The late Miocene to early Pliocene appears to be well represented by radiolarian faunas in formations in southern California. From radiolarian biostratigraphic studies in southern California the writers suggest that the base of the Mohnian (as represented by the section at Newport Bay) is equivalent to the base of the Ommatartus antepenultimus Zone of Riedel and Sanfilippo, and the top of Delmontian (as represented by the Malaga Mudstone at Malaga Cove) is within the Pterocanium prismatium Zone of these same writers. The uppermost occurrences of Prunopyle titan, Lychnocanium grande and Theocyrtis redondoensis and the lowermost occurrence of Lamprocyclas heteroporos occur relatively close together and represent the Miocene-Pliocene transition in the Malaga Cove section. The most reliable datum plane for the Miocene-Pliocene boundary in southern California might be the lowermost occurrence of Lamprocyclas heteroporos, for this appears to be an evolutionary event.

From radiolarian paleoecologic studies in southern California we suggest that the late Miocene to early Pliocene was a period of paleotemperature fluctuation with sea surface temperatures fluctuating as much as 10°C. The radiolarian diversities also fluctuated during this same period with a general trend toward a diversity decrease upsection. There were differential (selective) radiolarian extinctions during this same period in that “shallow water” (epipelagic) forms appear to be more severely affected than deep or tropical submergent forms.

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