Mammalian Fauna and Biostratigraphy of the Upper Part of the Wind River Formation (Early to Middle Eocene), Natrona County, Wyoming, and the Wasatchian-Bridgerian Boundary

Stucky, Richard Keith

Graduate Thesis Or Dissertation

Mammalian Fauna and Biostratigraphy of the Upper Part of the Wind River Formation (Early to Middle Eocene), Natrona County, Wyoming, and the Wasatchian-Bridgerian Boundary Pubblico Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/8w32r593g

Abstract

The mammalian fauna of the Wind River Formation of the northeastern Wind River Basin, Wyoming, has been used to define the middle to late Wasatchian Land Mammal Age of western North America. Detailed stratigraphic studies of the upper part of the Wind River Formation and analysis of the mammalian fauna allow for the recognition of two valuable biostratigraphic zones, the Lambdotherium range zone and the Palaeosyops borealis assemblage zone. Approximately 120 mammalian species are known from these two zones. Stratigraphic data suggests that previous lithostratigraphic and biostratigraphic correlations of the Lost Cabin Member of the Wind River Formation are in error. Geologic structure of the Wind River Formation complicates correlations between exposures. The Lambdotherium range zone is defined by the stratigraphic occurrence of Lambdotherium popoagicum. The Palaeosyops borealis assemblage zone is based on the first occurrence in the fossil record of P. borealis, Trogosus, Hyrachyus and Antiacodon. The Lostcabinian Land Mammal Subage of the Wasatchian is redefined on the basis of the Lambdotherium range zone of the Wind River Formation. The Palaeosyops borealis assemblage zone is included in the Gardnerbuttean Land Mammal Subage. The recognition of these two zones in the upper part of the Wind River Formation and a review of late Wasatchian and early Bridgerian mammal faunas suggest that several alternatives can be used to define the Wasatchian-Bridgerian boundary throughout western North America. The first alternative defines the boundary at the first occurrence of Palaeosyops borealis, Trogosus, Hyrachyus and Antiacodon, or at the base of the Palaeosyops borealis assemblage zone. The second alternative places the boundary at the first occurrences of Isectolophus, Microsus, Sciuravus and Limnohyops, and last appearances of Esthonyx, Didymictis, Coryphodon and Bunophorus. It is believed that the Wasatchian-Bridgerian boundary is best defined by the first occurrences of P. borealis, Trogosus, Hyrachyus and Antiacodon, and thus that the Gardnerbuttean Land Mammal Subage be included in the Bridgerian.

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