LATEST EARLY EOCENE FOSSIL SEDGES AND THEIR POTENTIAL RELATIONSHIP TO GRAZING MAMMAL EVOLUTION

A fossil sedge with a modern tussock growth habit characteristic of some forest and upland species of Carex (Cyperaceae) is reported from the latest early Eocene flora of Republic, northeastern Washington, USA. The fossils are preserved as compressions of plants with compact corm-like stems bearing tightly imbricated leaf bases and up to a dozen elongate leaves with multiple parallel veins. Corms are lobed basally, and one bears an attached vegetative offshoot, indicating these plants reproduced asexually. Fragmentary reproductive structures found in association with the plants are pistillate inflorescences with membranous glumes bearing scabrid awns. Each glume subtends a perigynium, the sac-like structure which distinguishes Carex from all other taxa within Cyperaceae.

The presence of Cyperaceae with a tussock growth habit in the Republic Flora has potential paleoecological significance. Although we have no exact modern analogue for the Republic locality, we have noted similarities with upland African forest communities, particularly ones with folivores. In forest-dominated communities of lowland Gabon and the Virunga Mountains of Rwanda, grazers, such as the ruminant buffalo (Syncerus caffer) with diets consisting mainly of Cyperaceae and Poaceae, are associated with open habitats.

Although the African savannah has often been used as a model for the radiation of middle Miocene grassland communities, the Afromontane forests may serve as a model for understanding preadaptation of grazing ancestral mammals. The presence of Cyperaceae during the early Eocene suggest that an earlier evolution within mammalian woodland grazers may have proceeded open savannah-land biota. A model for understanding the evolution of grazers in these systems would be to look at the diets of large ruminants today, in lowland Afrotropical rainforests and Afromontane forests. Buffalo provide an excellent initial analogue for developing a more robust model for the radiation of grazers into savannah habitats. We propose a model of grazers being a guild that evolved first within forests to take advantage of microhabitats such as wetlands and gap clearings dominated by sedges and grasses, and then radiated into savannahs.