Eighth Edition
by Harold L. Levin

Chapter 6 - page 8

Life on Earth: What do Fossils Reveal?

Fossils as Clues to Ancient Environments

Use of Fossils in Reconstructing Ancient Geography

Environmental limitations control the distribution of modern plants and animals.

Procedure:

1. Note locations of various fossil species of the same age on a regional map.
2. Interpret the paleoenvironment for each region (various marine, transitional, and continental environments). Depositional environments can be interpreted on the basis of rock types, sedimentary structures, and fossils.
3. Plot the environments to produce a paleogeographic map for that time interval.

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

Example: Plot locations of continental (non-marine) sedimentary rocks using locations of land-dwelling organisms such as dinosaurs or mastodons, fossilized tracks of land animals, and fossils of freshwater clams and land plants. Various non-marine environments can be recognized on the basis of rock types, sedimentary structures, and fossils:

1. Fluvial or river deposits and floodplains.
2. Aeolian environments or deserts.
3. Lacustrine environments or lakes.
4. Alluvial fans.
5. Swamps.

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

Land Bridges, Isolation and Migration

The migration and dispersal patterns of land animals can indicate the existence of:

• Former land connections or land bridges (such as the Bering Strait)
• Mountain barriers
• Former ocean barriers between continents (former isolation of South America from North America)

Migration of the camel family from North America to other continents.

Species Diversity and Geography

Species diversity is related to geographic location, particularly latitude.

High latitudes (polar regions) have low species diversity (few species), but may have high numbers of individuals.

Low latitudes (equatorial regions) have high species diversity (many species), but fewer individuals of each.

As a general rule, species diversity increases toward the equator.

Species diversity increases toward the equator.

Use of Fossils in the Interpretation of Ancient Climatic Conditions

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

1. Fossil spore and pollen grains can tell about the types of plants that lived, which is an indication of the paleoclimate.
2. Presence of corals indicates tropical climates
3. Plant fossils showing aerial roots, lack of yearly rings, and large wood cell structure indicate tropical climates
4. Marine molluscs (clams, snails, etc.) with spines and thick shells inhabit warm seas
5. Planktonic organisms vary in size and coiling direction according to temperature, for example the foraminifer Globorotalia
6. Compositions of the skeletons, for example shells in warmer waters have higher magnesium contents
7. Oxygen isotope ratios in shells. Oxygen-16 evaporates easier than oxygen-18 because it is lighter. O-16 falls as precipitation and gets locked up in glaciers, leaving sea water enriched in O-18 during glaciations. Shells that are enriched in O-18 indicate times of glaciation.