The Clarkforkian is an early Cenozoic North American Land-Mammal Age following the late Paleocene Tiffanian and preceding the early Eocene Wasatchian. Controversy has surrounded recognition of the Clarkforkian as a valid land-mammal age, but new investigations in the type area, the Clark's Fork Basin (northern Bighorn Basin) of northern Wyoming, enable the Clarkforkian to be precisely defined and its stratigraphic limits accurately determined. As a result of a University of Michigan project in this area, more than 250 vertebrate localities (134 of Clarkforkian age) have been discovered in the upper Polecat Bench Formation and the lower Willwood Formation, and most of these have been tied to measured stratigraphic sections. Thus the precise stratigraphic occurrence and range of all Clarkforkian mammals in the type area have been established. Seventy species of mammals are recorded, of which eight are new. The beginning of the Clarkforkian is defined by the initial appearance of Rodentia, together with the genera Esthonyx, Coryphodon, and Haplomylus, all of which first occur at about the same time. The beginning of the Wasatchian (upper boundary of the Clarkforkian) is defined by the first appearance of Artiodactyla (Diacodexis), Perissodactyla (Hyracotherium), adapid primates (Pelycodus), omomyid primates (Tetonoides, Teilhardina), and hyaenodontid creodonts in the northern Western Interior. The taxa defining both the upper and lower boundaries of the Clarkforkian appear to have been immigrants. The Clarkforkian itself can be subdivided into three zones, two of them based on evolutionary stages of Plesiadapis. They are, successively, the Plesiadapis gingerichi Zone, the Plesiadapis cookei Zone, and the Phenacodus-Ectocion Zone. (Go to the link and see more of this paper.)

Publisher: University of Michigan

Month of Publication: October

Year of Publication: 1981

Location: Ann Arbor, MI

Volume Number: 26

# of Pages: 197