From late in the Tiffanian (~57 million years ago) until the beginning of the Wasatchian (~55 million years ago), at least three successive waves of mammalian immigrants dispersed into North America. One result of these episodes of mammalian dispersal was a dramatic reorganization and modernization of the North American mammal fauna. Rodents, artiodactyls, perissodactyls, primates, and didelphid marsupials appeared in North America for the first time, while such archaic taxa as Plesiadapidae and Carpolestidae became extinct. Fuller documentation of this great faunal change requires better understanding of North American mammal faunas during the Clarkforkian and early Wasatchian Land Mammal Ages.

Two current field projects of the Carnegie Museum of Natural History aim to help fill in gaps in our knowledge of Clarkforkian and early Wasatchian mammals in North America. Since 1992 field crews from the Carnegie Museum have worked at the early Clarkforkian Big Multi Quarry in the northern Washakie Basin, Sweetwater County, Wyoming. The mammalian fauna obtained from this unique site is adding a great deal to our understanding of mammal diversity in the early Clarkforkian. Numerous taxa from Big Multi Quarry are new. Others are documented by extraordinarily complete fossils which reveal anatomical details that were previously unknown, thereby clarifying phylogenetic relationships. Rarely documented plant/animal associations are founded on a well-preserved fossil flora, which occurs a few inches above the mammal-producing horizon.

Figure 1: A Carnegie Museum field crew working at Big Multi Quarry in southwestern Wyoming in 1993.

The first diverse Wasatchian mammal fauna from eastern North America has been recovered from the Tuscahoma Formation in east-central Mississippi by field parties from the Carnegie Museum since 1990. Screen washing has yielded some 250 mammal teeth, including those of rodents, omomyid primates, insectivores, and many other taxa. This mammal fauna occurs in estuarine facies and is associated with sharks, rays, snakes, lizards, bony fishes, dinoflagellates, and pollen. Correlation with marine biochronologies suggests that the immigration of basal Wasatchian mammals into North America was virtually synchronous with the immigration of similar forms in the basal Sparnacian of western Europe.

Intercontinental Correlation of Paleogene Mammalian Faunas

Beard, K. C., M. R. Dawson, and A. Tabrum
Section of Vertebrate Paleontology
Carnegie Museum of Natural History

The latest Paleocene and earliest Eocene were times of major faunal turnover in the mammalian fossil record. Origins of modern mammalian orders were accompanied by climatic changes from a cool Paleocene to a warm Eocene. More precise intercontinental correlation of the mammalian faunas is crucial for the studies of phylogenetic evolution and paleobiogeography of mammals during the Paleocene–Eocene transition. Field studies in Mississippi, Wyoming, and China, using biostratigraphic and other methods, are aimed to refine the correlation of the mammalian faunas on a global scale, especially between North America and Asia.

Recent reference: Beard, K. C., M. R. Dawson, and A. R. Tabrum. 1995. First diverse land mammal fauna from the Early Cenozoic of the southeastern United States: The Early Wasatchian Red Hot local fauna, Lauderdale County, Mississippi. Geological Society of America, Abstracts with Programs, 1995:A-453.

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