indicative of the subfamily Chloridoideae and of C4 photosynthesis characteristic of tropical floras growing at an optimum temperature of 30 to 32°C (Thomasson et al., 1986). Such floristic studies suggest that the Late Miocene climatic regime in midcontinental North America resembled that of seasonally arid subtropical savannas in South and Central America today.
About 5 Ma, at the end of the Miocene, the long-term trend toward increasing aridity in midlatitudes led to the final breakup of the extensive North American savanna biome and its replacement in midcontinental North America by extensive grasslands or true steppe. Ungulates experienced a series of decimations that spanned the Hemphillian; and the first major decline (at about 12 Ma) affected browsers more than grazers (Webb, 1983a, 1984; see also Figure 11.4). By the end of the Hemphillian the number of ungulate genera was cut to less than half of its former peak. No new chronofauna was established, but a major immigration episode coincided with the final late Hemphillian extinctions.
Floristic changes at the end of the Miocene clearly record the expansion of grasslands in midcontinental and western North America. For example, Wolfe (1985, p. 371) notes that in the Hemphillian of Kansas "the fructifications include abundant grass, and the leaf-assemblages are the low-diversity type that typically occurs along streams in unforested grassland regions." In western Wyoming, Barnosky (1984) recognizes xeric shrub floras during the latest Miocene. The Mt. Eden flora in southern California suggests the beginning of steppe conditions (Wolfe, 1985). Axelrod's (1992) floristic review shows that in the Great Basin and the midcontinent, the most arid interval of the Tertiary occurred at about 6 Ma. Renewed Cordilleran tectonics during the Miocene-Pliocene greatly increased rain shadow effects and provincialism in many regions of temperate North America.
The vast herds of ungulates themselves probably accelerated the decline of savanna and the expansion of steppe in the Pliocene. In Africa during extended droughts, elephants and/or equids devastatingly transform savanna to steppe (Sinclair, 1983; Owen-Smith, 1988). In North America, many late Hemphillian and Blancan fossil quarries (e.g., the Hagerman Horse Quarry in Idaho) yield abundant samples of only one or two grazing equids. These suggest (but do not demonstrate) scenes of massive overgrazing. A positive feedback loop between faunal and floral evolution may have accelerated the process that established steppe conditions in the Early Pliocene.
It should be noted that the Early Pliocene expansion of steppe environments did not encompass all of North America. Provincialism is far more evident than in the Miocene. In the Pacific Northwest, faunal and floral evidence clearly indicates continuation of a mesic forest biome although it interdigitated with semiarid savannas in the Great Basin (Tedford and Gustafson, 1974). The biota around the Gulf of Mexico also indicates the continuation there of savanna conditions (Webb, 1989). The remarkable resemblances between such Hemphillian faunas as the Yepomera local fauna in Chihuahua, Mexico, and the Bone Valley fauna in Florida give evidence of a broad subtropical savanna south of latitude 30°N, which was perpetuated by the persistence of summer monsoons (Webb, 1977). Thus, the Pliocene shift to steppe biomes in the midcontinent produced greater provincialism among diverse regions of North America.
Onto such a scene of regional desiccation and increased provincialism burst a new wave of Asiatic immigrants. As indicated below (Table 11.2), these immigrations appear to be concentrated in the latest Hemphillian, about 5 Ma. Key groups of land mammals that crossed the Bering at this time were vole-like rodents, both terrestrial and aquatic (Repenning, 1987); at least six carnivores, including exotic groups such as hyaenids (Chasmaporthetes) and pandas (Parailurus), as well as more familiar groups such as Lynx and Ursus; and also at least two genera of deer, which gave rise to a substantial radiation in North and South America.
The secular trend of the Cenozoic toward colder, drier, and in other ways more extreme conditions led to the environments of the later Pliocene and Pleistocene. As the first Laurentide glaciation settled over North America about 2.5 Ma, forest tundra biomes became widespread in the Arctic Circle (Funder et al. 1985; Repenning, 1985). Mountain building in the Cordillera extended tundra corridors along high-elevation routes into temperate latitudes. Rain shadows helped extend cold-steppe habitats into many basins of western North America. In the Snake River Plain (as represented by the Hagerman fauna and flora), there were still broad-leaved deciduous forests. To the north and at higher elevations, however, conifers predominated (Lundelius et al., 1987). The southern plains (as represented by the Mt. Blanco fauna and the Rita Blanca flora) were characterized by a seasonally arid climate that produced grassy scrubland and an impoverished ungulate fauna dominated by grazers. Widespread caliche deposits in Late Pliocene deposits of the Great Plains have long been recognized as indicators of seasonal aridity (Hibbard and Taylor, 1960).
In Florida and presumably the rest of the southeastern United States, vertebrate faunas of late Blancan age carry