Paleofaunistics of Nonmammalian Vertebrates from the Late Pleistocene of the Mississippi-Alabama Black Prairie

Abstract

The Mississippi-Alabama Black Prairie is a distinct geologic, physiographic, and phytogeographic district within the Gulf Coastal Plain Province of southeastern North America. Residing in the alluvial portions of local Quaternary regolith are rare and diffusely distributed vertebrate fossils collectively possessing a Rancholabrean (NALMA) character. Derived from stream-concentrated lags, this seemingly loose association of vertebrate fossils is defined herein as the Late Pleistocene Black Prairie fossil assemblage (BPA). Although some systematic and interpretive work has been published in the past, much remains to be done, specifically with regard to defining the assemblage in terms of a local paleofauna and thereby exploring its usefulness as a paleoenvironmental tool. In the light of new species discoveries, the current paper explores assemblage paleofaunistics using the nonmammalian component.

The first part of this study provides a comprehensive and detailed morphometric and comparative description of the vertebrate remains in order to identify as many individual fossil specimens as possible from the very fragmentary BPA. In a stream-concentrated assemblage, turtle shell elements and snake vertebrae, for example, can provide much paleoecological information, but if heavily eroded by abrasive lag components and reworking, their identification can be labor intensive with many pieces remaining unidentified. Many assemblage and paleofaunal systematic accounts in the published literature consist of weak and even baseless attributions or of simple faunal lists alone. Such brief descriptive systematic treatments provide little if any justification for the taxonomic assignment. Therefore, the first half of this paper attempts to identify and describe important characters useful in the identification of certain non-mammalian vertebrates, primarily the freshwater turtles, which were not adequately covered in earlier accounts of the BPA.

The second part of this study is interpretive. It utilizes simple presence/absence (P/A) and relative abundance (RTA) of taxa in order to reconstruct hydrologic paleoenvironment in the Black Prairie and subdivide it into two fluvioriparian regimes. Before paleoenvironmental interpretation is attempted, however, the extent of taphonomic bias is explored. Although bias exists in the BPA with respect to relative species representation, taphonomic corrections and accommodations are made to legitimize fossil-modern and fossil-fossil assemblage comparisons, which form the basis for paleoenvironmental interpretations. Representational bias in the fossil assemblages and modern comparative faunal samples is identified and discussed. P/A and RTA are then used in simple community similarity analyses that compares.

The Black Prairie Late Pleistocene fossil assemblage is divided into two distinct subassemblages derived from two different fluvioriparian paleoenvironments—intermittent, or upland, streams and shallow perennial, or upper lowland, streams. This is most obviously determined from the remains of large fishes found at lower elevation sites and absent from higher elevation sites, which reinforces earlier findings of more aquatic mammals recovered from lower elevations. Modern habitat requirements of the fourteen chelonian taxa present in the Black Prairie assemblage, coupled with the comparisons to modern chelofaunas, have allowed further characterization of the different paleoenvironments suggested by the two subassemblages. There is considerable similarity in hydrologic requirements between the chelonian composition of fossil subassemblages and the modern streams from which each subassemblage is derived. Furthermore, the modern topographic position of each subassemblage is consistent with the paleoenvironment suggested by each. This in turn indicates little change in relative topography in the area since the Late Pleistocene, which is consistent with the region's geomorphic evolution.