Index, noun; a pointer, indicator, a thing that points out. (Webster’s Dictionary)
by Gordon Howard

Evolutionary paleontologists use ‘index fossils’ to assign an age to a layer of sedimentary rock and its associated fossils.

Evolutionary theory assumes that a particular creature evolved from its ancestors, lived successfully for a period, then became extinct as its descendants evolved better ways of surviving. In other words, that creature had a defined ‘evolutionary life-span’. We may be told, “It thrived in the Devonian period”. For example, we all ‘know’ that the dinosaurs ‘evolved’ about 230 million years ago, and died out 65 million years ago, don’t we?

Or do we? To ‘know’ that, people need to make two assumptions.

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