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Records of Life: Fossils and their geologic strata


Much of the history of life has been recorded in fossils- which are themselves a kind of happy coincidence, literally the accidental casts or reliefs of ancient plants and animals. Silt, clay, fine-grained sand, and dissolved minerals such as silica or iron often  gradually replace the actual once-living parts of buried organisms in casts formed by surrounding dirt. In time the dirt becomes sedimentary rock, containing the fossils.

These occur either as  positive or negative casts- the former, such as a shark’s tooth or a 
dinosaur leg bone (see fiigure of Iguanadon at right); the latter, such as the cast of a mollusk shell or the primitive, sponge-like Dickinsonia from 650 million years ago (seen below), leaving in the rock a negative relief or impression (fossa in Latin, hence the term “fossil”). 

For more information on paleontology, fossils, and related geology, see the online issue of Athena Review, "Records of Life."
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