Floristic implications of two contemporaneous inland upper Neogene sites in the eastern US: Pipe Creek Sinkhole, Indiana, and the Gray Fossil Site, Tennessee (USA)

Abstract

Late Neogene floras of North America are mainly represented by sites located along the east coast Piedmont and the Great Plains. To date, there are only two upper Neogene inland localities in the eastern half of North America, the Pipe Creek Sinkhole (Indiana) and the Gray Fossil Site (Tennessee). At both sites, a lacustrine environment was formed from sinkholes that preserve fossil assemblages including invertebrates, vertebrates, and plant remains. We reviewed the floristic record (micro and macrofossils) of each site to determine the late Neogene composition of the flora, its vertical stratification, and to provide insight into the changes associated with warmer and drier conditions in the eastern deciduous forest. The Pipe Creek Sinkhole flora when compared to extant pollen floras of the eastern US is nested within the Beach-Maple-Basswood forest type of Dyer; however, the Miocene forest is dominated by a pine-hickory or pine-hickory-cottonwood association. The Gray Fossil Site flora when compared to extant pollen floras of the eastern US is nested within the Mesophytic Forest and is dominated by an oak-hickory-pine assemblage. The Pipe Creek Flora and the Gray Fossil Site Flora lack the common sub story and understory taxa and are dominated by specific woody taxa and an array of herbaceous taxa that have high light requirements and indicate some disturbance. The lack of vertical forest structure indicates that at both sites, woodland to woodland savanna to prairie-like habitats may have prevailed. The common occurrence of charcoals in the lacustrine sediments indicates that fire was a frequent disturbance factor, which may have suppressed the development of a structured forest with a closed canopy at both localities. This was further exacerbated by the impact of large herbivores. Both fossil floras, despite changes in the vegetation composition and vertical stratification when compared to the extant indigenous deciduous forest, include the major elements from their respective extant mesophytic C3 floristic associations that currently exist at these locations, i.e. the Beech-Maple-Basswood (Pipe Creek Sinkhole) and the Hickory-Oak-Pine Mesophytic Forest (Gray Fossil Site). The Miocene-Pliocene climate, in comparison to the extant climate, affected the vegetation in eastern North America by the selective elimination of those taxa that could not tolerate the warmer and drier conditions; the concomitant increase in fire frequency and the impact of large herbivores further contributed to maintaining woodland/savanna communities at both localities.