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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 193 of Old World (petauristine) type, eomyid rodents, and the very significant cricetid rodent, Copemys. New carnivores include weasel-like and otter-like mustelids, while the first true cats in the New World are represented by the genus Pseudaelurus (see Table 11.2). Immigration of four very large mammals had a major impact on the Middle Miocene ecosystem: these were two rhinocerotids (short-legged, aquatic Teleoceras and long-legged, terrestrial Aphelops) and also two proboscideans (the browsing mammutid Miomastodon and the probable mixed-feeding gomphotheriid, Gomphotherium). It is now generally supposed that such "megaherbivores" played a formative role in modifying the landscape, as do elephants in modern savanna ecosystems (Owen-Smith, 1988). Detailed evidence on this point, however, has not been developed in the Miocene of North America. The mid-Hemingfordian immigration episode took place about 18 to 17 Ma and was the last major episode until the latest Miocene, a span of about 12 m.y. (Tedford et al., 1989). Clarendonian Chronofauna: Grassland Savanna Land mammal diversity in North America reached its zenith during the Barstovian mammal age (Webb, 1989). Savage and Russell (1983) recognized 16 families with 60 genera and 141 nominal species of land mammals in the Barstovian. The next highest numbers occurred in the Clarendonian (the next mammal age), with 55 genera and 117 species. These mammal ages are thought to indicate a savanna optimum in North America, with a rich mosaic of trees, shrubs, and grasses supporting an extraordinary variety of large and small, grazing and browsing, ungulates. It is not uncommon during this savanna acme to collect in a single site 20 genera of ungulates of which half are Equidae (Webb, 1983a; Voorhies, 1990). Savage and Russell (1983, p. 300) noted that during this interval "the mammalian fauna . . . appears singularly homogeneous throughout its geographic range." The Clarendonian chronofauna spanned three mammal ages, an interval of more than 10 m.y. (Tedford et al., 1987). The array of ungulates in the Late Miocene of North America is comparable in many respects to that living in Africa today. The resemblance is purely convergent: the two faunas have no genera in common, and members of the few shared families play substantially different roles. For example, the body form and paleoecological context of Teleoceras, the most distinctive rhinocerotid perossodactyl in the Clarendonian chronofauna, indicate a niche analogous to that of Hippopotamus (an artiodactyl) in Africa. Voorhies and Thomasson (1979) confirmed by direct evidence from the hyoid (throat) bones that Teleoceras consumed grasses and, from the biocenosis in pond deposits at Poison Ivy Quarry, that herds of young and old frequented water. Likewise in North America the role of the African giraffe is played by the native giraffe-camel, Aepycamelus (Webb, 1983a). The most plausible explanation for these convergences would seem to be that the North American fauna evolved in a broadly comparable manner because of environmental conditions similar to those in Africa, namely, grassland savanna. Webb (1983a) and Janis (1984) independently attempted comparisons between North American Miocene and African Recent ungulate faunas, with emphases on body size and height (or volume) of cheek teeth. They found similar numbers of ungulate species distributed as browsers, roughage feeders, and mixed feeders. A rough estimate of biomass distribution among these categories, based on quarry censuses from major North American Miocene sites, also gave results that were remarkably similar to actual census data from African game reserves: browsers accounted for less than 10% of all ungulate biomass, whereas roughage feeders (with high hypsodonty indices) made up some 60 to 80% of the total. In a related study, Hulbert (1982) showed that population structure in a Clarendonian species of the grazing horse Neohipparion indicates that it underwent seasonal migrations, an essential feature of the strategy of African grazing ungulates. African ecological studies show that savanna biotas live where annual rainfall ranges between about 400 and 1000 mm and that, within this range, biomass is positively correlated with rainfall (Coe et al., 1976). Furthermore the number of ungulate species and the relative abundance of larger-bodied species interact with and are roughly correlated with primary productivity (McNaughton et al., 1988). These general observations in Africa shed light on mechanisms governing species richness and size-frequency distribution of ungulates in Miocene local faunas of midcontinental North America before and after the savanna optimum in the Barstovian. In midcontinental North America the Late Miocene ungulate diversity decline presumably tracks a decrease in mean annual rainfall. Floristic evidence supports the faunal evidence of widespread grassland savanna in the Middle Miocene of North America and increasingly arid climate in the Late Miocene. The Tehachapi flora of Hemingfordian age in California, the Stewart Valley flora of Barstovian age in Nevada, and the Kilgore flora of Barstovian age in Nebraska each has distinctive regional features, yet together they justify the conclusion (Wolfe, 1985, p. 371) that "during the Early to Middle Miocene, savanna developed in low latitude areas that are presently dry." Studies of as many as 18 species of fossil grasses directly associated with mid-Hemphillian ungulates in the Minium Quarry in Kansas also strongly indicate a grassland savanna biome (Thomasson, 1986; Thomasson et al., 1990). In the same floras are leaves with stomata and other detailed features
