National Natural History Collections, Berman Building, Edmond J. Safra campus, The Hebrew University of Jerusalem, Givat Ram, IL-91904 Jerusalem, Israel; National Collections of Natural History, Tel Aviv University, IL-69978 Tel Aviv, Israel.
The lentic Basommatophora molluscs and hygrophilous land snails of the Early-Middle Pleistocene site of Gesher Benot Ya'aqov (GBY), 0.78 Ma, crossed by the Matuyama-Brunhes chron boundary [MBB] were studied in order to reconstruct their specific habitats and possible reactions to climate change along the site sequence. Samples of equal sizes from 27 of the 46 layers along the 100 k.yr. time-span of the site were examined. About 2000 specimens of 21 lentic and hygrophilous species belonging to five families: Planorbidae (11), Lymnaeidae (6), Acroloxidae (2), Carychiidae (1) and Succineidae (1) were identified. The family with the largest biodiversity is the Planorbidae and of these, the most abundant species include Gyraulus piscinarum (937), Planorbarius corneus (210) and Radix labiata (160). The recent known zoogeographic origin of 81% of the species is Palaearctic and Holarctic. The MBB coincides with remarkable environmental changes reflected in molluscs, other faunal and floral elements and stable isotopes. The Planorbidae and Lymnaeidae reach greater abundance (90% and 80% of their assemblages, respectively) pre-MBB, while Acroloxidae, Succineidae and Carychiidae are more abundant (74%, 64% and 90%) post-MBB. Our data indicate that GBY molluscs show a two-phase pattern (shallow and deep lake) in each of the five defined cycles. Their numbers increase during the shallow water phases, thus the site climate changes from cold and humid in the oldest layers, to dry and cold up to the MBB and few succeeding layers. Between cycles 2 and 3, post-MBB, we see a short period of warm and humid climate that enables the influx of African and Asian elements. At the same time, cold climate species of Euro-Siberian and Palaearctic origin disappear. The succeeding layers indicate a cooler and humid climate. Finally, the sequence indicates deep water in the oldest layers and desiccation towards MBB and deeper water post-MBB.