Biostratigraphy is a method of relative dating that utilizes the presence of fossil organisms. It is based upon the idea that a particular time interval can be characterized by a distinctive assemblage, or group, of organisms. For example, if one knows that dinosaurs lived from 228 to 65 Ma, then any rock containing a dinosaur fragment must fall within that age range. Although biostratigraphy cannot provide ages in years before present, the fact that many species of organisms have existed on Earth for 1-2 million-year intervals enables them to be
used as powerful dating tools. For example, Tyrannosaurus rex lived for only about 2 million years, from 67 to 65 Ma. Therefore if we found a T. rex fossil (a good find, indeed), we would know that, no matter where that dinosaur was found, it could be correlated with T. rex-bearing sediments in North America that have been well dated at 67-65 Ma.
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