MAMMAL EXTINCTIONS, BODY-SIZE, AND PALEOTEMPERATURE

There is a general inverse relationship between the natural logarithm of tooth area (a body size indicator) of some fossil mammals and paleo temperature during approximately 2.9 million years of the early Eocene in the Bighorn Basin of northwest Wyoming, When mean temperatures bec ame warmer, tooth areas tended to become smaller. During colder times, larger species predominated; these generally became larger or remaine d the same size. Paleotemperature trends also markedly affected patter ns of local (and, perhaps, regional) extinction and immigration. New s pecies appeared as immigrants during or near the hottest (smaller form s) and coldest (larger forms) intervals. Paleotemperature trend revers als commonly resulted in the ultimate extinction of both small forms ( during cooling intervals) and larger forms (during warming intervals). These immigrations and extinctions mark faunal turnovers that were al so modulated by sharp increases in sediment accumulation rate.