MAMMALIAN BODY SIZE CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM IN THE POWDER RIVER BASIN, WYOMING

The Paleocene-Eocene Thermal Maximum (PETM) represents a rapid global warming at the Paleocene-Eocene (PE) boundary. Research by others in Wyoming’s Bighorn Basin has demonstrated that certain mammalian taxa (e.g., the ‘condylarths’ Apheliscus and Hyopsodus, and the perissodactyl Sifrhippus [=Hyracotherium]) experienced a decrease in body size during the height of the PETM. To date, this phenomenon has not yet been documented in North American PE boundary-spanning mammalian faunas outside of the Bighorn Basin.

East of the Bighorn Basin in Johnson County, Wyoming, strata of the ‘Wasatch’ formation in the Powder River Basin contain among the more complete terrestrial records of the PETM in North America, with mammalian faunas (in direct superposition) spanning latest Paleocene (late Clarkforkian) through earliest Eocene (early Wasatchian, including Wa0 and Wa1) time. New research from localities spanning the PETM allows us to test whether the dwarfing of mammalian taxa observed by other researchers in the Bighorn Basin occurs in the Powder River Basin (and is therefore a more regional pattern).

Tooth crown area was measured for several taxa in the Powder River Basin included by others in the Bighorn Basin studies. The inclusion of the order Rodentia in our study adds an additional facet to the analysis of body size changes during the PETM. Using tooth crown area as a proxy for body size, we looked for patterns of body size change during the PETM. Preliminary results from the ‘condylarths’ Apheliscus and Hyopsodus suggest that dwarfing occurred in the Powder River Basin during the Wa0 interval, similar to what others noted for this taxon in the Bighorn Basin. However, the rodent Acritoparamys seems to show an increase in body size from late Clarkforkian to earliest Wasatchian (Wa0) time; however, this may be a result of small sample size. The recovery of additional teeth and further analysis of fossils from the Powder River Basin is critical to testing whether body size changes in mammalian taxa during the PETM extend beyond the boundaries of the Bighorn Basin. From a larger perspective, studies such as this should provide insight into the impacts of current and future global warming on today’s mammalian fauna.