The Oligocene-Miocene boundary has not been clearly defined, and its stratigraphic position in Europe is controversial. Because it is a boundary between international stratigraphic units, it should be defined by international agreement in an area with suitable fossils, preferably with planktonic foraminifera, at a horizon that can be identified widely throughout the world. The boundary should have a standard section, and if a suitable standard section cannot be found in Europe, it should be selected elsewhere. A suitable horizon that would be relatively easy to correlate throughout the world could be defined within the succession of zones demarked by forms of the Orbulina bio-series.

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