Early Eocene biotic and climatic change in interior western North America

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.