Early Eocene biotic and climatic change in interior western North America

Abstract

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 {sup 40}Ar/{sup 39}Ar data from the Willwood Formation in the Bighorn Basin of Wyoming establish an age of 52.8 {plus minus} 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 {approximately}55.7 Ma. This new information allows the authors to estimate the durations of the Wasatchian ({approximately}5 m.y.) and the Lostcabinian ({approximately}2 m.y.) and shows that the continental biotas are coeval with the acme of Cenozoic warmth inferred from {delta}{sup 18}O measurements of foraminifera. From 58 to 50 Ma, paleoclimate in the continental interior at about 45{degree}N was warm and equable, but patterns of temperature change inferred from continental floras do not track precisely the marine {delta}{sup 18}O record.