Integrated Late Quaternary chronostratigraphy for San Salvador Island, Bahamas: Patterns and trends of morphological change in the land snail Cerion

Reconstructing the phylogeny and biogeography of the Caribbean land snail Cerion requires a robust stratigraphic and chronological framework. To this end, we have determined the stratigraphic succession on San Salvador, a Bahamian island with a rich fossil and modern Cerion fauna. A primary purpose of this paper is to independently verify this succession through whole-rock and Cerion aminostratigraphies and AMS 14C-based age models. Over 150 individual Cerion shells were age-ranked from 140 ka to modern using stratigraphic position and reverse-phase HPLC (RPC) amino acid racemization, which was sufficiently sensitive to resolve stratigraphic subunits within the Holocene and late Pleistocene. A secondary purpose of this paper is to assess broad changes in the gross morphology (height, width) of supersets of Cerion from age-ranked lots spanning this ~ 140 kyr chronostratigraphy. Through each of the three interglacial sequences (i.e., marine isotope stages 5e, 5a, and 1), between-sample trends in mean gross morphology often greatly exceed within-sample variances (± 1σ). Live-collected Cerion exhibit a range in gross morphology that nearly encompasses that of the entire fossil sequence. A trend of increasing gross shell size characterizes each of the interglacial phases, with a major step-decrease between marine isotope stages 5a and 1. While between-unit variation is often great in Cerion from SSI, within-unit variation appears unimodal through the record.