Holocene molluscan succession from NE Morocco: Palaeoenvironmental reconstruction and biogeographical implications

Fluvio-lacustrine Holocene sequences from the semi-arid region of eastern Morocco are rich in shells and show similar sedimentary levels that could correspond to synchronous environmental variations. The high diversity of Moroccan continental malacofaunas makes them a very suitable fossil group for palaeoenvironmental reconstructions and palaeobiodiversity implications. The malacological study of Blirh is the first in Morocco to be based on a systematic sampling providing a complete faunal record in order to reconstruct palaeoenvironmental successions. Three biozones have been recognized. They illustrate occurrence of a marsh and a lake from shortly before 10,600 cal. BP followed by an abrupt drying up around 5000 cal. BP. This dramatic environmental change is not synchronous with lithological variations as stated by previous studies based on geological and palynological records. The Blirh lacustrine phase fits into the humid period that follows full glacial hyperarid conditions during the latest Pleistocene in North Africa from 14,800 cal. BP. The abrupt shift to dry conditions around 5000 cal. BP inferred from the malacological succession correlates with the contraction of Saharan lakes. During the last biozone, despite disappearance of the main water-body, marshy species persist and demonstrate occurrence of very wet environments in the Middle Moulouya plateau until historical times. Several species, currently absent from the region, were found within the Holocene assemblages. At least two new species of aquatic molluscs (Hydrobiidae) have been discovered. Sampling of modern malacofaunas from wells and springs in the area has demonstrated that one of them is still living in the water-table. The Blirh malacological record provides an insight of the palaeo-biogeographical changes affecting this group during the Holocene and demonstrates the very recent increase of human impact on the regional biodiversity.