the marine record and thus with global climatic episodes (Webb, 1984). When chronofaunal stability prevails, member taxa generally experience stabilizing selection. Only when climatic change perturbs the terrestrial ecosystem sufficiently does it produce an RTE.
Wing and Tiffney (1987) described similar patterns in floristic stability and turnover. They found that "the chief mechanism for drastic reorganization in community structure" was "severe climatic change." A similar view was formally proposed as the turnover-pulse hypothesis by Vrba (1985a, p. 232) in the following terms:
Speciation does not occur unless forced (initiated) by changes in the physical environment. Similarly, forcing by the physical environment is required to produce extinctions and most migration events. Thus, most lineage turnover in the history of life has occurred in pulses, nearly (geologically) synchronous across diverse phylogenies, and in synchrony with changes in the physical environment.
This view underlies our analysis of immigration episodes in land mammal faunas. We attempt to test such terrestrial turnover patterns by comparing them with the isotopic record of Cenozoic climate change, which is derived independently from marine sediments.
In the following analysis we take the number of immigrant genera in each land mammal age as a key to faunal turnover episodes. Immigrants (first appearances) are recognized on the basis of two criteria: (1) the absence of that genus or closely related genera in prior North American records and (2) the presence of related forms (sister group) earlier in another continental fauna. Mammalian biostratigraphers have increasingly focused on immigrant taxa as the best diagnostic basis for defining land mammal ages, as well as informal subages or zones (Woodburne, 1987). Thus, for both its environmental and its stratigraphic significance, the record of immigrant mammals in North America has been well documented.
Figure 11.1 identifies major episodes of mammalian immigrations into North America during the Tertiary; we designate as first-order immigration episodes those that number nine or more genera. Second-order episodes are those with five to seven genera. These data are more fully explained in the following sections on results. Biostratigraphy and chronostratigraphy follow Woodburne (1987). These data will continue to be refined, by addition of immigrant genera, by extension of their known ranges into earlier intervals, and by refinements in the geochronological framework.
Three land bridges controlled the access of Cenozoic land mammals to North America. In the latest Paleocene and early Eocene the North Atlantic Thulean route between Europe and North America provided the basis for a strong faunal resemblance between these two continents. The next major immigration episode into North America took place in the Late Eocene. The mammals of this episode may have come across the Bering Strait, but more probably they again tracked the Thulean land bridge, which did not founder to great depth until the Oligocene (Rowley and Lottes, 1988). Subsequent episodes of faunal dispersal from the Old World must have crossed via the Bering land bridge. The very high latitude of this route (at the North Pole in the Cretaceous) placed increasingly severe ecological constraints on the immigrants that traversed it. Even as Beringian access shifted to somewhat lower latitudes during the Neogene, climatic regimes became more severe, so that Beringia continued to function as a narrow ecological filter. During the Pliocene (Blancan land mammal age) the Bering land bridge provided passage for temperate woodland species, such as an extinct deer (Bretzia) and an extinct panda (Parailurus), to the Pacific Northwest (Tedford and Gustafson, 1974). By the Pleistocene, however, the Bering filtered out all but steppe and steppe-tundra species, with the exception of humans who by then had developed highly specialized cultural adaptations to life in Beringian steppe-tundra.
A third route for intercontinental access to North America came from the south via the Panama (isthmian) land bridge. By Late Pliocene time it facilitated immigrations from South America into North America, mainly representing species adapted to subtropical savanna (Stehli and Webb, 1985).
In the following section we review the environmental history of mammalian faunas in North America, with an indication of the predominant biome during each chronofauna. We also briefly discuss the immigration (and faunal turnover) episodes that punctuate the chronofaunal succession. A major unresolved problem stems from the different durations of the land mammal ages in current use. The Duchesnean, for example, may span as much as 4 m.y. and the preceding Uintan as long as 6 m.y. Although it might be appropriate to treat immigrations as a rate by making the assumption that immigration events were uniformly distributed through a mammal age, in the present context we make no a priori assumptions about how immigrations may have been distributed within a mammal age subdivision, placing them simply at the stratigraphic level advocated by the best available empirical studies. We follow with the caveat that long intervals are more likely to produce artificially high immigration numbers, and note the need for continued refinement of fossiliferous local sections, especially in the mid-Cenozoic. Before we examine the possible correlations between immigration episodes and global climatic shifts, we briefly review the history of changing biomes in North America.