The fauna of the Hoxnian interglacial deposits of Nechells, Birmingham

Abstract

The described fauna has been extracted from the sediments of the Nechells channel of Hoxnian Interglacial age, whose stratigraphy and flora have already been discussed. It comprises a small number of species of fish, ostracods, Cladocera, mites, molluscs and polyzoa and a considerable number of insects with beetles predominant. The distribution of these has been related to the floral zones of Kelly (1964). The earliest sediments are those of a glacially dammed lake and the limited fauna associated with these are ostracods, Daphnia, Cristatella and, amongst the insects, the larvae of Sialis , trichoptera and chironomids. Shortly after, fish and molluscs appear and by Zone IN 3, beetles first become significant. After IIN a , beetles form the predominant element of the fauna. Calcareous and phosphatic organisms disappear after IIN a , corresponding to a change in the nature of the bottom sediments, but this is largely explained by post-depositional solution. The time and climate distribution of Daphnia and Cristatella mucedo , both of which have winter resting stages, is discussed in relation to climate, as are beetles of the genus Rhynchaenus in relation to the occurrence of food plants, and the chironomids in relation to temperature and limnology. The coleopterous fauna, the first of considerable size to be described from a period of the Pleistocene as old as the Hoxnian, is still essentially British and the identity with present-day species is striking. An exceptional case of this identity is provided by the aedeagus dissected from Hydraena riparia . Only three non-British species have been named and two of these, Platypus oxyurus and Brachytemnus submricatus , from near the top of the sequence, have now a disjunct occurrence in southern Europe. Micropeplus caelatus has been identified and this now has a sporadic distribution in central Europe and is restricted in Britain to the extreme south-west of Ireland. There are no beetles characteristic of cold climate but the earliest fauna as a whole clearly indicates glacial conditions. The climate ameliorates to reach its optimum, a little warmer but not much different from that of today, towards the end of the borehole sequence, and the deposits do not include a fauna characteristic of the second (cooling) half of an interglacial.