Late Paleocene biostratigraphy, isotope stratigraphy, and mammalian systematics of the northern Bighorn Basin, Wyoming.

Strata in the northern Bighorn Basin preserve a rich sequence of superposed mammalian assemblages of late Paleocene age that have been widely used as a basis for regional biostratigraphic zonation and for evolutionary studies. From new collections and previously undescribed material, I diagnose 15 new mammalian species, most of small body size, and document morphologic detail, variability, and stratigraphic occurrences of other species. Previous biozonation for most of the late Paleocene is based on evolutionary lineages of Plesiadapis. Because of ambiguities in recognition of the first occurrence of Plesiadapis fodinatus, however, I abandon the P. fodinatus zone (Ti-5a) and recognize two new consecutive zones (Ti-4b and Ti-5a revised, respectively) based on the first occurrences of Phenacolemur and Probathyopsis . I also abandon the late Clarkforkian Phenacodus-Ectocion acme zone (Cf-3) and recognize a new zone (Cf-3 revised) of similar age based on the first occurrence of Copecion. Rarefied mammalian diversity from eight quarries indicates a stepwise decrease in diversity from the late Torrejonian (To-2) to the middle Tiffanian (Ti-3), followed by a stepwise increase from the middle Tiffanian to the Clarkforkian (Cf-2). The diversity trends generally follow temperature trends inferred from the benthic marine record and from continental floras, suggesting that diversity was influenced by global temperature change. The first appearance of rodents in North America at the beginning of the Clarkforkian has been attributed to global warming at high latitudes that opened a northerly corridor from Asia. To test this, a stable oxygen isotope stratigraphy using the phosphate component of Phenacodus tooth enamel was constructed. If continental temperatures were elevated during rodent immigration a corresponding elevation in the delta18O of mammal teeth is expected, reflecting elevated delta18O in meteoric water. Phenacodus teeth have not been found at the level where rodents first occur, but mean delta18O (n = 9) from slightly later (∼75 k.y. later) is elevated ∼1.0‰ above the preceding zone (Ti-6). These data are consistent with a short-term temperature increase, but are complicated by increased variability suggesting greater fluctuation in temperature or aridity, and unstable climatic conditions at or near the first appearance of rodents.Ph.D.Earth SciencesGeologyPaleoecologyPaleontologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124312/2/3137941.pd