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Most radiolarian biostratigraphic datums and zonations currently used (as well as other microfossil datums, and zonations) appear to be time transgressive due to their being based upon warm-water shallow living radiolarians with disjunct distributions whose geographic ranges and local first and last occurrences are controlled mainly by paleoclimatic conditions. The Miocene-Pliocene transition appears to be a time of worldwide cooling (including a cooling in the type locality) and this has probably led to elevations and depressions of the paleontologically recognized boundary in many areas.

In an equatorial Pacific core studied (V24-59) Nigrini's (1971) equatorial Pacific radiolarian Zones 1, 2, and most of 3 occur within the Brunhes Magnetic Epoch, and Zone 4 extends from the Jaramillo Event to the Gilsa or Olduvai Event. Riedel and Sanfilippo’s Pterocanium prismatium Zone (Riedel and Sanfilippo, 1970, 1977 and 1978) from the Gilsa or Olduvai event into the upper Gauss, the Spongaster pentas Zone extends from the upper Gauss to around Gilbert b, and their Stichocorys peregrina Zone begins around Gilbert b. These “warm-water” datums, when compared to radiolarians believed to have been tropical submergent or deep living suggest that tropical submergent or deep living forms may be very useful for establishing “cosmopolitan” datum planes. Two such datums approximating the Miocene-Pliocene boundary are the first occurrences of Lamprocyclas heteroporos and Sphaeropyle langii at about 4.2 and 5 m.y.a. respectively.

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