index fossil, guide fossil(noun)
a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found
Any ancient remains of a plant or animal that lived during a specific geological period and that can be used to date the containing rocks.
Index fossils are fossils used to define and identify geologic periods. They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived, then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor. Ammonites fit these demands well, and are the best-known fossils that have been widely used for this. Other important groups that provide index fossils are the corals, graptolites, brachiopods, trilobites, and echinoids. Conodonts may be identified by experts using light microscopy such that they can be used to index a given sample with good resolution. Fossilized teeth of mammals have also been used.
The numerical value of index fossil in Chaldean Numerology is: 9
The numerical value of index fossil in Pythagorean Numerology is: 1
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"index fossil." Definitions.net. STANDS4 LLC, 2017. Web. 28 May 2017. <http://www.definitions.net/definition/index fossil>.