Based on nannofossils, a carbonate core from the South Pacific (Eltanin 21-5; 36°41^primeS; 93°38^primeW; length 480 cm; depth 3,121 m) is late Pliocene to early Pleistocene in age. The Pliocene-Pleistocene boundary (325 cm) is placed at the last appearance of most discoaster species in the core including D. pentaradiatus and D. surculus. Discoaster brouweri extends higher, to 225 cm where it also becomes extinct. Above the top of the Pliocene, the presence of D. brouweri and absence of Gephyrocapsa oceanica indicate an earlier Pleistocene age; the middle Pleistocene and much of the late Pleistocene are missing in unconformity near the core top.

This core is significant in showing alternations of dominantly keeled and dominantly nonkeeled populations of the Globorotalia truncatulinoides - G. tosaensis plexus. The lower (425-480 cm) and upper (0-130 cm) core sections contain populations dominated (>78%) by keeled forms referable to G. truncatulinoides, whereas intermediate intervals between 198 and 400 cm contain populations dominated (>80%) by nonkeeled forms which agree well with topotypes of G. tosaensis. Transitional populations are present between 145 and 180 cm.

Globorotalia truncatulinoides is associated in the core only with marginal tropical foraminiferal faunas including Globorotalia menardii, Globigerinoides conglobatus, and "Globigerina" dutertrei whereas G. tosaensis is associated with a cooler water planktonic foraminiferal assemblage lacking these species and with higher frequencies of Globorotalia inflata and right coiling Globigerina pachyderma. Likewise, the coccolith Unbilicosphaera leptopora which prefers warm waters, exhibits marked increases in frequency in the upper and lower core sections containing G. truncatulinoides.

Although not decisive, this sequence suggests that during the late Pliocene-early Pleistocene, at least in this area, G. truncatulinoides and G. tosaensis were either phenotypic variants or separate subspecies or species with distinct environmental preferences. It also provokes speculation as to whether the G. tosaensis to G. truncatulinoides evolutionary bioseries near the Pliocene-Pleistocene boundary in tropical deep-sea areas, including the Gulf of Mexico, is the result of ecologic or oceanographic change.

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