A major change in deep-water oceanic circulation occurred near the end of the Eocene forming the present psychrosphere and basic two-level water-mass system. This event is especially felt in the South Atlantic and Indian Ocean as shown by ostracods found in DSDP cores. Faunas traceable back to the Cretaceous rapidly evolved into the modern deep-sea faunas at this time. Several important ornate genera undergo diachronous changes suggesting that the flooding of the world ocean basins with colder waters happened in stages. The displacement of Tethyan water masses coming from the north by newly formed Antarctic bottom waters can be followed by changes in the ostracods.

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