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→Geological age
===Geological age===
===Geological age===
The geological age of strata determines which fossils may be present. Nannoplankton first evolved in the Late Triassic ({{Ma|Late Triassic}}). Planktic foraminifers evolved during the Late Jurassic ({{Ma|Late Jurassic}}) and were very diverse during the Cretaceous and Tertiary ({{Ma|Cretaceous|Quaternary}}). Diatoms are found in rocks as old as Early Cretaceous ({{Ma|Early Cretaceous}}), and radiolaria have a spotty occurrence back to the Ordovician ({{Ma|Ordovician}}). Ostracodes and benthic foraminifers can be found in marine rocks as old as Ordovician. Phosphatic conodonts are locally abundant from Ordovician to Triassic strata. Of the organic-walled fossils, a variety of marine groups and spores are found throughout the Paleozoic. Marine dinoflagellates were common during the Jurassic. Fossil pollen is no older than Early Cretaceous. Megafossils, large benthic foraminifers, and calcareous algae can be found in most marine strata of the Phanerozoic.
The geological age of strata determines which fossils may be present. Nannoplankton first evolved in the Late Triassic ({{Ma|Late Triassic}}). Planktic foraminifers evolved during the Late Jurassic ({{Ma|Late Jurassic}}) and were very diverse during the Cretaceous and [[Tertiary]] ({{Ma|Cretaceous|Quaternary}}). Diatoms are found in rocks as old as Early Cretaceous ({{Ma|Early Cretaceous}}), and radiolaria have a spotty occurrence back to the Ordovician ({{Ma|Ordovician}}). Ostracodes and benthic foraminifers can be found in marine rocks as old as Ordovician. Phosphatic conodonts are locally abundant from Ordovician to Triassic strata. Of the organic-walled fossils, a variety of marine groups and spores are found throughout the Paleozoic. Marine dinoflagellates were common during the Jurassic. Fossil pollen is no older than Early Cretaceous. Megafossils, large benthic foraminifers, and calcareous algae can be found in most marine strata of the Phanerozoic.
===Paleogeographic setting===
===Paleogeographic setting===