 This boundary is closely linked with the Oligocene Oi-1 event, an oxygen isotope excursion that marks the beginning of ice sheet coverage on Antarctica.The "Grande Coupure", or "great break" in continuity, with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian species.The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations.