PLANT AND MAMMAL DIVERSITY IN THE PALEOCENE TO EARLY EOCENE OF THE BIGHORN BASIN

Abundant plant and vertebrate fossils have been recovered from fluvial sediments deposited in the Bighorn Basin, Wyoming, during the first 1 3 m.y. of the Tertiary. Here we outline and discuss changes in the com position and diversity of floras and faunas during this period, which includes the recovery of terrestrial ecosystems from the K/T boundary extinctions, and later, during the Paleocene-Eocene transition, the gr eatest global warming of the Cenozoic. Floral diversity has been studi ed at three levels of spatial resolution: sub-local (at individual col lecting sites), local (along a single bed or stratigraphic horizon), a nd basin-wide (regional). Sub-local diversity shows a moderate increas e from the early to late Paleocene, followed by a decrease across the Paleocene/Eocene boundary, then an increase into the later early Eocen e. Local heterogeneity was lower in Paleocene backswamp floras, althou gh distinct groups of species dominated in different local fluvial set tings such as backswamps and alluvial ridges. Heterogeneity of backswa mp forests increased by about 65% from the early to late Wasatchian (e arly Eocene). The number of plant species inferred from the Bighorn Ba sin dataset rose gradually from the Puercan to an early Clarkforkian p eak of about 40 species, declined sharply to about 25 species by the C larkforkian/Wasatchian boundary, then rose through the Wasatchian to a bout 50 species. A regional analysis of mammalian genera shows high tu rnover and a rapidly increasing number of genera within a million year s of the K/T boundary (10-50 genera), a slight decline to 40 genera by the early Clarkforkian, then an increase from 40 to 75 genera by the late Wasatchian. Our analyses found no major extinctions in mammals du ring the Paleocene and early Eocene in the Bighorn Basin, but a one-th ird decrease in the number of plant species at about the Paleocene/Eoc ene boundary. Rates of taxonomic turnover were much higher for mammals than plants. The diversity trends for plants and mammals show little congruence, implying that the two groups responded in a very different manner to post K/T extinction opportunities. There is also little con gruence between plant diversity levels and change in mean annual tempe rature (MAT) as inferred from foliar physiognomy.