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The Tiffanian North American land-mammal age is an informal biochronologic unit that spans 4.5 myr of the middle and late Paleocene. It is preceded and succeeded by the Torrejonian and Clarkforkian land-mammal ages, respectively. The Tiffanian was initially based on a small collection of mammals from southern Colorado, but faunas of Tiffanian age are now known throughout the Rocky Mountain region. The richest and most complete sequence of middle and late Tiffanian faunas known occurs in the northern Bighorn Basin. These faunas are the primary basis for regional Tiffanian biochronology. The Bighorn Basin sequence is also important because it preserves the only known detailed record of faunal change across the Tiffanian-Clarkforkian boundary, which is marked by the first appearance of rodents in North America. Geomagnetic polarity reversals, a carbon isotope excursion marking the Paleocene-Eocene boundary, and a recent Ar40/Ar39 age from a volcanic ash provide a strong geochronologic framework for the faunal sequence. Thus, this record affords a unique window into the evolution of North American mammals that can be compared globally with other biotic, paleoecological, and paleoclimatic records.
In spite of the importance of this Tiffanian record, numerous mammalian species represented by fossils collected over the last seventy-five years have not been described and the variability of many other species is poorly documented. This is especially true of small-bodied mammals. To remedy this, a compendium of Tiffanian and early Clarkforkian mammals that occur in the Bighorn Basin is presented. Most specimens are curated at the University of Michigan, but selected specimens curated at the Yale-Peabody Museum, originally collected by Princeton University crews, are also included. Seventeen new mammalian species are diagnosed and named, including two Leptictidae, two Pantolestidae, six Lipotyphla, one Plagiomenidae, four plesiadapiform Primates, and two Arctocyonidae. A new arctocyonid genus is also named. In addition, the size and morphologic variability of numerous others species is documented. The stratigraphic occurrences of all Tiffanian and early Clarkforkian species known from the Bighorn Basin are documented, and the faunal content of each biostratigraphic zone is summarized. This serves as the basis for a recently revised biostratigraphic zonation of the Tiffanian, and is intended to aid future studies of mammalian biochronology and evolution.
Changes in mammalian diversity (species richness) from the middle to the late Paleocene are compared with changes in global climate, inferred from the marine oxygen isotope record. The diversity record of the Bighorn Basin was combined with that of the Crazy Mountains Basin of Montana, since some temporal intervals are better represented in the latter. Localities were divided into clay-gall quarries, mudstone quarries, and surface collections. The number of individuals per species was estimated for each biozone or quarry, and the number of species was then normalized for sample size using rarefaction. Results indicate that species richness decreased from the Torrejonian to the early Tiffanian, reached a low point in the middle Tiffanian, and then began a steady increase to the middle Clarkforkian. The richness curve mirrors the marine temperature curve. Richness is significantly correlated with temperature suggesting that temperature had an important influence on the species diversity of Paleocene faunas.