a number of "holdover taxa"—such as the peccary, Mylohyus; the protoceratid, Kyptoceras; and the three-toed horse, Cormohipparion—which were already extinct in the midcontinent. The ecological balance of these holdovers suggests persistence of seasonal savannas and extensive mesophytic forests in warmer, more productive settings than in the High Plains (Robertson, 1976; Webb, 1978; Hulbert, 1987). In general, the Late Pliocene array of biomes in North America approximated the degree of provincialism seen at present, although the extremes of freezing winters and arid deserts were absent.

In the Late Pliocene (Blancan 2) the first large wave of immigrants from South America appeared, including glyptodonts, chlamytheres, armadillos, ground sloths, capybaras, and porcupines. At the same time, another large wave of immigrants came from Asia; among them were the spectacled bear (Tremarctos) and several rodents (Synaptomys, Pliopotamys, and Mictomys) (Lundelius et al., 1987). This final immigration episode of the Tertiary coincided with expansion of the Bering bridge. It clearly correlates with the onset of Northern Hemisphere glaciation, sea-level lowering, and evidence of pre-Nebraskan till in western Iowa (Lundelius et al., 1987; Repenning, 1987).

The late Hemphillian and early Blancan immigration episodes just considered adumbrate an increasingly active series of Quaternary immigration episodes. High latitude Quaternary studies on both sides of the Beringia land bridge record trends among microtine rodents, as well as many large mammals, such as mammoths and musk oxen, toward steppe-tundra adaptations (Sher, 1974; Repenning, 1985; Herman, 1989). The complex pattern of faunal dynamics among Quaternary mammals is well summarized by Tedford et al. (1987, p. 192): "The accelerating pace of immigration from Hemphillian into Pleistocene time reflects the availability of dispersal routes to North America and increasing environmental instability, particularly at high latitudes, that provided the goad for the movement of mammals."

We analyzed the record of immigrations in successive North American land mammal faunas, as a possible signal of major environmental changes in the history of that continent. We selected this continent because it has the most complete Cenozoic record of terrestrial mammalian history. We tallied the number of land mammal genera that apparently immigrated in each of 52 informal subdivisions of 19 land mammal ages following the biostratigraphic scheme presented in Woodburne (1987). In this review, we have not dealt with other diversity changes due to extinctions and endemic radiations. We found that the two largest episodes (Wasatchian 1 and Hemingfordian 1) involved 15 and 14 genera, respectively; five other episodes also involved nine or more genera (Table 11.1). We arbitrarily designate these seven largest as first-order immigration episodes. We categorize another cluster of seven episodes, consisting of five or seven genera each, as second-order episodes. All smaller counts were considered third-order or "background" immigration patterns. The numbers of genera in first-order and second-order episodes are documented in Table 11.1. The only third-order episodes we were able to tally accurately are the five from the Miocene. The genera counted in the seven first-order immigration episodes are listed in taxonomic order in Table 11.2.

An unexpected result of this analysis is the paired pattern among six of the seven first-order immigration episodes. Except for the Duchesnean, each first-order episode occurs within about two million years of another first-order episode. The three pairs are Clarkforkian with Wasatchian, Hemingfordian 1 and 2, and Hemphillian 3 and Blancan 2. The first-order status of the Duchesnean immigration episode may be artifactual. This interval, and the preceding Uintan, span about 10 m.y., much of it represented by sparsely fossiliferous sediments. The nine immigrant genera recognized here are thought to be clustered in the late Duchesnean (Emry, 1981; Krishtalka et al., 1987), but this may well be an artifact of a relatively