Fossil coleopteran assemblages as sensitive indicators of climatic changes during the Devensian (Last) cold stage

Abstract

Coleoptera are abundant fossils in Quaternary deposits laid down under freshwater or terrestrial conditions. They display a remarkable degree of evolutionary stability and reasons are adduced for believing that this morphological constancy is associated with physiological constancy. Thus whole communities of species have been assembled in the past, drawn together by common ecological preferences, so that the species composition of fossil assemblages resembles that of modern faunas. Marked changes in the geographical distribution of Coleoptera during the last glacial-interglacial cycle conform to an orderly pattern of climatic fluctuations. The Coleoptera contribute most information about the Devensian climates during warmer interstadial periods because during the colder episodes conditions in Britain became more or less intolerable to insect life and the fossil content of the sediments approaches zero. The term interstadial is here used for an interlude of milder climate in an otherwise cold period which either does not attain temperatures equivalent to those of the present day or which attains temperatures as warm, or even warmer than those of today but which does not last long enough for floral and faunal equilibrium to become established. During the Chelford Interstadial, at the limit of acceptable radiocarbon dating but possibly about 60 000 years (a) ago, the climate in central Britain was rather cooler than now with a moderate degree of continentality. The Upton Warren Interstadial complex, between about 45 000 and 25 000 a ago, reached its thermal maximum at about 43 000 a before present when temperatures were rather higher than those of the present day and the climate was moderately oceanic. This episode may have been as short in duration as 1000 a. After this the interstadial is characterized by a period of much lower temperatures, with a greatly increased degree of climatic continentality, lasting for about 15 000 a. Few insect faunas are known from the period of maximum ice expansion but the scant evidence supports an interpretation of a climate of arctic severity. During the closing phases of the Devensian cold period there is faunal evidence for only one major climatic oscillation - here called the Windermere Interstadial. The sharp rise in the thermal environment at its beginning took place rather before 13 000 a ago but later than 14 000 a ago. Thermal maximum was attained almost immediately with temperatures during the summer at or above their present day level. Moderate oceanicity of the climate at this time means that winter temperatures were not much lower than those of the present day. At least during the earliest parts of this interstadial a temperate insect fauna was associated with a flora almost entirely dominated by herbs. The decline of the Windermere Interstadial from its thermal maximum seems to have been more or less synchronous from southern to northern England and to have taken place at about 12 200 a ago. A cool temperate phase then ensued for over one thousand years with summers about 3 $^\circ$C cooler than during the thermal maximum. This episode corresponds in time to the Allerod oscillation. The Loch Lomond Stadial between 11 000 and 10 000 a ago saw the return of arctic faunas to the British Isles even as far south as Cornwall. The presence of Asiatic species, though not abundant, suggests that the climate at this time may have been rather continental. The timing and intensity of the climatic changes during deglaciation show close parallelism to the changes in oceanic circulation in the eastern Atlantic now being interpreted from cores of ocean bottom sediments.