GEOLOGY

Detailed explanation-1: -Index fossils are fossils that are widespread but only existed for a short period of time. Index fossils help scientists to find the relative age of a rock layer and match it up with other rock layers. Living fossils are organisms that haven’t changed much in millions of years and are still alive today.

Detailed explanation-2: -An ideal index fossil lived during a short period of time, was geographically and environmentally widespread, and is easy to identify. Some of the most useful index fossils are hard-shelled organisms that were once part of the marine plankton.

Detailed explanation-3: -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.

Detailed explanation-4: -Why are the best index fossils from species that only existed for a short period of time? Because once they were very common they disappeared after the mass extinction event.

Detailed explanation-5: -Best index fossils are common, widely distributed, and easy to identify. Some of the most common index fossils are ammonites, trilobites, and graptolites.