In attempting to understand ecosystems of the Eocene-Oligocene, one may ask which plants the abundant browsing and mixed-feeding herbivores might have eaten. Since grass is still rare in the Eocene-Oligocene, it may be necessary to postulate an emphasis on woody shrubs such as Sarcobatus, Ephedra, and Equisetum. Huber (1982) postulates that just such herb-dominated savannas precede an abundance of true grass savannas in Tertiary floristic history. On the other hand, grass was present and may be underrepresented due to taphonomic conditions in paleofloral sites. Grasses were established in the mid-Eocene of Australia (Truswell and Harris, 1982, cited in Wolfe, 1985) and Europe (Litke, 1968). On present evidence, Wolfe (1985, p. 364) recognized ''. . . a distinctive Eocene savanna (i.e., shrubs and widely spaced trees but no grass on the interfluves)." The extent of grasses within this chaparral and woodland savanna setting remains in doubt.
The quantum jump in mixed-feeding herbivores noted for the White River chronofauna probably helped expand the extent of woodland savanna. The new fauna and flora probably formed a positive feedback loop: large herbivores preferred feeding in more open woodlands; in turn, expansion of open formations facilitated evolution of mixed-feeding herbivores. Among the subtle changes within the history of that chronofauna ("chronofaunal creep") was a decline in the rich array of browsers (e.g., loss of titanotheres after the Chadronian), and an increase in the body sizes and numbers of shrub eaters (quasi-grazers). Such changes indicate an increasing percentage of open-country patches, and tend to be corroborated by the limited botanical and pedological data.
The next major immigration episode in the record of North American mammals takes place in the Early Miocene. Miocene immigration patterns in North American mammals were thoroughly revised by Tedford et al. (1987). Their analysis recognizes six immigrant genera in the latest Arikareean and 14 genera in the earliest Hemingfordian (20 Ma). Native members characteristic of the Runningwater chronofauna of the late Arikareean and early Hemingfordian include flat-incisored beavers (notably Palaeocastor with its deep corkscrew-shaped burrows), hypsodont oreodonts such as Merychyus and Merycochoerus, oxydactyline and protolabine camels (especially Michenia), and the mixed-feeding horse, Parahippus. Several immigrant groups also constitute a vital and characteristic part of the Runningwater chronofauna, notably the primitive bear, Phoberocyon; such genera of amphicyonids (an extinct family also known as "bear-dogs") as Cynelos and Amphicyon; raccoons including Edaphocyon and Amphictis; and several mustelid carnivores including Leptarctus. The first representatives of three ruminant families arrived from Asia at this time: Blastomeryx represents the Moschidae (family of the living Chinese musk deer); Paracosoryx is the scion of the autochthonous North American family Antilocapridae; and Barbouromeryx establishes the presence of the giraffe-like family Palaeomerycidae. Each of these immigrant ruminant groups evolves and diversifies in the course of the Miocene.
The paleoecology of Arikaree assemblages in the midcontinent is thoroughly analyzed by Hunt (1990). Earlier in the Arikareean, widespread eolian distribution of tuffaceous sediments in a seasonally arid climate played an important role in preserving rich samples of mammals in burrows and other nontransported settings. Both the mammalian taxa and the sedimentological evidence indicate ". . . well vegetated stream border communities, spatially separated by open interchannel reaches occupied by grassland, or possibly low shrub savanna or open savanna-parkland environments" (Hunt, 1990, p. 107).
The trend toward more open country habitats continued from the Early into the Middle Miocene at least in the midcontinent. Another substantial wave of immigrants from Asia altered the makeup of the North American mammal fauna and spurred the course of its evolution. It is awkward from a nomenclatural view that this turnover episode falls within the Hemingfordian land mammal age and is not very far removed in time from the previous turnover episode. Tedford et al. (1987) comment as follows:
It is important to note that an important faunal discontinuity exists within the Hemingfordian, signaling an abrupt shift in the Great Plains from the chronofauna characteristic of the late Arikareean and early Hemingfordian to one typifying the late Hemingfordian and early Barstovian . . . . The differences between the faunas of the Box Butte through Sheep Creek interval and the chronofaunally related younger faunas here referred to the Barstovian (Observation Quarry through Lower Snake Creek) can be ascribed mainly to anagenetic change in persistent lineages, making it necessary to distinguish these ages at the specific rather than the generic level.
Major advances within the autochthonous fauna of Hemingfordian age indicate continued evolution toward more open-country adaptations. The best example is the transition from Parahippus, a browsing horse, to Merychippus, an early grazer (Hulbert and MacFadden, 1991). Similar trends are suggested by the increased hypsodonty and quantum increase in diversity of pronghorn antelopes.
The mid-Hemingfordian wave of immigrants from Asia, like the preceding episode, represents a broad ecological spectrum. Among the diverse rodents are flying squirrels