Neogene Bivalve Rangia (Miorangia) Johnsoni; Taxonomy, Depositional Facies and Stratigraphic Range

Abstract

The study of numerous specimens of a fossil bivalve, collected from a Chickasawhay River outcrop in southeastern Mississippi, included statistical tests of valve elongation ratios. Despite their great morphological variety, the molluscan valves were determined to represent only a single species, Rangia johnsoni. Valve shape variability appears related to ontogenic development from the young juvenile to adult stages. Diminutive shells, designated previously as "R. microjohnsoni," do not belong to a separate, valid species. The term Miorangia, employed by Dall and Gardner, is a valid subgeneric tax on. Further use of the terms Miorangia microjohnsoni, M. johnsoni and R. microjohnsoni is unwarranted. Rangia johnsoni inhabited not only reduced salinity, estuarine-lagoonal depositional facies but also extended into inner neritic environments. Although most abundant in the Late Miocene units of the Gulf Coastal Plain, its age range includes the upper Middle Miocene and perhaps even the earliest Pliocene.