The modern environments of Molluscs in southern Mesopotamia, Iraq: A guide to paleogeographical reconstructions of Quaternary fluvial, palustrine and marine deposits

摘要

The Quaternary, mainly freshwater sediments of the Lower Mesopotamian plain include a thin transgressive marine unit which extends inland some 250 km from the present coastline, the Hammar Formation. The identity and respective areal extent of continental and transgressive units are based mainly on their molluscan faunas. Those faunas reflect the several environments of the Middle Eastern biogeographic province: fluviatile, lacustrine, estuarine and lagoonal. The ecological requirements of the more common subfossil and modern species of the malacofauna were studied during a month of field reconnaissance.The freshwater portion comprises only four bivalve species (Corbicula, Unio, Anodonta and Pseudodontopsis). The principal gastropods include Theodoxus (Neritaea), Melanopsis, Melanoides, Bellamya, Gyraulus, Lymnaea (Radix) spp. Corbicula characterizes both fluviatile channels and lacustrine environments; Melanopsis, Melanoides, Theodoxus and the Unionaceae prefer shallow, sparsely vegetated, freshwater lakes. The greatest diversity was seen in the slowly flowing water of the channels draining the marshes; small Planorbids are rarely abundant whereas viviparid and lymnaeids may accumulate near the margins of marshes due to floatation during floods.The brackish water faunas are characterized by Neritina (Dostia), the potamid Cerithidea (Cerithideopsilla), and Stenothyra. The new combination Theora mesopotamica (Annandale, 1918) is introduced here as the exact equivalent of Abra cadabra Eames et Wilkins, 1957. The abundance of these species is indicative of a restricted, low energy marine environment, confirmed by great numbers of the foraminiferan Ammonia gr. beccarii and the ostracod Cyprideis gr. torosa.Reworked Hammar Fm marine fossils, first recorded by the Annandale (1918) collection from a lacustrine deposit near Nasiriyah, have been observed in river, marsh and lake sediments too, usually in association with scarce foraminifera. The limited occurrence of these macrofossils (abundant in their life environment) indicates reworking.