OCR Text |
Show 8/ 14/ 97 Smith and Forester 2 Within the Great Basin, ostracode distributions fall into two biogeographical categories: endemic and pandemic ( or cosmopolitan) distributions. Endemic species include hypogean taxa found in the deep aquifers, whereas the pandemic species are typically epigean taxa living in the lakes and wetlands. The hypogean taxa are characterized by " exotic" carapace morphologies, a feature seen in ostracode populations living in large, stable, isolated systems elsewhere in the world, such as Lake Baikal in the Former Soviet Union, Lake Tanganyika in Africa, and the Edwards Aquifer in Texas. The species from deep aquifers in Nevada are not known to occur anywhere else in the world, and represent members of a stable, isolated ecosystem in which the effects of climate change are damped. Consequently, they are not useful as paleoclimate indicators, but have great potential as indicators of aquifer hydro- geochemistry through isotopic analysis of their shells. Endemic species of the deep aquifers can be contrasted with the epigean taxa of the lakes and wetlands. Common epigean species known today in the Great Basin ( e. g. Limnocythere ceriotuberosa, L. bradburyi, L. staplini, L. sappaensis, Candona caudata, and others) appear to have maintained modern- style assemblage relationships throughout the Quaternary, and in some cases for at least the past 5 million years. Their modern biogeographic distributions follow regional and continental scale climate patterns controlled by air mass position and global atmospheric circulation. For example, in North America, L. bradburyi lives today south of the " frost line" ( average daily minimum temperature in January equal to 0° C), and occupies the middle and northern extent of the average position of the Hadley circulation cell. L. ceriotuberosa and Candona caudata occupy the range of the Ferrel circulation cell, with L. ceriotuberosa distributions limited by the boundaries of the dry Pacific Air mass. Cytherissa lacustris, found in the fossil record of lakes in the Great Basin, is living today within the range of the Arctic circulation cell, as well as in the profundal zones of the Great Lakes and deep lakes in Yellowstone National Park. The presence of C. lacustris in fossil assemblages in Great Basin lakes indicates that past air mass configurations and their associated combinations of precipitation and temperature have been quite different from those we see today. These epigean species are truly cosmopolitan, in that they have worldwide distributions in similar climatic regions. The importance of these species to paleoclimatic reconstruction lies in the fact that their fossilized shells record not only lake paleochemistry, but also past changes in air mass configuration. The Walker Lake ostracode record provides an example of the kind of paleoclimate information obtainable from fossil ostracode distributions, indicating the changing air mass patterns through Pleistocene time. Thus, endemic ostracode species provide very different types of information compared with pandemic or cosmopolitan species. The two groups appear to be driven by very different types of selection pressure. For pandemic epigean species, physical accommodation to changing environments seems to be the driving selection pressure. For endemic species of the deep aquifers, living in large and geologically stable environments, selection shifts from physical accommodation to biological accommodation, as environmental variability plays a lesser role. |