Qualitative and quantitative planktonic foraminiferal trends have been examined across the Oligocene/Miocene transition in three DSDP sites in the South Pacific ranging from the equator to temperate regions. The evolutionary appearance of Globoquadrina dehiscens represents one of the most reliable datums for interregional correlation of the Oligocene/Miocene boundary. This datum occurs within the biostratigraphic range of Globorotalia (Fohsella) kugleri and marks the boundary between Zones N4A and N4B. Globigerinoides first evolved during the late Oligocene and is not coincident with the Oligocene/Miocene boundary.

Unlike the Eocene/Oligocene boundary, the Oligocene/Miocene boundary is not marked by a crisis in the Oligocene planktonic foraminiferal assemblages. Most Oligocene forms continue their range upward into the early Miocene where most are replaced by typical Neogene forms.

The evolution of Globoquadrina dehiscens effectively heralds the beginning of the major Neogene evolutionary radiations in planktonic foraminifera including: (1) the evolutionary radiation of Globigerinoides into a number of species; (2) the initial evolution of Globorotalia (Globoconella) incognita, which forms the earliest ancestral form of Globoconella; (3) the evolution of the Globigerina (Zeaglobigerina) woodi group; (4) the evolution of Sphaeroidinellopsis disjuncta, which is the ancestral stock of the Sphaeroidinellopsis-Sphaeroidinella lineage; and (5) the evolution of Globorotalia (Fohsella) peripheroronda from Globorotalia (Fohsella) kugleri. These evolutionary radiations are reflected by a general increase in simple species diversity through the early Miocene. In all sites, species diversity is lowest in the interval near the Oligocene/Miocene boundary. Amongst the sites examined, diversity is highest, not in the tropical site, but in warm-subtropical Site 208, because at this latitude the faunas include both tropical and temperate forms.

The earliest Miocene, immediately following the boundary, is marked by high frequencies of Globorotalia (Fohsella) kugleri in tropical areas and Globoquadrina dehiscens in warm subtropical to temperate areas.

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