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 70°N latitude (Wolfe, 1985; Wing and Tiffney, 1987). During the Tiffanian (ca. 63 Ma), cooler climates—evidenced by increased percentage of deciduous trees and decreased diversity—produced "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records the earliest first-order immigration episode in the history of North American land mammals, it is soon superseded by an even larger episode.
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).
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).