Quantitative analysis of middle Eocene-early Oligocene bathyal deep-sea benthonic foraminifera was carried out on samples from DSDP Sites 77, 292 (Pacific Ocean), 219 and 253 (Indian Ocean), 363 (Atlantic Ocean), and Eureka 67-128 (Gulf of Mexico) and compared with benthonic foraminiferal stable isotopic data to determine the effects of deep-water circulation changes on the faunas, Faunal changes (first and last occurrences) are found throughout the sequences, and a catastrophic turnover of the benthonic foraminiferal fauna at the Eocene-Oligocene boundary does not occur. A few distinct events do occur associated with inferred coolings at the middle/late Eocene and Eocene/Oligocene boundary. For example, Nuttallides truempyi, an important middle Eocene species, has an iso hronous last occurrence within the Globigerinatheka semiinvoluta zone in Sites 219, 253, 292, and 363 and coincides with a 3° deep-water cooling inferred from the O¹⁸ record.

During the late Eocene and early Oligocene these bathyal sites are marked by a remarkably uniform assemblage dominated by Oridorsalis tener, Globocassidulina subglobosa, and Cibicidoides ungerianus. In Sites 292 and E67-128 additional species that are important are Bulimina alazanensis, Buliminella grata, and Bulimina tuxpamensis. This relatively uniform bathyal faunal assemblage in these Atlantic, Indian, and Pacific sites is similar to an assemblage found previously in North and South Atlantic bathyal sites. This faunal pattern, as well as the isochronous last appearance of N. truempyi, suggests that a relatively uniform and widespread bathyal water mass extended throughout the world ocean during middle Eocene-early Oligocene time.

The faunal data show three responses to the sharp deep-water coolings at the middle/late Eocene and late Eocene/early Oligocene boundary: (1) a dominant species may have a last occurrence as a direct result of the cooling, (b) an increase in species abundance precedes the cooling followed by a sharp decrease associated with a decrease in deep-water temperatures, or (c) a species is largely unaffected by the temperature change.

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A comparison of the present data and previous work shows that the greatest occurrence of extinctions is found at the Paleocene/Eocene boundary with a lower number o extinctions at the middle/late Eocene and Eocene/Oligocene boundaries. The lack of a major catastrophic change at the Eocene/Oligocene boundary may be a result of previous faunal events at the Paleocene/Eocene and middle/late Eocene boundary eliminating stenothermal species, leaving environmentally tolerant species in the late Eocene that were largely unaffected by the 3°C (37°F) temperature decrease at the Eocene/Oligocene boundary.

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