Environmental Changes Over Time

Environmental Changes Over Time

Changing Environmental Conditions Over Time

If there is any one thing the fossil record reveals to us, it is that there has been considerable
environmental change over time. Fossils reveal to us more than an image of the remains of a past plant
or animal, it also gives us an idea of the environment in which it lived. To delve deeper into this, we have
to remember that "the present is the key to the past" (Principle of Uniformitarianism).

Principle of Uniformitarianism

We are unable to go into the past to observe what made a rock layer the way it is or determine exactly
how a dinosaur appeared. It is a fact that there are many things beyond the reach of science. So,
what we as scientist do is the next best thing. We look at natural processes that occur in the present.
For instance, when we dig a trench across a sand dune, we find a pattern to how the layers of sand
build up one over another. When we go to an ancient bed of sandstone, such as the Coconino
Sandstone in north central Arizona, and find the same pattern, we believe the same processes that
occurred then as occur now. The very basic assumption is that nature behaves exactly the same in
the past as it does now. This is where we get the phrase: The present is the key to the past.
Click here for additional information on the Principle of Uniformitarianism.

Coconino Sandstone showing
cross-bedding as it would
appear in a sand dune.

Fossilized Sand Dune

The Coconino Sandstone was
believed to be the remains of a
desert that existed over 200
million years ago. It stretched
from Canada to Mexico and was
as large as the Sahara.

Major Types of Environments we see Today

When we study ecology, we look at several major types of environments found on the earth today. We label
these environments by the types of living things we find there. These mixes of plants and animals are
mainly the result of temperature and moisture. In warm climates with abundant moisture, we end up with a
jungle-type landscape. If we take away the moisture, we end up with desert. We call these biomes.
Click here to learn more about Biomes.

Although the most dramatic fossils are those of land creatures such as the dinosaurs and the mastodon,
the largest variety were ocean dwelling creatures. There are also different environments found there. For
this page, it is best to think of them as shallow water vs. deep water dwellers; floaters and swimmers vs.
bottom crawlers; and beach dwellers. Some required the still waters found in protected bays, others
needed the wide oceans to float in.

Evidence of Past Life

The fossilized remains of past life are found in rocks from around the world. The most ancient are the very
old remains of cell-like spheroids and rod-shaped forms found in the Fig Tree Chert of South Africa. These
date back to 3.5 billion years ago. More recent fossils of around 2 billion years old are found in North
America. The surface environment of the Earth was very different then than it is now.

Some of our most recent are found in volcanic ash, like the eruption of Pompeii.

Credit: Trevor George, with permission.
www.fossils.demon.co.uk

Fossil Shrimp

Fossil cephalapods: The long straight fossils are straight-shelled
orthocerids, the circular are ammonites. This rock is most likely
Silurian-Devonian from England.