How do index fossils help determine age of rock layers?
index fossils have a very short life span which means they only can be found in a particular age.
index fossils are those fossils which have wide geographical distribution and short life span. so if they are found in a particular age, it means they belong in that age and they cannot be found in formations younger then that. for example if there is an index fossil of Permian age, we can corellate the permian age formations