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.
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.
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, d18O 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