Six unusual morphotypes of the silicoflagellate subspecies Distephanus speculum speculum constitute the "pseudofibula plexus," the distribution of which is concentrated in the uppermost Miocene-lowermost Pliocene sediments centered around the Antarctic continent. The occurence of this plexus in Antarctic and Subantarctic waters seems to correlate with the late Miocene-earliest Pliocene glaciations of the continent. Therefore some type of ecologic control is suspected to account for its distribution in time and place. Distribution maps show that members of the plexus dominate silicoflagellate assemblages closest to the continent (class I province) but that their numbers diminish away from the continent (through class II and class III provinces), as does the thickness of the interval they occupy; thus the stratigraphic boundaries of the plexus may be diachronous. Only at Ocean Drilling Program Site 704 on Meteor Rise does the plexus occur in a continuous pelagic carbonate sequence with well-developed stable isotope and magnetobiostratigraphy. Its first abundance peak at that site, just above the base of the Messinian Stage, corresponds with a major interglacial event recorded in the planktonic and benthic foraminiferal isotopic records, an episode believed to have produced significant melted of the continental ice sheet and the injection of low-salinity meltwaters across the surface of the Southern Ocean. The major abundance peak of the plexus at this site occurs farther upsection, where it follows closely another anomalous negative excursion in the planktonic oxygen isotope record. At the conclusion of the latest Miocene-earliest Pliocene glaciations, however, the Pseudofibula plexus disappeared abruptly form the Southern Ocean, suggesting that its existence was closely tied to glacial/interglacial events on the continent. Injections of meltwaters into a nutrient-rich upwelling environment may have triggered ots blooms.