A joint evening meeting with the Linnean Society, 17th January 2002

The Pleistocene Record: Faunal Changes in Response to Quaternary Ice Ages

R. C. Preece - Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ

Amongst the invertebrates, molluscs are unusual in having a relatively good fossil record. This talk, like the one to follow, will focus on the history of non-marine taxa in NW Europe. Our understanding of the climatic history of the last 2 million years has been completely transformed by detailed analyses of continuous sedimentary sequences, particularly oxygen isotope records from deep sea cores. These have essentially vindicated the Astronomical theory and have confirmed that orbital variations are the pacemaker of the Quaternary ice ages. In the Early and early Middle Pleistocene the climatic signal is dominated by 41 kyr obliquity cycles caused by variations in the axial tilt of the Earth. After about 800 kyr BP, the climatic periodicity changed to a 100 kyr rhythm, reflecting the switch to eccentricity as the primary forcing mechanism. This change in mode resulted in an intensification of the climatic signal; the cold stages were accompanied by major expansion of ice sheets and the interglacial stages appear to have been longer and warmer. The recent work on ice core records from Greenland has revealed that events within the last cold stage were also far more complex than previously thought. Some two dozen short-lived climatic oscillations have now been recognized, controlled not by orbital parameters but by the internal dynamics of the ice-sheet and changes in thermohaline circulation.

Difficulties in correlating the fragmentary continental sequences with the more complete marine records will be discussed. The faunal responses of Pleistocene non-marine molluscs will then be examined against this background. Molluscs themselves have played an important role in differentiating interglacial stages, as well as providing clues as to the climatic character of each stage. British records from the Early Pleistocene are poor, since they based on occasional shells carried into the shallow marine deposits of the crags of East Anglia. The Dutch records from the Tiglian are better but most of these remain to be published. Many of the species from these deposits are strong thermophiles but about 20% are extinct. Climatic oscillations are discernible and cold-adapted species such as Columella columella occur in parts of the late Tiglian. Extinct taxa (e.g. Tanousia runtoniana and Valvata goldfussiana) persist into the early Middle Pleistocene and have proved useful in subdividing the 'Cromerian Complex', an important period during which humans spread into NW Europe.

Many extinct taxa vanished before the Anglian glaciation that covered much of southern Britain. The ensuing Hoxnian interglacial appears to have been extremely warm and supported a rich molluscan fauna. A characteristic assemblage of land snails consisting of a mixture of biogeographical elements is known from tufas of this age. This assemblage includes species such as Platyla polita and Clausilia pumila (both central European), Platyla similis (south-east European), Perforatella subrufescens and Leiostyla anglica (western European), Laminifera pauli (western Pyrenean) and Retinella (Lyrodiscus), a subgenus now confined to the Canary Islands. Fluvial assemblages also yielded the so-called 'Rhenish' taxa (i.e. Theodoxus danubialis, Viviparus diluvianus) that suggest a fluvial connection with the continent midway through the stage. These data hint that Britain may not have been an island throughout the entire stage. The Hoxnian is the first of four interglacial stages that are thought to have followed the Anglian, which does not seem to have caused a major extinction. Evidence for the later stages may be found in successive river terraces, which have yielded different faunas. The Last interglacial, for example, is famous for its Hippopotamus fauna but it also differs from earlier ones in lacking the bivalves Corbicula fluminalis and Pisidium clessini. The talk concludes with an examination of faunas from the last cold stage, which must now be viewed as a period of remarkable climatic instability that will have left a genetic legacy on modem faunas.

Illustration

The Late Glacial and Holocene Record:

Faunal Succession during the last 13,000 Years

R. A. Meyrick, Forschungsinstitut und Naturmuseum Senckenberg, Forschungsstation für Quartärpaläontologie, Steubenstraße 19 a, D-99423 Weimar, Germany

Deposits of Late-glacial and Holocene age, which cover the last 13,000 ¹⁴C years, are more widespread than those of earlier periods and allow faunal successions to be studied at much higher temporal resolution. An additional advantage is that they fall within the range of radiocarbon dating, enabling the age of events to be determined with far greater precision.

The Late-glacial period (13,000 - 10,000 ¹⁴C yr BP) has been intensively investigated during the last 40 years. Many of the important Late-glacial land snail successions came from colluvial deposits on the chalk of south-east England. These sediments showed a tripartite sequence, with a soil horizon developed between two units of chalky colluvium. A general biostratigraphical pattern emerged, whereby assemblages from the early part of the Late-glacial consisted of impoverished communities containing no more than about 10 taxa. Additional species, including several with southern modem ranges (Helicella itala, Abida secale) appeared later and these, together with the episode of pedogenesis, were taken to indicate climatic amelioration. Interestingly, these southern taxa did not disappear above the soil, which suggested that the climate may not have been as severe as that from the earlier part of the Late-glacial. However, new work on the waterlogged sediments at the site of the Channel Tunnel has allowed the recovery of organic fossils (plant remains and insects). These show that temperatures in the early part of the Late-glacial were relatively warm and that the climate deteriorated slowly during the "Allerod" phase before plummeting in the Younger Dryas.

Temperatures rose abruptly at the beginning of the Holocene. Many of the best land snail successions have been obtained from tufa sequences, which preserve detailed fossil mollusc records, often at high temporal resolution. The assemblages become progressively richer in terms of species and open ground elements give way to those of woodland. Faunas from the mid Holocene contain elements such as Spermodea lamellata and Leiostyla anglica (in the UK) or Platyla polita, Sphyradium doliolum and Perforatella incarnata (in western Germany). These, and other, taxa clearly had far wider ranges during this period than they do today. Anthropogenic forest clearance in the late Holocene may have caused some range contraction, but it seems likely that climate was also partly responsible for this phenomena, although it can often be difficult to distinguish between the relative influence of these controlling factors.