Different fossil groups are biostratigraphically useful for different geological time intervals based on their evolutionary history and preservation: graptolites are classic Paleozoic (especially Ordovician-Silurian) zone fossils; ammonoid cephalopods are premier Mesozoic index fossils; nummulitic (larger benthic) foraminifera are diagnostic of the Paleogene (especially Eocene); and planktonic foraminifera are widely used for Cenozoic biostratigraphy, particularly refined for the Neogene.
Matching each fossil group to its primary biostratigraphic time interval: (a) Graptolites correlate with (r) Paleozoic, as they are index fossils for the Ordovician through...
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