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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 199 Although Anchitherium came from North America, it reached Europe by way of Asia. Early Orleanian correlates with lower Hemingfordian and with the first-order immigration episode at about 20 Ma in North America. The Vallesian-Astaracian boundary has been the subject of controversy, and that controversy is entwined with evidence regarding the first appearance of Hipparion and other hipparionine horses from North America via Asia. The genus Hipparion appears on the margins of the Mediterranean at 11.5 Ma in Chron C5R, which would make the base of the Vallesian at least this old. This date correlates precisely with the base of the Clarendonian assigned by Tedford et al. (1987) on the basis of North American biostratigraphy and many radiometric dates. Few immigrants, however, appear in North America at that time. The base of the Turolian is marked by many immigrant appearances such as the rabbit, Alilepus, and the proboscidean, Anancus, and is well dated at 9 Ma (Berggren et al., 1985; Mein, 1989). This date coincides with the beginning of the Hemphillian in the North American record and with the second-order immigration episode here recognized as Hemphillian 1 (Table 11.2). The beginning of the European Ruscinian mammal age correlates with the latest Hemphillian in North America (Tedford et al., 1987). The Ruscinian mammal age, approximating the Miocene-Pliocene boundary, is now well dated in the Mediterranean area, where that sea refilled. Ruscinian is defined by the many new appearances that occurred as a consequence of the Messinian sea-level cycle and the severe climatic deterioration that accompanied it. In North America, we recognize Hemphillian 3 as a first-order immigration episode, correlated with the Messinian sea-level cycle (Webb, 1983b). The early Villafranchian is marked in Europe by a major turnover episode including the arrival of Equus there (as well as in Asia), accompanied by the appearance of Elephas, various large deer, and also as Azzaroli (1989, p. 341) notes, "the demise of a forest assemblage characterized by Tapirus, Sus, and Ursus." This episode may be correlated with the first-order immigration episode known as Blancan 2 in North America. Presumably these equivalent events were driven by the same climatic mechanisms, namely, the onset of Northern Hemisphere glaciation. By the Pleistocene, North America and Europe lack any meaningful correspondence between their respective mammal ages. Possibly the frequency of faunal migrations and climatic shifts after the Gauss Chron will require a much more refined system than the present broadly defined mammal ages. Repenning (1987) and others have found much finer-tuned rodent zonations to be quite useful in relating mammal faunal turnover to global climatic changes in the Quaternary. Chinese Land Mammal Record Correlations of the rich land mammal succession in China are rapidly improving and already can test, in some instances, the hypothesis of global synchroneity of rapid faunal change episodes among land mammals. Current studies of Oligocene faunas in China suggest that Asia may have been the source of many immigrants in the European Grande Coupure after the marine seaway retreated from the Turgai Straits. In his review of Neogene mammalian biochronology in China, Qiu (1989) recognizes four major immigration episodes that alter the character of the Asian fauna. First, the nearly simultaneous immigration of Anchitherium from North America and Gomphotherium from Africa marks a major change in the Early Miocene fauna. Qiu (1989) also notes the brachyerycine insectivores as another trans-Beringian immigrant group that appears in the Early Miocene of China. The second major Miocene immigration episode from outside Eurasia was the appearance of Hipparion from North America. The importance of the Hipparion datum is widely recognized throughout Eurasia and may occur at about 12 Ma. Thirdly, the Gaozhuang fauna of Miocene-Pliocene age in the Yushe Basin is under intensive study and represents one of the best understood and best-dated immigration episodes in Asia. Among key immigrants are an arvicoline rodent Germanomys; the rabbit Hypolagus; the canid Nyctereutes; the pig Sus; and the camelid Paracamelus (Qiu, 1989; Tedford et al., 1989). There is a fairly close correlation between the Gaozhuang fauna in China and the Ruscinian in Europe. Finally, the Late Pliocene immigration episode in China closely corresponds to that in Europe, key immigrations being those of Elephas from Africa and Equus from North America. As in North America, this episode occurs at about 2.5 Ma near the base of the Matuyama Chron (Qiu, 1989). Ding et al. (1992) show that Hipparion-bearing red clays were transitional to the loess sequence in which these immigrants appear and that these events were coupled with major uplift of the Himalayan Plateau. Indian Land Mammal Record One of the best-studied mammal-bearing sequences in the world is that of the Siwalik Hills of Pakistan and India. Long, nearly continuous sections, extensively exposed and richly fossiliferous, yield a virtually complete magnetostratigraphic column for the Miocene. It is quite interesting, therefore, that Barry and Flynn (1989, p. 567) attribute the pulse of faunal turnover in that region primarily to immigration episodes triggered by land bridges; they summarize that relationship as follows: