The phrase describes organisms that formed index fossils as they are extinct.
What do you mean by Index fossils?
Index fossils may be defined as those fossils which are used to identify or date the specific rock layer in which it is found.
Index fossils are extinct, had hard parts, and were alive only for a very short period of time. They are also known as guide fossils.
Trilobites may be an ideal example of an index fossil.
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