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What is a Fossil? - Page 1 of 2
By Ed Strauss, Washington

How do we know that the Earth is evolving? How do we know that it used to be very different than today? How do we know how long ago things changed?

These are general questions that can be answered with one general answer. We know from the study of geology and fossils (paleontology).

A fossil is defined as : The remains of a once living organism, generally one that lived prior to the last glacial period (e.g. 10,000 years before present). They include skeletons, tracks, impressions, trails, borings and casts. Fossils are usually found in hard rock, but not always (e.g. amber). (emphasis added) Allaby, M., editor, 1992, The Concise Oxford Dictionary of Botany; Oxford University Press.

It is not known who the first human was to discover a fossil or what they thought of it. They could well be a fossil themselves by now. It is known that a Greek named Xenophanes from the 6th century BC noted laurel leaf impressions. Scientists from the Renaissance such as DaVinci also mentioned leaf fossils. In the 19th century scientists from Europe and America were reporting discoveries of dinosaur skeletons. When Darwin’s theory of evolution was published and later widely accepted it all fit together. Fossils are the physical record of the history of evolution. A history of the changes that have taken place in the plant and animal kingdoms.

Fossil skeletons, impressions, and casts tell us what used to exist and tracks, trails, and borings tell us a little about how they existed. A fossil skeleton is not hard to understand, it is mineralized bone. Impressions are similar to pressing a leaf in a book to dry and preserve it. A cast is a little different.

A cast is defined as : The secondary rock or mineral that fills a natural mold, producing a replica of a fossil. (emphasis added) Bates, R. L., Jackson, J. A., editors, (prepared by the American Geological Institute), 1976, Dictionary of Geological Terms, third edition; Anchor Books (Doubleday).

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