The Eocene/Oligocene boundary in Europe was long equated with a major episode of faunal turnover called the "Grande Coupure" (Stehlin, 1909; Savage and Russell, 1983; Russell and Tobien, 1986; Legendre, 1987; Legendre et al., 1991). In North America the Eocene/Oligocene boundary was believed to correspond to the boundary between the Duchesnean and Chadronian Land-Mammal Ages (Wood et al., 1941). With the advent of high-precision 40Ar/39Ar dating and the correlation of the Eocene/Oligocene boundary in the marine record with the extinction of the planktonic foraminiferan family Hantkeninidae (Hooker et al., 2004), the position of the epoch boundary has been revised on both continents. The boundary is now generally placed within magnetochron C13r at a little less than 34 million years ago (Berggren et al., 1992, 1995b; Prothero and Swisher, 1992; Prothero and Emry, 2004). This time coincides with the boundary between the Priabonian and Rupelian stages (see Fig. 1.7).
The principal faunal turnover at the Grande Coupure took place between the Priabonian and Rupelian Stages (Paleo-gene mammal reference levels MP 20-21; see the section on European Land-Mammal Ages, below), although it is now acknowledged that it was a protracted event. It involved extinction of more than 50% of the indigenous fauna, together with an influx of numerous immigrants from Asia. Disagreement persists over how closely the turnover coincided with the Eocene/Oligocene boundary and whether it was caused by climatic cooling or other factors (see Berggren and Prothero, 1992; Hooker, 1992a; and Legendre and Hartenberger, 1992, for contrasting views). However, the epoch boundary, based on the foram extinction noted above, is now known to be slightly older than the major cooling event that correlates with the Grande Coupure; consequently these events are now dated as earliest Oligocene (Hooker et al., 2004). In fact, there were several major phases of faunal turnover in Europe beginning in the middle Eocene and extending into the early Oligocene (Legendre, 1987; Hooker, 1992a; Legendre and Hartenberger, 1992; Franzen, 2003), but none appears to correspond precisely with the Eocene/ Oligocene boundary as now recognized. Nevertheless, this revision is so new that most recent accounts continue to place the Eocene/Oligocene boundary at the beginning of the Rupelian Age (Paleogene mammal reference level MP 21) at about 34 million years ago.
In North America, as a result of the revised Eocene/ Oligocene boundary, the Chadronian, long considered equivalent to early Oligocene, is now situated in the late Eocene. The Orellan Land-Mammal Age is early Oligocene, and the Chadronian/Orellan boundary coincides with the Eocene/Oligocene boundary (Prothero and Emry, 2004).
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