The molluscan fauna of the Cromerian type site at West Runton, Norfolk

Abstract

An updated and revised list of the non-marine molluscs of the West Runton Freshwater Bed (WRFB), the type site of the Cromerian Stage, is presented. A minimum total of 73 species is now known, consisting of 46 aquatic and 27 terrestrial taxa. The molluscan fauna suggests a sluggish, base-rich, stream choked with aquatic vegetation, upstream of any brackish influence. The banks were bordered with marshland. Sandy facies with a greater proportion of fluvial species indicate more active flow within small channels. Four aquatic and 5 terrestrial species no longer live in the British Isles, and 3 (6.5%) of the aquatic species (Tanousia runtoniana, Borysthenia goldfussiana and Pisidium clessini) are extinct. The Cromerian molluscan fauna lacks southern thermophiles and, apart from the extinct taxa, is remarkably similar to modern faunas from eastern England. Indeed, 91% of the species of aquatic mollusc found in the WRFB still inhabit East Anglia. A number of the species that no longer live in Britain have ranges that are central or eastern European, implying that the climate may have been more continental. The stratigraphical ranges of both B. goldfussiana and T. runtoniana extend into the late Early Pleistocene in The Netherlands, and in Britain they seem to occur only in temperate stage deposits falling within the early part of the ‘Cromerian Complex’ or earlier.