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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 186 Present societal concerns about global climatic change focus primarily on possible drastic alterations of continental ecosystems. Paradoxically, the most useful perspective on climatic history comes from the marine record of the Cenozoic Era, particularly from stable isotopic analyses of deep-sea cores. Such marine records do not directly yield the greater extremes of temperature and other environmental signals that continents experience. Accordingly, a critical area of current research concerns avenues for linkage between marine climatic records and the behavior of terrestrial ecosystems. That is why the record of land mammal faunas, by providing sufficient geochronological precision to be correlated with the marine record, holds special relevance to current considerations about global change. Land mammal faunal succession may reflect climatic change in two somewhat distinct manners. First, the adaptive features of a given assemblage may be viewed in relation to the presumed conditions for which it was adapted; also, by analogy with living communities of similar structure, its composition may indicate the kind of landscape in which it lived. Secondly, in a largely independent and nonbiological manner, quantitative analysis of faunal turnover helps pinpoint times of ecosystem stability or, alternatively, of rapid reorganization. Rapid faunal turnover episodes may be correlated with key events in floristics, sedimentary regimes, isotopic signals, or other independent evidence of climate change episodes. We pursue both of these approaches in the context of Cenozoic land mammals of North America. First, we review the land mammal record of North America and, together with other botanical and sedimentological evidence, indicate the general succession of ecosystems and landscapes. Second, we quantify the record of intercontinental immigration to North America, placing special emphasis on first-order episodes. Finally, we correlate this continental record with the marine record of global climate change. We show that the continental ecosystem did not respond uniformly to climatic episodes indicated in the marine record. We conclude that such shifts were a necessary but not a sufficient condition for faunal turnover and terrestrial ecosystem reorganization. LAND MAMMAL FAUNAS AND THEIR RESPONSE TO CLIMATIC CHANGE The richest continental record of Cenozoic mammals is that of North America. We concentrate on that record and methods of recognizing its response to climatic change. Several recent reviews of the mammalian succession may be noted. Woodburne (1987) offers a modern geochronological revision of the North American mammalian succession, and Lindsay et al. (1990) refine and update the biostratigraphy of Neogene mammal faunas in Europe. Savage and Russell (1983) provide comprehensive coverage of all continental and marine mammal faunas. Stucky (1990) reviews paleoecology and diversity change in North American Paleogene faunas, and Webb (1977, 1983a, 1989) treats Neogene faunal dynamics in North America. Figure 11.1 presents the geochronological and biostratigraphic framework of North American land mammals during the Tertiary. Chronofaunas and Turnover Pulses Vertebrate paleontologists refer to persistent faunas of essentially uniform taxonomic composition and stable diversity as chronofaunas following Olson (1952). In ecological terms, chronofaunas are thought to represent stable coadapted sets of species, and their detailed histories (under various perturbations) shed light on the theory of how communities or community complexes are structured (Olson, 1983; Webb, 1987). Five chronofaunas account for most of the stable intervals of the North American Cenozoic. They are the Eocene chronofauna (Krause and Maas, 1990); the White River chronofauna (Emry, 1981; Krishtalka et al., 1987); and in the Miocene, the Runningwater, the Sheep Creek, and the Clarendonian chronofaunas (Webb, 1983a; Tedford et al., 1987). Only in the extreme phases of the icehouse world, during the Pliocene and the Pleistocene, did the stability and persistence of mammalian chronofaunas breakdown. These stable chronofaunas are followed by relatively brief episodes of rapid faunal turnover. Such rapid turnover episodes (RTE) have been recognized and studied during the past decade, following the establishment of relatively precise geochronometry. Several students of mammalian faunas have documented that even during an RTE the correlation between numbers of first appearances and last appearances is maintained, indicating an equilibrium process in which faunal interactions maintain a balance between gains and losses (Gingerich, 1984; Webb, 1989; Stucky, 1990). Gingerich (1984) proposed that the high peak of land mammal extinctions in the late Pleistocene of North America was an equilibrium response compensating for the high number of immigrations in the Late Pliocene and early Pleistocene; the 2-m.y. time lag tended to conceal the correlation. Others have explained RTE patterns in terms of climatic forcing. Webb (1983b) distinguished two types of rapid turnover episodes, the criterion being whether extinction waves or immigration waves peaked first, and suggested that the former (E-type RTE) were probably caused by climatic changes. He also showed that during the Neogene, six land mammal extinction episodes (including the two largest in North America) were correlated with positive oxygen isotope excursions in
GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS Figure 11.1 Land mammal immigration episodes in the Cenozoic of North America. Mammal ages, as correlated with Cenozoic Epochs, magnetic time scale, and time in millions of years, follow Woodburne (1987). First-order immigration episodes (with eight or more genera) are represented by triangles; second-order episodes (with five to seven genera) are indicated by rectangles. 187