GEOLOGY

Detailed explanation-1: -Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area.

Detailed explanation-2: -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.

Detailed explanation-3: -Index fossils are the remains of plants and animals that lived for a limited period. Best index fossils are common, widely distributed, and easy to identify. Some of the most common index fossils are ammonites, trilobites, and graptolites.

Detailed explanation-4: -Answer and Explanation: Index fossils are used to provide a relative date for sedimentary rock strata, in which the fossils are found.

Detailed explanation-5: -index fossil, any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment. A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time.