* Index fossils are fossils of organisms that:
* Lived for a short period of time.
* Were geographically widespread.
* Had easily recognizable features.
* How they work:
* If an index fossil is found in a rock layer, we know that rock layer formed during the time the index fossil lived.
* If other fossils are found in the same rock layer, we can infer that those fossils also lived during the same time period.
* Key point: Index fossils don't tell us the exact age of the rock layer in years. They tell us the relative age compared to other rock layers containing the same index fossil.
Absolute age is determined by methods like radiometric dating, which measure the decay of radioactive isotopes in rocks.
