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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 200 In the Siwaliks in situ evolution appears to occur in only a few lineages and is therefore thought to be unimportant in most groups. Immigration and extinction events tend to be correlated and together were the principal cause of faunal change. As immigration events often precede extinctions, and in some cases can be inferred to have caused them, immigration and the resulting ecological disruption may have been the primary cause of community change. Many of the immigrant species probably originated in Africa, Europe, or other parts of Asia. Faunal turnovers are thus also intervals of faunal exchange and indicate times when land connections were established. Nearly all of these episodes show approximate correlations to global climatic, oceanographic, and tectonic events, and these through their effects on sea-level, intercontinental connections, and vegetation, may have controlled movement of mammals into the Siwalik province. The earliest major immigration episode had already occurred before the first Neogene mammalian fauna is recorded in the Siwalik Hills, and led to the establishment of bovids and other ruminants (the dominant large herbivores and muroid rodents). Barry et al. (1985) estimate that this first episode occurred between 18 and 22 Ma, and Barry and Flynn (1989) correlate it approximately with the TB 2.1 sea-level fall of Haq et al. (1988). This episode coincides with similar (first-order) episodes of mammal immigration between Eurasia and North America, and between Europe and Africa. A second major immigration episode (between 15 and 13 Ma) involving muroid rodents and ruminant artiodactyls was accompanied by an abrupt rise in species numbers (Barry et al., 1991). This coincides approximately with the high diversity "savanna optimum" in North America, but does not correlate with any major immigration episode in North America. The next major faunal turnover episode observed in the Siwalik mammal succession occurs at 9.5 Ma when hipparionine horses appear and murids become dominant over cricetid rodents. Barry et al. (1985) suggested that this probably represents one of the major Middle Miocene isotopic and sea-level events discussed by Barron (1985). A major climatic shift toward increasing aridity and seasonality took place between 7.5 and 7.0 Ma, and caused a major turnover in Siwalik faunas, notably the replacement of large hominoids by cercopithecoid monkeys. The pattern of increased aridity, however, was time transgressive, so that contemporaneous faunas 300 miles to the southeast retained a more tropical woodland aspect (Barry and Flynn, 1989). Unfortunately, the Siwalik record does not adequately record the major faunal turnover episodes at the end of the Miocene. African Land Mammal Record In Africa, much of the Cenozoic record of mammalian immigrations and faunal turnover episodes is interrupted by too many hiatuses to represent a coherent record of immigration episodes. Nevertheless, during selected intervals, Africa provides an exceedingly valuable sample of a continent notable for being the evolutionary cradle of such important mammalian groups as proboscideans, ruminants, and hominoids. At the beginning of the Neogene, for example, Savage (1989, p. 592) recognizes in the Early Miocene at Gebel Zelten in Libya " . . . a tropical regime with open shrubland faunas . . . communicating across Tethys with Eurasia." Among the mammal groups with shared genera of about 20 Ma (Orleanian age of the European land-mammal scheme) are proboscidea, creodonts, carnviores, rhinoceroses, suids, and bovids (Vrba, 1985b). Particularly significant immigration episodes occurred in Africa at about 5.0 and 2.5 Ma. At the former time, apparent immigrants are the antelope groups (bovid subfamilies) Ovibovini, Bovini, and probably Reduncini; at the latter time, caprines (goats) and the genus Oryx constitute key immigrant groups (Vrba, 1985b). Coincidence of first-order immigration and rapid turnover episodes across several continents adds great force to the significance of North American episodes. At present, however, such generalizations can be developed securely only for three intervals in the Neogene. In absolute terms, these times are about 20, 5, and 2.5 Ma. These three well-corroborated rapid turnover episodes correspond to the following first-order immigration episodes in North America: Hemingfordian 1, Hemphillian 3, and Blancan 2. Oxygen Isotopes and Mammal Immigrations In an influential paper, Fischer (1983) introduced the concept of two Phanerozoic supercycles in which the Earth's climate has alternated between a greenhouse state and an icehouse state. In the Cenozoic the Earth has experienced its most recent change from greenhouse to icehouse. The broad outlines of this change are well known and have been revealed by shifts in the oxygen isotope ratios extracted from tests of foraminifera in marine sediment cores. The Cenozoic record for benthic foraminifera clearly displays a secular trend to more positive isotopic values, reflecting major cooling of bottom waters in the world ocean. There is general agreement that in the Early Eocene, Î´18O values were zero or negative; that these values became more positive throughout the Eocene; and that they declined sharply in latest Eocene, and again in Middle Miocene and latest Miocene. Another independent approach to the history of Cenozoic climates derives from seismic stratigraphic studies of
GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 201 passive continental margins. Haq et al. (1988) provided a recent summary of the apparent eustatic curve compiled from many well-dated seismic stratigraphic studies. The Cenozoic is represented by parts A and B of the Tejas megasequence and includes seven second-order supercycles. Each of these supercycles spans about 10 Ma and terminates at a sequence boundary of major magnitude. Shackleton et al. (1988), Williams (1988), Christie-Blick et al. (1990), and Miller et al. (1991) have carefully considered the relationships between isotopic and seismic studies as they bear on the history of Cenozoic climate. Potential pitfalls for the seismic method are confounding the effects of local sedimentary load or local tectonism with more global eustatic effects. For these reasons, we focus on the isotopic record as the more reliable signal of global climatic change during the Cenozoic. It should be noted, however, that for many intervals, there is substantial correlation between the seismic and isotopic approaches. Shackleton and Opdyke (1977) showed that during the late Pleistocene, oxygen isotope ratios covaried between tropical planktic and benthonic foraminifera. They interpreted the simultaneous changes in surface and bottom-dwelling foraminifera as evidence of growth and decay of ice sheets on the continents. Therefore, the Î´18O record represents change in both ocean temperature and ice volume. The problem has been to partition these two effects. Miller et al. (1991) recently extended the covarying isotopic studies to sediments of mid-Tertiary age, recognizing key intervals of ice buildup by covariant increases in heavy oxygen ratios in Miocene and even Oligocene cores. Prentice and Matthews (1988) have attempted to monitor Cenozoic sea-level change by analyzing oxygen isotope ratios in planktic foraminifera from equatorial regions. The efficacy of this approach depends on the assumption that equatorial sea- surface temperatures (away from upwellings) have not changed during the Cenozoic. Thus, they reason that observed isotopic changes wholly reflect waxing and waning of glacial ice. Figure 11.2 juxtaposes North American land mammal immigration episodes with the trace of Cenozoic oxygen isotope ratios based on planktic forams from equatorial regions. We discuss each of the continental first-order immigration episodes in relation to the marine record. The earliest pair of first-order immigration episodes are the Clarkforkian and early Wasatchian, straddling the Paleocene- Eocene boundary. Although they fall generally within the warmest climatic interval of the Cenozoic, they correlate with small cooling events in the isotope curve at about 59 and 56 Ma. The Clarkforkian immigration episode correlates particularly well with the abrupt cooling episode at the end of the Paleocene (59 Ma), demonstrated by Kennett and Stott (Chapter 5, this volume) in high-resolution data based on planktonic forams from the southern ocean. Miller et al. (1987) postulated a sea-level drop at this time, and Haq et al. (1988) recognized a second-order drop in sea-level (TA 222-24) in the Thanetian. The very large Wasatchian land mammal immigration episode reflects plate tectonic effects in the North Atlantic that established a very broad, low latitude corridor across the Thulean route and thus produced extremely close faunal resemblance between North America and Europe as discussed above. The next first-order episode occurs in the early Duchesnean (Late Eocene) at about 40 Ma (Emry, 1981; Krishtalka et al., 1987). This episode corresponds well to a number of global Late Eocene events. Miller et al. (1987) show a major oxygen isotope increase at about 40 Ma based on benthic forams, and this event is also seen in planktonic forams from equatorial cores presented by Prentice and Matthews (1988). These isotopic events correlate with the beginning of the major sea-level drop TA4 (Priabonian) of Haq et al. (1988). According to Hallam (1984) and others cited therein, this latest Eocene sea-level drop is greater than that of the Late Oligocene. The profound global climatic shift of the Late Eocene correlates with a strong increase in the Earth's thermal gradient due to cooling in the southern ocean (Kennett and Barker, 1990). During the Neogene, land mammal immigration episodes increased markedly in North America. Because of their importance and frequency, these Miocene and Pliocene immigrations were subjected to detailed analysis by