Several decades before the evolution of horses was documented in western North America, Kowalevsky (1873), working in Europe, recognized that hypsodonty in hipparionine horses reflected the spread of grasslands in the Miocene. Such reconstructions of environmental history from fossil mammal assemblages rest on two fundamental premises. The first is that the fossil record adequately samples the kinds and numbers of land mammal genera that lived during successive stages of the Cenozoic. The second premise is that samples of once-living mammals, through their morphologies, distributions, and numbers, and also through their sedimentary and taphonomic contexts, bear evidence of their environments. Such relationships are well established between living mammals and present-day environments; for example, the diversity and structure of African ungulate faunas reflect the vegetative cover and rainfall in which they range (Coe et al., 1976; Sinclair and Norton-Griffiths, 1979; Vrba, 1985b; McNaughton and Georgiadis, 1986). Webb (1984) and Janis (1984) independently analyzed the distribution of body size and hypsodonty in fossil ungulate faunas from North America to develop convincing analogues with modern ungulate faunas in Africa. More generally, Andrews et al. (1979) empirically determined the patterns of body size and other gross features that characterize modern mammalian faunas from various terrestrial biomes regardless of the continent on which they occur.
One particular method of faunal inference that has been applied with considerable success in recent years is called the cenogram. A cenogram displays the whole spectrum of estimated body sizes for all species from a relatively complete faunal sample, for comparison with other such cenograms representing known biomes. Legendre (1986) convincingly used this method to show that the land mammal cenogram for the Late Eocene Phosphorites of Quercy in France had the same pattern as a modern equatorial rain forest fauna, notably a predominance of small-bodied (arboreal) forms, and Gingerich (1989) applied the same method with excellent results to still earlier Eocene (Wasatchian) faunas in Wyoming. Other examples are reviewed in Behrensmeyer et al. (1992).
Actualistic studies of taphonomic processes in the deposition of mammal remains provide the critical link between modern and fossil ecosystems (Behrensmeyer and Hill, 1980; Damuth, 1982; Behrensmeyer et al., 1992). Thus, a legitimate train of logic and experiment connects the comparison of modern ecosystems with depositional settings in the recent past. It should be stressed that such relationships must be interpreted cautiously, and that data from ancient mammalian faunas apply most reasonably at the biome level in well-studied parts of the fossil record.
Another largely independent approach involves the study of ecomorphology. For example, the teeth of mammals are among their most important adaptations, and many features of their energetic and often highly specialized life styles depend to a great extent on their masticatory capabilities. The relationship between tooth morphology and ecology is particularly clear among large herbivores that must adaptively reconcile their long lives and high metabolic activities with relatively poor-quality protein resources (often further complicated by high fiber and high toxin contents). Fortunately, the fossil record of herbivore teeth is excellent and provides a clear history in diverse mammalian taxa of progressive change in their capacity to process increasingly coarse fodder. The dental adaptations of various mammalian herbivores are cited frequently in the following discussion as evidence of the prevailing conditions in North American environments. Such studies were well summarized by Scott (1937).
The most obvious such dental adaptation that many herbivore groups develop is hypsodonty (i.e., high-crowned teeth). A hypsodont animal may be defined as one in which several of its teeth (typically the molars but not uncommonly a whole battery of cheek teeth) have the crown height greater than one dimension of the crown wear surface (length, width, or the average of the two). If one imagines a cubic tooth in an unworn condition it would have a hypsodonty index of one and would be on the threshold of attaining hypsodonty. In modern ungulates and rodents it is not uncommon to find teeth with crown heights many times greater than their crown surface dimension, and some groups develop hypselodonty (i.e., ever-growing teeth).
The adaptations of large mammalian herbivores also contribute strongly to shaping their environments. For example, large mixed herds of grazers facilitate the food resources of one another and also help maintain optimum savanna settings, sometimes by grazing and sometimes by fostering fires (Sinclair and Norton-Griffiths, 1979; McNaughton et al., 1988; Owen-Smith, 1988).
With these interpretive tools in hand we scan the history of land mammal faunas and associated floras to envision the major succession of biomes in North America.
The earliest mammal faunas of the North American Cenozoic consisted of diverse small to medium-sized arboreal and scansorial forms. Most were frugivorous, omnivorous, or insectivorous; specialized carnivores and herbivores of larger body sizes came somewhat later. Four