The term "index fossils" refers to varieties of fossils which can be used in relative dating to approximate the (unknown) time period of another fossil. Index fossils are characteristically widespread in terms of geographic space but exist during a rather narrow window of time. In this way, if one particular kind of fossil is found all over the planet but only during a certain time period, we can associate this fossil with that particular time...
The term "index fossils" refers to varieties of fossils which can be used in relative dating to approximate the (unknown) time period of another fossil. Index fossils are characteristically widespread in terms of geographic space but exist during a rather narrow window of time. In this way, if one particular kind of fossil is found all over the planet but only during a certain time period, we can associate this fossil with that particular time period. Let's imagine a scenario where paleontologists are digging in a particular area and they find two fossils-- one of which they know the age of, and one they do not. Using the first fossil as an index, the paleontologist could infer that the second fossil was formed during the same time period.
Ammonoids, also called ammonites, are the best known index fossils and are regularly used to provide approximate dates for other fossils. Because ammonoids were so widespread and underwent a variety of morphological changes, they can be reliably used to approximate dates for other fossils based upon already known variations through time. Ammonoids can be used to date from about the Devonian Period (419 million years ago) to the Cretacious Period (66 million years ago), when they disappeared along with numerous dinosaur species.
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