms live?

Each species has particular environmental requirements that can be described in terms of limiting factors such as:

Some organisms live on the land, and some live in the water.

  1. Land-dwellers are called terrestrial organisms
  2. Water-dwellers are called aquatic organisms
    1. Marine (inhabit saline sea water) - salinity of sea water is about 34 - 36 parts per thousand total dissolved solids, or about 3.5% salt.
    2. Non-marine (inhabit freshwater) - salinity of freshwater is about 1 part per thousand total dissolved solids, or about 0.1% salt. Includes rivers, freshwater lakes, springs, caves, wells, groundwater.
    3. Brackish (inhabit water of intermediate salinity) - brackish water is a mixture of fresh water and sea water, and may be found in bays, deltas, lagoons, estuaries, harbors, etc.
    4. Hypersaline - water of very high salinity, such as the Great Salt Lake, Dead Sea, and some tropical bays and lagoons with high evaporation rates.

Marine organisms inhabit the pelagic realm - all of the water overlying the ocean floor. It can be divided into:

Terrestrial organisms may inhabit one of the following environments:

  1. Lacustrine - lakes and ponds
  2. Fluvial - rivers and streams
  3. Paludal - swamps
  4. Eolian (or aeolian) - deserts

Modes of life

Mode of life consist of:

  1. nutrition
  2. locomotion
  3. reproduction

Within the neritic zone, animals may have varying modes of life:

Bottom dwellers capable of movement are called vagile or vagrant.
Bottom dwellers which do not move are called sessile.


Methods of obtaining nutrients:

  1. Primary Producers or autotrophs - produce their own food through photosynthesis, and supply food and energy for other organisms. Examples?
  2. Consumers or heterotrophs - cannot produce their own food and must eat.

Predation - one species eating another.

    1. Herbivores - heterotrophs that eat plants
    2. Carnivores - heterotrophs that eat herbivores and other carnivores
    3. Other feeding modes which may fit into the above categories:
      1. Parasites - derive nutrition from other organisms without killing them
      2. Scavengers - derive nutrition from dead organisms
      3. Suspension feeders - filter small food particles from the water
      4. Deposit feeders or detritus feeders - ingest sediment and extract small food particles
      5. Grazers - scrape food from the substrate
      6. Decomposers and Transformers - bacteria which break down organic matter converting it into a form which can be utilized by other organisms (nutrients)

The food chain or food web