LATE PALEOCENE SEASONALITY AND POSSIBLE SHORT-TERM WARMING ASSOCIATED WITH EARLIEST CLARKFORKIAN APPEARANCE OF RODENTS IN NORTH AMERICA: INFERENCES FROM STABLE ISOTOPES IN THE NORTHERN BIGHORN BASIN, WYOMING

Oxygen in enamel from large-bodied mammals can be used to estimate d¹⁸O of drinking water, which is sensitive to d¹⁸O of meteoric water and, in turn, to temperature change. We analyzed enamel phosphate of Phenacodus teeth, the largest (ca. 32-40 kg) common mammal available, using teeth primarily from the P. grangeri-P. intermedius lineage ranging from middle Tiffanian (late Paleocene) to early Wasatchian (early Eocene) in age. To avoid weaning effects, we used only upper and lower P4 and M3. Serial samples for five P4-M3 sets from single individuals exhibited progressive temporal increases in d¹⁸O, both within teeth and from M3 to P4 (P4 forms ontogenetically after M3), indicating a minimum seasonal fluctuation of 3.0‰ for the late Tiffanian, and 4.4‰ for the Clarkforkian (Cf). Time-averaging within enamel samples and muting of seasonal signals in surface waters lead us to expect that actual variation was probably greater.

Rodents first appeared in North America in the late Paleocene (beginning of Cf-1; ca. 56.2 Ma), along with tillodonts and Coryphodon, probably as immigrants from Asia. Change to a warmer more equable climate, allowing for establishment of populations across Beringia, is often cited as a mechanism facilitating Clarkforkian (Cf-1) and Wasatchian (Wa-0) immigration. Using M3s, we found elevated d¹⁸O in 3 of 4 teeth (16.3, 16.4, 16.8, 14.0‰) from a 10 m interval at the top of which rodents first appear, indicating an increase of ~2‰ (meteoric) above the late Tiffanian mean. This represents a temperature increase of ca. 3.5º C, based on modern water-temperature relationships. The possibility of elevated values from diagenesis cannot be discounted as teeth in this interval often occur in sandstone, a lithology conducive to alteration. However, mechanisms to elevate d¹⁸O are not obvious. We found a similar increase of 2.1‰ above the Clarkforkian mean in Wa-0, the interval when perissodactyls, primates, and artiodactyls first appeared in North America, based on one M3 (16.3‰). M3 variation at a given level is typically ca. 1‰ but can be higher. We infer that late Paleocene Cf-1 and early Eocene Wa-0 immigration events both coincided with short-term warming events, but because shifts in d¹⁸O can occur from causes other than temperature, further testing is necessary.