THE VEGETATIONAL CONTEXT FOR RODENT EVOLUTION IN THE PACIFIC NORTHWEST: MIDDLE MIOCENE PHYTOLITHS THE MASCALL FORMATION OF EASTERN OREGON

Plant silica (phytolith) records from the north and central Great Plains indicate that the relatively closed, wooded habitats of the early Oligocene changed to open, grass-dominated vegetation by the early Miocene. The resulting savanna or woodland mosaic initially consisted of a non-analog mix of mainly cool-temperate C₃ pooid grasses with woody dicotyledons and warm- and wet-loving palms and gingers. In the late Miocene landscapes became progressively more open and C₄ grasses rose in abundance in grass communities.

In contrast, Miocene vegetation changes on the west coast of North America are not fully understood. Here we present a new phytolith-based vegetation record from the middle Miocene Mascall Formation in the John Day Basin, eastern Oregon. The Mascall Formation is 350 m thick and ranges from <16 to 13 Ma in age, covering the Middle Miocene Climatic Optimum (17-14 Ma). The Mascall consists of diatom-rich lacustrine deposits with a diverse, warm-temperate megaflora, overlain by tuffaceous alluvial and floodplain deposits with well-developed alfisols, andisols, and vertisols. These strata contain no megafloras, but phytoliths are well-preserved throughout the section, providing a high-resolution vegetation record. Specifically, phytoliths can inform grass-tree dynamics, grass community composition (e.g., C₃ vs. C₄) and other aspects of vegetation type.

Comparisons of forest-indicator phytoliths and grass phytoliths in Mascall deposits suggest that Oregon was mostly forested in the middle Miocene, with palms and ginger phytoliths pointing to warm and humid conditions; the diversity of aquatic type phytoliths also indicate extensive wetlands. Although relatively rare, the grasses included a diverse mix of pooids, warm-adapted PACMAD grasses, and shade-loving bamboos.

Phytolith composition is relatively stable through the section, indicating little climatic or hydrologic change during this time. The inferred forest of the Mascall Formation contrasts sharply with coeval phytolith assemblages from the north and central Great Plains, which show dominantly open, grass-dominated ecosystems. We suggest that the differences reflect an east-west gradient in vegetation structure and composition, potentially linked to orographic effects from the rising Rocky Mountains.