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OVERVIEW AND RECOMMENDATIONS 13 Not only during the Pliocene but throughout much of the Cenozoic Era, mammals experienced pulses of evolutionary turnover that produced stepwise net increases in the relative number of species adapted to savannas, grasslands, or steppes. The first pulse of evolutionary turnover came at the end of the Eocene, when the extinction event that also affected the marine realm removed numerous browsers, including the huge, rhino-like titanotheres. New mammalian taxa included numerous taxa adapted to eating coarse fodder (see Webb and Opdyke, Chapter 11). By mid-Miocene time, the diversification of taxaâadapted to grassy habitatsâhad produced the greatest North American land mammal diversity of all time, in savannas that were the biotic equivalents of those in Africa today. Continuation of the trend produced drier grasslands and steppes with lower mammalian diversities later in the Neogene. Although post-Eocene pulses of turnover for Cenozoic mammals have not yet been shown to correlate well with particular floral shifts, they have been correlated with isotopic evidence of glacial expansion. Efforts to associate turnover with global climatic changes are complicated by regional trends produced by major tectonic events, such as the uplift of the Sierra Nevada, the Colorado Plateau, and the Himalayan Plateau. Similarly, although the spectacular diversification of grasses and other plants adapted to dry, seasonal habitats clearly resulted from the general post-Eocene climatic trends, intervals of diversification have not as yet been associated with pulses of extinction of moist-adapted forms. Delayed Recovery Severe extinctions that are not largely offset by simultaneous immigration or speciation result in impoverished ecosystems that sometimes persist for millions of years. Several factors can contribute to delayed recovery. Sometimes a delay results from a dearth of taxa capable of responding to the opportunity created by severe extinction. A striking example is the absence throughout Mississippian and Pennsylvanian times of a framework-building reef community to replace the tabulate-stromatoporoid community that had been devastated in the Late Devonian mass extinction (James, 1984). Contrasting with this situation was the rapid diversification of sclerophyllous terrestrial plants (forms with reduced leaves and thickened cuticles) in Australia after the Eocene (see Christophel, Chapter 10). These taxa seem to have originated in nutrient- poor soils at the margins of Paleogene rain forests and were, in effect, poised for rapid evolutionary response when, because of aridification, soils deteriorated over a broad area of the continent. Similarly, the mammals' evolutionary recovery from severe Late Eocene extinction in the Northern Hemisphere was accelerated by the fact that a variety of mammalian taxa with high-crowned teeth adapted for grazing on coarse vegetation had already evolved during the Eocene, prior to the severe climatic change (see Webb and Opdyke, Chapter 11). For reasons that remain to be explained, small brachiopods that occupied the chalky seafloor of western Europe attained their former diversity within about 1 m.y. after the terminal Cretaceous extinction (see Figure 2). In general, delayed recovery from severe extinction has typified the marine realm. After the severe extinction of Late Eocene and Early Oligocene times, for example, marine faunas remained relatively impoverished throughout the Oligocene. Delayed marine recovery appears to have two primary causes. One is the inherently slow rate of adaptive radiation that characterizes many taxa of marine animals. The other is the typical failure of postcrisis conditions in the marine realm to stimulate the adaptive radiation of new kinds of taxa adapted to these conditionsâto provide a new resource base comparable to productive savannas on the land (see Stanley and Ruddiman, Chapter 7). Even as overall mammalian diversity declined in North America after mid-Miocene time, certain mammalian and other taxa favored directly or indirectly by aridification underwent spectacular adaptive radiations: songbirds and Old World rats and miceâtwo groups that included many species that fed on the seeds of the
OVERVIEW AND RECOMMENDATIONS 14 Figure 2 Diversity of small brachiopods in the chalk of western Europe. About three-quarters of the species died out suddenly in the terminal Cretaceous extinction, but a larger number of new species then originated very rapidly, during the next million years (from Johansen, 1988). Figure 3 Proliferation of adaptive radiation upward from the base of the food web in terrestrial ecosystems that were favored by the global trend toward aridification during the past 25 m.y. (from Stanley, 1990). Many songbirds and Old World rats and mice feed on the seeds of taxa of grasses, herbs, and weeds that diversified dramatically during the spread of grasslands. The large majority of modern snake species belong to the family Colubridae, and many of these feed on rats and mice or on songbird eggs and chicks.