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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 190 orders, Multituberculata, Insectivora, Primates, and Condylarthra, held over from the Cretaceous, comprise almost the entire Paleocene fauna. Analysis of virtually complete skeletal remains of the multituberculate genus Ptilodus demonstrated that its arboreal adaptations converged closely on those of a tree squirrel (Jenkins and Krause, 1983). The Condylarthra doubled and redoubled their generic numbers every few million years, and soon accounted for several distinct families including some moderately large herbivores (Van Valen, 1978). The distant forerunners of Carnivora (sometimes placed in their own extinct order, Creodonta) appeared by mid-Paleocene. By Late Paleocene, three relatively rare orders of larger mammals immigrated from Asia: the Pantodonta were partly amphibious molluscivores; the Taeniodonta were precociously hypsodont herbivores; and the Dinocerata were the first horned, browsing, herd animals of the Cenozoic. The Torrejonian mammal age included seven immigrant genera if one combines Torrejonian stages 2 and 3, as registered by Stucky (1990). Predominant environments of the Paleocene land mammals were multistratal evergreen forests and cypress swamps extending north to at least 70Â°N latitude (Wolfe, 1985; Wing and Tiffney, 1987). During the Tiffanian (ca. 63 Ma), cooler climatesâevidenced by increased percentage of deciduous trees and decreased diversityâproduced "dramatically lower species richness and evenness" (Rose, 1981, p. 386), presumably because there were less reliable vegetational resources on a year-round basis. Krause and Maas (1990) introduced an enlarged data set on this interval that may weaken this conclusion. The cooler climate and lower mammal diversity persisted into the Clarkforkian. Small mammals that had previously predominated became relatively rare, while larger forms such as phenacodont condylarths and carnivorous mammals increased in abundance. More importantly, the beginning of the Clarkforkian registers a first-order immigration episode, including Asiatic origins of the extinct order Tillodontia, the modern order Rodentia, and the pantodont family Coryphodontidae (Rose, 1981; Krause and Maas, 1990). Nine immigrant genera reached North America, if one combines Stucky's (1990) records for units 2 and 3. Although the Clarkforkian records the earliest first-order immigration episode in the history of North American land mammals, it is soon superseded by an even larger episode. Eocene Chronofauna: Subtropical Forest The wave of immigrations that began in the Clarkforkian intensified in the earliest Eocene or Wasatchian land mammal age. Rose (1981, p. 379) commented as follows: The most striking aspect of the Wasatchian assemblages is their domination by several taxa that were unknown or exceedingly rare before the Wasatchian. Most of these taxa appeared for the first time in North America at or near the beginning of the Wasatchian and rapidly became the most common members of the fauna. . . . They include the first known members of the orders Perissodactyla (Hyracotherium) and Artiodactyla (Diacodexis), the primate families Adapidae (Pelycodus) and Omomyidae, and the creodont family Hyaenodontidae. Land mammal immigrants of the early Wasatchian (see Figure 11.3 and Table 11.2) constitute the largest immigration cohort in the North American record. In Wasatchian 1 alone Stucky (1990) records 10 immigrant genera. Krause and Maas (1990, p. 90) more cautiously acknowledge that "many of the early Wasatchian first appearances appear to be immigrants from other continents." Mammals evidently crossed the North Atlantic freely in both directions via the Thulean bridge. According to Savage and Russell (1983), at least 50% of mammalian genera in the Sparnacian of Europe were shared with the Rocky Mountain region, by far the closest degree of transatlantic resemblance that occurred at any time during the Cenozoic. The early stages of the Wasatchian are well constrained by stratigraphic studies in the Bighorn Basin of Wyoming. There the base of the earliest Wastachian lies within the reversed magnetic interval below stratigraphic anomaly 24. For this reason, its age lies between 56 and 57 Ma (Butler et al., 1991). Climatic conditions inferred from faunas and floras of this time suggest that warm equable conditions had returned during the Early Eocene (Rose, 1981; Wolfe, 1985). Primates and other arboreal mammals reach peak richness and abundance by the Middle Eocene, declining thereafter (Stucky, 1990). Rodents and Perissodactyla diversify profoundly, each accounting for about 20% of known mid-Eocene species (Savage and Russell, 1983). Even within the Arctic Circle on Ellesmere Island, diverse arboreal, frugivorous mammals, such as prosimian primates and dermopterans (frugivores distantly related to Asiatic flying foxes), persist along with a subtropical flora (West and Dawson, 1978). White River Chronofauna: Woodland Savanna In the continental record for North America a major faunal and environmental shift took place within the Late Eocene (late Duchesnean). Chronometric control on this episode needs further refinement, but it lies between 37 and 42 Ma, and so on an interim basis we call it 40 Ma. For a full discussion of the problem, see Krishtalka et al. (1987).
GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 191 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 10Â°C drop to about 12.5Â°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.