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The Fossil Record

Most of what we know about ancient populations comes from the records these animals left in the form of fossils. In the Natural Selection Unit, you learned about the importance of fossils in discovering evolutionary processes.  Fossils help us to learn about the populations of yesterday as well as today.  

Fossils reveal and help us understand:
  • Mass extinctions of species at several times in past,
  • Periods when many new forms appeared in a short time,
  • Family relationships of living species to one another,
  • Rapid environmental changes in the past,
  • Effects of human-caused changes to the earth's environment, and
  • Effects of environmental changes on biological diversity and ecological structure.


21 million year old fossil beaver burrow in Nebraska.

We can literally unearth information about species that lived millions of years ago. In layers of earth that were formed around 100-200 million years ago, scientists have found numerous fossils of various types of dinosaurs and fossils of many plant species that dinosaurs could have eaten. Therefore, we conclude that this was an Age of Dinosaurs. But rocks younger than about 65 million years ago contain no dinosaur fossils. Because there are no dinosaurs today and because there is no fossil record of them between now and 65 million years ago, we conclude that dinosaurs became extinct about that time. Click here for a neat Web site on fossils.

Have you ever heard the saying, “History repeats itself”?  Can we predict future extinctions and the future of our planet’s current ecosystems? That would be very valuable indeed! We do know that there have been massive Ice Ages and worldwide tropical conditions at various times - long before humans were around. Unfortunately, we do not understand weather well enough yet to know what caused these global changes nor how they might predict future weather for us.

Sources of Error in Fossil Records

An obvious source of error is that some species do not easily become fossils (bacteria, for example). The distribution may be uneven around the world and scientists may not have taken samples from the right places. The actual world-wide population may be greatly underestimated or overestimated.

Radioactive dating can also produce error. Recall that a fossil or the rock within which it is found can be dated by radioactive dating. These methods are very accurate, but a small error can mislead. If the measurements have an error of 1 percent, for example, an age determination of 100 million years could actually be wrong by a million years too low or too high. Age estimates are confirmed when several isotopes are tested in series. 

Radioactive elements used in dating of earth formations and fossils.

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