In the Middle Eocene the first indications of seasonal aridity appeared in the Rocky Mountain region. The great lakes of the Green River region, such as Lake Gosiute, formed extensive evaporites and were encroached on by deeply oxidized redbed deposits. By the Late Eocene, woodland savanna had become the predominant biome in the midcontinent (Webb, 1977; Wing and Tiffney, 1987). The dramatic shift from subtropical forest to predominantly woodland savanna in the Rocky Mountain region soon led to major faunal changes. It is unfortunate that this important turnover interval is not well documented by continuously fossiliferous sedimentary sequences.
The Duchesnean land mammal age is marked by a major faunal turnover episode, including the last appearances of such archaic groups as Condylarthra, Tillodontia, and Dinocerata. More important are the arrivals of the first eubrontothere (Duchesneodus) and several more modern taxa evidently dispersed from Asia (Emry, 1981; Krishtalka et al., 1987). Stucky (1990) recognizes a total of nine genera in Duchesnean 1 and 2. After the Duchesnean the number of browsing herbivore genera rose from 8 to about 40 (Stucky, 1990), and the species numbers of all herbivores cited in Savage and Russell (1983) rose from less than 40 to about 90 during the Eocene-Oligocene transition and remained at this level throughout the Oligocene (Webb, 1989). Emry (1981) labeled this persistent mammalian fauna of the Late Eocene and Oligocene the "White River chronofauna."
The Duchesnean immigration was preceded by a number of other immigration events apparently spread through the Uintan. Unfortunately this interval is about 6 m.y. long, and particular first appearance data are not tightly correlated. We arbitrarily place a second-order episode near the mid-Uintan. The following modern mammal families appear in North America during the Late Eocene: soricid insectivores; sciurid, castorid, cricetid, and heteromyid rodents; leporid lagomorphs; canid and mustelid carnivores; and ungulates-camelids, tayassuids, and rhinocerotids. Several of the newly appearing groups can be shown to have entered North America from Asia, among them the rabbits (Mytonolagus), the amynodont rhinocerotids, and most of the selenodont artiodactyls including camelids, hypertragulids, leptomerycids, and oreodonts (Webb, 1977; Webb and Taylor, 1980; Emry, 1981).
Most of the Late Eocene immigrant herbivores are characterized by adaptations for masticating coarse fodder. At least some members of the following groups developed hypsodont dentitions during the Late Eocene: taeniodonts, leporids, castorids, eomyids, rhinocerotids, hypertragulids, oromerycids, and "oreodonts" (Webb, 1977). For example, Wood (1980, p. 38) characterized the cheek teeth of the eomyid genus Paradjidaumo as ". . . more hypsodont and progressively more lophodont than in Adjidaumo." Also, although most oromerycids are brachydont, Prothero (1986, p. 461) observed that in the new genus Montanatylopus "the molars are much more hypsodont than in any other oromerycid." Thus it is fair to recognize the Late Eocene and Oligocene White River chronofauna as the first in North America to sustain a substantial diversity of hypsodont herbivorous mammals.
The larger mammalian herbivores may be divided into two broadly distinct habitat groups: one group lived primarily along watercourses; the other lived mainly on the interfluves. The flat-skulled leptaucheniine "oreodonts" and the trunk-bearing amynodont rhinos were short-legged, semiamphibious forms that cropped lush vegetation in or near stream courses (Scott, 1937; Wall, 1982). On the other hand, most selenodont artiodactyls, as well as the horse Mesohippus, had relatively long slender limbs and ranged widely in open habitats. Several studies of faunal facies in the White River deposits, reviewed in Webb (1977), provided direct statistical evidence that selenodont artiodactyls and rabbits occurred predominantly in open-country or upland habitats.
Several of the White River ungulates, notably Leptomeryx, ranged together in herds (Clark et al., 1967). The adaptive relationships among social behavior, body size, and feeding mode, developed by Estes (1974), Jarman (1974), and others on the basis of the modern African ungulate fauna, suggest that the appropriate comparison for leptomerycids and other selenodonts is with moderate-sized herding forms such as the gazelles (Jarman's category C). Such forms are mixed feeders, relying on grasses only in their most nutritious new-growth stages and shifting to browsing in the dry season (Janis, 1982).
A third adaptive zone, that of the small burrowing herbivore (rhizovore) and insectivore, is extensively occupied during the White River chronofauna. This implies extensive development of well-drained soils supporting shrubby vegetation. At the same time the diversity of arboreal mammal genera declines (Webb, 1977; Stucky, 1990).
With such mammalian evidence in mind, one may look for other indications that savannas were opening the landscape of the Late Eocene and Oligocene. Hutchison (1982) recognized the severe impact of increasing aridity and seasonality on the aquatic reptile fauna during that interval in the Rocky Mountain region, and in Early Oligocene floras of North America, notably the Florissant in Colorado, the dramatic decrease in the percentage of entire-margined leaves indicates approximately a 10°C drop to about 12.5°C mean annual temperature (MacGinitie, 1962; Wolfe, 1985). Retallack's (1983) pedological studies of White River sediments provide a fascinating look at local paleosols underlying various habitats, and hint at an overall trend toward increasingly grassy and shrubby environments following the Early Oligocene climatic deterioration.