Malacological evidence relating to the insularity of the British Isles during the Quaternary

Abstract

Abstract The marine molluscan faunas from different temperate stages of the Pleistocene of the North Sea vary enormously both in terms of species richness and in the diversity of their biogeographical composition. The marine assemblages from the Middle Tiglian, Eemian and Holocene have all yielded c. 100 species or more, including many with southern or ‘Lusitanian’ affinities. It is thought that during these stages the Strait of Dover was open, allowing entry of these southern taxa into the southern North Sea. Conversely, the temperate stages from the Late Tiglian up to and including the Holsteinian have yielded relatively impoverished faunas (no more than about 40 species) virtually lacking any of the southern elements. During these stages it would appear that the Strait of Dover was closed, so preventing the spread of marine molluscs into the North Sea. Examination of the history of fluvial molluscs (particularly prosobranchs and larger bivalves) on either side of the English Channel supports this interpretation. Fluvial provinciality is recognized during the stages when Britain is thought to have become an island. Non-provinciality, pointing to fluvial exchange, occurs during the other stages.