Tedford et al. (1987). In Figure 11.3, we juxtapose the full array of Neogene land mammal immigration episodes (including third-order episodes) with the Î´18O excursions numbered by Miller et al. (1991). As suggested by Opdyke (1990) the episodes correlate remarkably closely with oxygen isotope events in the marine record. Only one isotope event (namely Miocene 3) fails to correlate with an immigration episode in North America. On the other hand, several immigration episodes in the Miocene of North America fail to correlate with any of the positive isotope excursion numbered by Miller et al. (1991). One first-order immigration episode (Arikareean 2 at 20 Ma) fails this correspondence test; as noted below it does correlate with a small (unnumbered) positive isotope excursion. Blancan 1 (at 4.8 Ma) also appears to be unrequited, but in fact it may correspond with Pliocene 1 of Miller et al. (1991). Other lesser immigration episodes at 14.5, 7.0, and 6.0 Ma do not correspond to numbered isotope excursions. The Early Miocene records the largest set of generic immigrations in the history of the North American land mammal fauna. Three land mammal dispersal episodes fall near the boundary between Arikareean and Hemingfordian: they begin with a second-order episode at 21 Ma
GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS Figure 11.2 First- and second-order land mammal immigration episodes in the Cenozoic of North America juxtaposed with the marine record of Î´18O from equatorial planktic foraminifera after Prentice and Matthews (1988). If sea-surface temperatures have not changed in equatorial regions during the Cenozoic, this curve is thought to reflect ice buildup and downward shifts in global sea-level. Note especially the significant correlations at about 40 and 20 Ma. 202
GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 203 followed by first-order episodes at 20 and 18 Ma. The episodes at 21 and 18 Ma coincide with isotope events Mi1b and Mi1c of Miller et al. (1991). The immigration episode at 20 Ma corresponds to a positive isotope excursion in both benthic and planktonic forams as identified by Prentice and Matthews (1988). According to the latter, sea-level at 20 Ma reached depths not attained again until the Pleistocene. Miller et al. (1987) show a possible erosional interval at 20-21 Ma, and Haq et al. (1988) record a first-order drop in the seismic curve between 20 and 21 Ma (TB2). On the other hand, the episode at 18 Ma does not correspond to any major sea-level drop noted in the seismic curve. Figure 11.3 Land mammal immigrations in the Miocene of North America, including first-order (triangles) as well as second- and third-order (rectangles) episodes, juxtaposed with Î´18O positive excursions as numbered by Miller et al. (1991) based on detailed study of the benthic foram record. Early Miocene land mammal immigrants, including three groups of ruminants, participated in the transition to savanna biomes in midcontinental North America, accompanied by a shift to seasonally arid climates and loess deposition, as discussed above. Presumably this Early Miocene savanna transformation was driven by a fundamental change of state in the global climatic pattern, most notably intensified deep water flow with opening of the Drake Passage and concomitant development of the Antarctic Circumpolar Current (Kennett and Barker, 1990). Further major shifts are evident in the Middle Miocene, but in North America, these do not include significant numbers of immigrants. Instead, the acme of land mammal diversity, dominated by horses and other savanna herbivores, is attained in the Barstovian. This savanna optimum on the continent correlates with the largest Middle Miocene shift in the Neogene Î´18O record. The strong string of positive isotope shifts in the marine record is widely believed to represent the buildup of permanent glaciers on East Antarctica (Kennett and Barker, 1990). Haq et al. (1988) record a major sea-level drop at 16 Ma with additional drops at 14 and 12 Ma. In Figure 11.4, we juxtapose the Neogene portion of the benthic foram isotope record after Miller et al. (1987) with the detailed record of large herbivore diversity and feeding modes in North America. The latter record, more fully discussed by Webb (1983a), shows the acme of herbivore diversity in the Barstovian, followed by a steep stepwise decline during the Clarendonian and early Hemphillian. The losses affected primarily browsing ungulates and, secondarily, grazing ungulates. Environmentally, these Middle and Late Miocene faunal changes represent the shift from savanna to steppe biomes, presumably a result of declining rainfall. A remarkably similar transition occurs concur Figure 11.4 Decline of ungulate genera in Miocene of North America (after Webb, 1983a) compared with Miocene record of Î´18O based on benthic foraminifera (after Miller et al., 1987). Note that the major Middle Miocene cooling episode coincides with the major extinction of browsers and mixed feeders about 12 Ma.