The extinction of Discoaster barbadiensis and D. saipanesis, the last representatives of the highly successful low- and mid-latitude group of Paleogene rosette-shaped discoasters, close to the Eocene-Oligocene boundary, has been unfortunately seen by some as evidence of a major extinction "event" in the calcareous nannoplankton. These extinctions (and a few others that occur at about the same time) should be more properly viewed against the larger background of calcareous plankton biostratigraphy and paleobiogeography.

A compilation of DSDP Paleogene calcareous nannoplankton data from low, middle, and high latitudes shows maximum diversity values (ca 120 species) during the middle Eocene (NP14-15) and minimum diversity values (ca 37 species) in the early Oligocene (NP22). The approximately 70% reduction in diversity occurs over an interval of approximately 7 m.y. in a stepwise fashion with the most abrupt reduction (~ 50%) occurring between the late middle Eocene (NP16) and the early late Eocene (NP18). A further reduction of about 20% occurs across the Eocene-Oligocene boundary (NP18 to NP22), and the values for the remainder of the Oligocene (NP23-25) stabilized at about 50+ species.

A quantitative investigation has been made of late Eocene-early Oligocene calcareous nannoplankton assemblages at 4 DSDP sites in the North (Site 292, 16°N) and South (Site 277, 52°S) Pacific, Indian (Site 219, 9°N) and South Atlantic (Site 363, 20°S) Oceans. An equatorward migration of high latitude floral assemblages occurred during the late Eocene and early Oligocene. This migration was initiated in the late middle Eocene (NP16-17) and the replacement of a tropical flora (dominated by discoasters and Reticulofenestra reticulata) by mid-high latitude assemblages (dominated by reticulofenestrids, R. bisecta, R. hesslandi, R. umbilica, R. hampdenensis) was not completed until the early Oligocene (NP22; see Figure 1). The extinction of the rosette-shaped discoasters and R. reticulata is related to the late Eocene-early Oligocene isotopic cooling event but does not reflect its maximum. These species became extinct when sea surface temperatures reached a threshold value below which they could not survive.

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Isotopic records on benthic and planktonic foraminifera from the same sites (Keigwin, work in progress) show a strong parallelism between climatic changes (sharp cooling in the late middle Eocene, at the Eocene-Oligocene boundary, maximum cooling within the earliest Oligocene) and calcareous nannoplankton migrations. The extinctions of the Reticulofenestra umbilica group and equilibrium within the floral migration patterns in the calcareous nannoplankton coincide with the maximum isotopic cooling event in the earliest Oligocene. Our data indicate that extinctions among the calcareous nannoplankton occurred in sequential, step-like manner over a several million year interval. They may best be explained as reflecting the gradual but clearly defined trend towards decreasing temperature v lues during the late Eocene to early Oligocene.

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