The Wasatchian-Bridgerian land mammal age boundary (early to middle Eocene) in western North America

Mammalian faunas from western North America of late Wasatchian through early Bridgerian age (late early Eocene to early middle Eocene) are reviewed. Faunas of late Wasatchian age (Lostcabinian) are characterized by the presense of Lambdotherium. It is suggested that the Wasatchian-Bridgerian boundary be placed at the first occurrence in the fossil record of Palaeosyops (including Eotitanops), Hyrachyus, Trogosus, Helaletes, Omomys, Helohyus, Pantolestes, Homacodon, Washakius, and/or Smilodectes. Most all of these genera overlap in biostratigraphic occurrence with taxa typical of the Wasatchian, including Coryphodon, Esthonyx, Palaeosinopa, Didymictis, Hapalodectes, Bunophorus, Diacodexis, Phenacodus, Hyracotherium, Heptodon, and Shoshonius, all of which last appear in either Gardnerbuttean faunas or Bridger A, B or C. The Gardnerbuttean is recognized as earliest Bridgerian; marked faunal turnover occurred during this time interval.