Natural Selection Image Map

Fossils

Ever been to the petrified forest in Arizona's national park?  If you have, you would have seen whole forests that have become fossils. Some of these fossils are so well formed that you can determine the age of trees the same way we do on a tree that we cut down. The number of rings around the trunk tell us how old the tree is.

How are fossils formed?

If trees or other plants or animals get buried under sediment before they decompose, ground water can dissolve silica in the soil above and carry it into the dead plant or animal. The silica will crystallize in the dead plant or animal and stay trapped there. Thus, the detailed structures are revealed and preserved. When rivers cut through the sediment and wash it away, fossils become uncovered and you can find them.

So how do fossils relate to Natural Selection? If nothing alive today resembles a fossil we assume that the species is extinct.  Fossils show us that many species (perhaps as many as 90% of all species) no longer live. Why did they become extinct? Something must have forced them out of existence.  Forces of nature seem to be the obvious answer. These species might have

  • lost their niches
  • failed to survive when niches changed
  • lost out to competing or predator species
  • died from disease or some natural disaster


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