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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 204 rently in the land mammals of the Indian subcontinent: from the herbivore acme in woodland savanna environments, a stepwise decline takes place that also represents a shift toward more arid environments between 16 and 12 Ma (Barry et al., 1991). The Middle Miocene patterns on land and sea strongly support an interpretation of climatic forcing (driven by attainment of permanent glaciation in East Antarctica) of land mammal faunal evolution in temperate latitudes. In the Late Miocene of North America, beginning with the Clarendonian/Hemphillian boundary at about 8.5 Ma, three second-order immigration episodes occur. The episode at 8 Ma coincides with isotope event Mi7 of Miller et al. (1991) and roughly approximates the drop in sea-level recognized as TB3.2 of Haq et al. (1988). Of the two further migratory episodes at roughly 7 and 6 Ma, only the latter corresponds to a reported marine event, namely that shown by Miller et al. (1991) at 5.8 Ma. Kennett and Barker (1990) suggest that the later Miocene was the interval during which West Antarctica became permanently glaciated. The Miocene-Pliocene boundary is associated with a first-order immigration episode in land mammal faunas of both North America and Europe. This episode correlates well with a positive shift in δ18O (Hodell et al., 1986), but does not seem to correlate with any episode of sea-level lowering in the Haq et al. (1988) curve. Possibly sea-level is beginning to fluctuate at high frequency in the Late Miocene and Pliocene, with the consequence that it becomes difficult to define any particular pulse with seismic stratigraphy. On the other hand, the Messinian is well known in the Mediterranean region and in the isotopic record as a major glacioeustatic cycle (Adams et al., 1977; Kennett, 1985; Hodell et al., 1986). As emphasized by Webb (1983b), the Messinian can be tightly correlated with the mid-Hemphillian extinction and faunal turnover episode in temperate North America. The last second-order episode of the Tertiary sweeps through land mammal faunas on most continents at 2.5 Ma in the Late Pliocene. Such major immigrations are clearly related to the onset of Northern Hemisphere glaciation recognized in the δ18O record and also indicated by glacial sediments (Ruddiman and Raymo, 1988; Shackleton et al., 1988; Raymo and Ruddiman, 1992). All of these features correlate with marked cooling and sea-level lowering indicated by the oxygen isotope record (Shackleton and Opdyke, 1977). Generally speaking, land mammal immigration episodes in North America and other well-studied continental areas correlate with positive excursion of the oxygen isotope record from tropical marine forams. Other continental regions clearly corroborate the significant turnovers recorded at about 20, 5, and 2.5 Ma. In some cases, however, the terrestrial turnover pattern gives no evident response to major cooling events. The two most glaring examples are the Early Oligocene (White River chronofauna) and the Middle Miocene (Clarendonian chronofauna) in North America when, despite major global changes, the mammal faunas show remarkable stability and continuity. Possibly no land bridges were available during the Early Oligocene. In the Middle Miocene, this excuse cannot serve, because a few Asiatic immigrants did trickle in and Beringia was emergent. Evidently, during these two stable chronofaunal stages the North American mammalian fauna was resilient and not open to immigrants. CONCLUSIONS The rich record of Cenozoic land mammals in North America documents a large-scale succession of terrestrial ecosystems. In the midcontinent the predominant biomes changed from subtropical forest in the Eocene through woodland savanna in the Oligocene, to park savanna and then grassland savanna in the Miocene, to steppe in the Pliocene, when increased provincialism became evident. This broad march of faunal change has been inferred not only from dental adaptations and body-size distributions in the mammalian faunas, but also from independent lines of terrestrial evidence including floral succession, sedimentology, and stable isotopes. Clearly the continental biota in North America broadly tracks the global climatic trends of the Cenozoic. More specific faunal analyses of the well-calibrated North American record reveal a remarkably irregular progression of faunal change. Immigration of land mammals from Europe, Asia, and South America, totaling some 140 genera in the course of the Cenozoic, are concentrated mainly in seven first-order episodes. Pairs of first-order episodes bracketing 57, 19, and 4 Ma suggest that a major interval favoring immigration somehow produced an ''echo" effect. The largest of all immigration sets occurred in the Early Miocene when two first-order episodes and one second-order episode arrived between 21 and 18 Ma. Land mammal records from other continents tend to replicate strong mammal immigration episodes at 20, 5, and 2.5 Ma. Other resemblances include a Middle Miocene acme in the Siwaliks, possibly correlated with the Barstovian acme in North America, but this interval has a notably low rate of intercontinental immigrations. Causal considerations lead to two possible hypotheses linking mammal immigrations to patterns of global climatic change. The first hypothesis postulates greater availability of land bridges due to low sea-levels, given the correlation between positive oxygen-isotope excursions and eustatic drops in the marine record. The second, more
