TURTLE FOSSILS FROM THE MISSISSIPPI BLACK PRAIRIE AS ECOLOGIC AND PHYSIOGRAPHIC INDICATORS OF LATE PLEISTOCENE PALEOENVIRONMENTS

Nine chelonian taxa, including extinct and extralimital forms, were previously reported from the Pleistocene of the Black Prairie Province, east-central Mississippi (33.5N) (J. M. Kaye). Among the more interesting fossils recovered was a single partial nuchal of Emydoidea blandingii, a species limited to the northern Midwest and extreme northeastern U.S. (>36.5N) since the end of the last Ice Age (c. 10 ka). Additional collecting and extensive comparison has resulted in the recovery of many more remains and four other chelonian taxa: wood turtle (Glyptemys insculpta), now limited to the Northeast and Great Lakes region (>37.0N); musk turtle (Sternotherus), 2 spp./1 extralimital(?); and a large form of map turtle (Graptemys). The first two additions are actually quite common in the assemblage, each even more so than Emydoidea; Graptemys is rare.

The Black Prairie chelofauna consists largely of disarticulated and fragmentary shell remains found in late Pleistocene and younger stream deposits; it represents a subset of a diverse riparian vertebrate assemblage containing both terrestrial and aquatic elements. Turtle remains from the study area comprise two distinct subassemblages, each representing a different riparian environment. One is composed very dominantly (>95%) of an extinct(?) subspecies of Terrapene carolina, which, next to the giant testudinine Hesperotestudo, is the least aquatic of all turtles represented; turtle fossils are found in clays at modern elevations ranging from 185-320 ft. The other is much more diverse with at least 65% composed of at least 11 other species, including aquatic emydids; fossils are found in sandy clays at 160-185 ft elevations. Each subassemblage is based on occurrence and composition with relative abundance determined by interspecific proportions of individual shell elements (or fragments thereof) and attention to taphonomic biases. The low-diversity, higher-elevation subassemblage with abundant Terrapene is interpreted as representing ancient upland, intermittent stream environments, and the high-diversity, lower-elevation subassemblage with greater proportions of aquatics as representing lowland, perennial stream environments. This correlation between ancient and modern elevations suggests minimal change in local relative topography since the late Pleistocene.