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Laser-fusion 40Ar/39Ar dating and magnetostratigraphy have significantly changed our conception of the temporal duration and correlation of Paleogene North American land mammal "ages." The Wood Committee (1941) originally divided the Paleocene Epoch into five land mammal "ages." Current age estimates of their time spans are: Puercan, 65–63.8 Ma; Torrejonian (including the "Dragonian"), 63.8–61 Ma; Tiffanian, 61–56 Ma; and Clarkforkian, 56–55.2 Ma. The Paleocene/Eocene boundary, long placed in the Clarkforkian, occurs in the earliest Wasatchian, based on correlations using mammals, pollen, and terrestrial carbon isotopes.

The Wood Committee (1941) divided the North American Eocene Epoch into four land mammal “ages”: Wasatchian (originally thought to be early Eocene), Bridgerian (thought to be middle Eocene), and Uintan and Duchesnean (both once thought to be late Eocene). The earliest Wasatchian is now considered Paleocene age, and the Wasatchian/Bridgerian boundary is about 50.4 Ma in age. The Bridgerian, Uintan and Dudnesnean land mammal “ages” are all middle Eocene age. The Bridgerian/Uintan boundary occurs in magnetic Chron C21n, about 47 Ma. The Uintan/Duchesnean boundary occurs within Chron C18n, and lies above an ash dated at about 40 Ma. The Duchesnean/Chadronian boundary lies within Chron C16n, about 37 Ma.

Finally, the Wood Committee (1941) divided their concept of North American Oligocene sequence into three land mammal “ages”: the Chadronian, Orellan and Whitneyan (supposedly early, middle, and late Oligocene). The Chadronian/Orellan transition occurs just above a date of 33.9 Ma, late in Chron C13r; it is slightly younger than the Eocene/Oligocene boundary, and this makes the Chadronian mostly late Eocene, not early Oligocene age. The Orellan/Whitneyan boundary occurs in the middle of Chron C12r, just below a date of 31.8 Ma. The Whitneyan/Arikareean boundary occurs within Chron C11n, above a date of 30.0 Ma. Consequently, the Orellan and Whitneyan are both early Oligocene, and most of the Arikareean (long considered early Miocene) is late Oligocene age. These new age estimates and correlations differ greatly from the time scales published as recently as 1987.

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