A new earliest Wasatchian mammalian fauna is described from twenty localites in the Bighorn and Clarks Fork basins of northwestern Wyoming. These localities are in greenish gray sandstones and brightly colored mudstones within the Clarkforkian-Wasatchian boundary sandstone complex, or in brightly colored mudstones correlative with and lateral to the boundary sandstone. The fauna represents a new basal zone of the Wasatchian, here called zone Wa₀
Thirty-five mammalian genera and 41 mammalian species are known from zone Wa_0. Three genera and ten species are new. The Wa₀ fauna includes the earliest North American records of Artiodactyla, hyaenodontid Creodonta, Perissodactyla, and true Primates, but the Wa₀ fauna is dominated by hyopsodontids and phenacodontids, and it is compositionally intermediate between those of the preceding Clarkforkian land-mammal age and succeeding Wasatchian land-mammal age. Indices of diversity are high, and the pattern of rank abundance of species fits a model of biotic dependence. High diversity and biotic dependence are characteristics of Wasatchian but not Clarkforkian faunas. The Wa₀ fauna includes eleven small species with larger Clarkforkian and/or Wasatchian congeners. All of the small species appear in the Wa₀ fauna without Clarkforkian precursors intermediate in size. The depositional setting of Wa₀ mammals and associated fauna can be used to test the idea that a sudden appearance of small forms is due to rapid evolution in situ. (Go to the link and see more of this paper.)

Publisher: University of Michigan

Month of Publication: November

Year of Publication: 1989

Location: Ann Arbor, MI

Volume Number: 28

# of Pages: 97