ANATOMY

Detailed explanation-1: -index fossil: A fossil that can be used to determine the age of the strata in which it is found and to help correlate between rock units.

Detailed explanation-2: -Index fossils (also known as guide fossils or indicator fossils) are fossils used to define and identify geologic periods (or faunal stages). Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends.

Detailed explanation-3: -Certain fossils, such as ammonites, trilobites and graptolites, are known as index fossils. Because they were so abundant and widespread during a short period of geological time, their presence helps date rocks from that period.

Detailed explanation-4: -Index fossils are fossils that can be used to date the rock in which they are found. The best examples are fossils of animals or plants that lived for a very short period of time and were found in a lot of places. Ammonites, shelled relatives of today’s octopus, make ideal index fossils.

Detailed explanation-5: -Index fossils are used to define geological periods. These fossils can be defined as “commonly found, widely distributed fossils that are limited in time span.” If one finds an index fossil in a given layer, then one has bounds on the age of the layer. Using index fossils, the geological periods are defined.