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

The Quaternary, mainly freshwater sediments of the Lower Mesopotamian plaininclude a thin transgressive marine unit which extends inland some 250km from the present coastline, the Hammar Formation. The identity andrespective areal extent of continental and transgressive units arebased mainly on their molluscan faunas. Those faunas reflect theseveral environments of the Middle Eastern biogeographic province:fluviatile, lacustrine, estuarine and lagoonal. The ecologicalrequirements of the more common subfossil and living species of themalacofauna were studied during a month of field reconnaissance. Thefreshwater portion comprises only four bivalve species (Corbicula,Unio, Anodonta and Pseudodontopsis). The principal gastropods includeTheodoxus (Neritaea), Melanopsis, Melanoides, Bellamya, Gyraulus,Lymnaea (Radix) spp. Corbicula characterizes both fluviatile channelsand lacustrine environments; Melanopsis, Melanoides, Theodoxus and theUnionaceae prefer shallow, sparsely vegetated, freshwater lakes. Thegreatest diversity was seen in the slowly flowing water of the channelsdraining the marshes; small Planorbids are rarely abundant whereasviviparid and lymnaeids may accumulate near the margins of marshes dueto floatation during floods. The brackish water faunas arecharacterized by Neritina (Dostia), the potamid Cerithidea(Cerithideopsilla), and Stenothyra. The new combination Theoramesopotamica (Annandale, 1918) is introduced here as the exactequivalent of Abra cadabra Eames et Wilkins, 1957. The abundance ofthese species is indicative of a restricted, low energy marineenvironment, confirmed by great numbers of the foraminiferan Ammoniagr. beccarii and the ostracod Cyprideis gr. torosa. Reworked Hammar Fmmarine fossils, first recorded by the Annandale (1918) collection froma lacustrine deposit near Nassiriyah, have been observed in river,marsh and lake sediments too, usually in association with scarceforaminifera. The limited occurrence of these macrofossils (abundant intheir life environment) indicates reworking.