Imprecise correlation of the marine and terrestrial fossil records has been a major obstacle to understanding migration and extinction of continental biotas and early Cenozoic climate change. New 40Ar/39Ar data from the Willwood Formation in the Bighorn Basin of Wyoming establish an age of 52.8 ±0.3 Ma for earliest Lostcabinian (late Wasatchian) faunas and coeval early Eocene floras. Strata just beneath earliest Wasatchian faunas can be correlated with the NP9/NP10 boundary in marine sedimentary units, which has an interpolated age of ∼55.7 Ma. This new information allows us to estimate the durations of the Wasatchian (∼5 m.y.) and the Losicabinian (∼2 m.y.) and shows that the continental biotas are coeval with the acme of Cenozoic warmth inferred from δ18O measurements of foraminifera. From 58 to 50 Ma, paleoclimate in the continental interior at about 45°N was warm and equable, but patterns of temperature change inferred from continental floras do not track precisely the marine δ18O record.

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