Radiolarian assemblages have been quantitatively examined for two equatorial Pacific time-slices (22 and 16 Ma) and three time-series (DSDP Sites 71,173 and 289) spanning the early Miocene from 22–15 Ma. A subset of 43 radiolarian species that range throughout the 22–15 Ma interval was used to compare the time-slices. Census data tabulated for the time-series were treated with a Q-mode factor analysis that identifies five faunal provinces.
The early Miocene was a time of very high evolutionary radiation, especially among the suborder Nassellaria. However, by the end of the middle Miocene 72 percent of the Nassellarian species present in the greater than 63μm fraction of the sediment at 15 Ma had become extinct. Therefore, the middle Miocene was a time of Nassellarian extinctions. In contrast, the Spumellaria were relatively unchanging throughout the early to middle Miocene, and only 16 percent had become extinct by the end of the middle Miocene. These Spumellaria are inferred to have been mainly cooler water/deeper dwellers and were ubiquitous. Their position as generalists in the fauna probably allowed them to be less affected by changing ocean conditions.
By 16 Ma, diversity of the radiolarian assemblage was lower than at any other time during the Neogene in the tropical Pacific. At this time a dramatic change was observed in the western tropical assemblage. One species, Stichocorys wolffii, reached maxima of over 50 percent of the population and other species that had previously been abundant disappeared. These characteristics seemed to have resulted from extinctions of Nassella-ria and biogeographic changes within the assemblage.
The radiolarian faunal groups responded to and reflect broadscale paleoceano-graphic changes. As Neogene Indo-Pacific circulation became restricted, some radiolarian species, such as the spyroids, moved eastward and some, such as the stichocorids, moved into the temperate/subarctic.