The radiolarian Prunopyle titan Campbell and Clark has been shown by Ingle to be an index of the upper Miocene in California. Its greatest abundance is within the upper Mohnian of southern California where it occurs with sinistrally coiled populations of Globigerina pachyderma (Ehrenberg), a cold-water index. In deep-sea cores from high latitudes, Prunopyle titan has an upper stratigraphic limit within what is termed by some authors the “Gauss Normal Magnetic Epoch” and a bottom limit below what has been termed the “Gilbert Reversed Magnetic Epoch.” The range of Prunopyle titan is below the zone of Pterocanium prismatium Riedel; the latter is considered to be an index of the Pliocene.
In land sections, K-Ar dates indicate that the upper limit of Prunopyle titan is about 9–10 m.y. before the present; in deep-sea cores the upper stratigraphic level of P. titan is equated with an age of about 3 m.y. or less. At least three upper Miocene radiolarians are associated with P. titan in both its occurrences in deep-sea cores and in the upper Miocene of southern California. No Pliocene radiolarian indices occur in the P. titan zone of deep-sea cores. If the paleomagnetic record of deep-sea cores is correctly related to that of volcanic rocks, and the radiometric dates of the latter are valid, the radiometric dates for the later Tertiary marine section of California are about three times too old.