‘Little ice ages’, such as the cold period that lasted from about 1650 to 1850, occur at more frequent intervals. They are caused by a temporary decline in sunspot activity combined with an increase in terrestrial volcanic activity, which produces a veil of dust in the atmosphere that reduces the solar heat reaching the earth. Major glaciations, however, only occur when the earth periodically ‘wobbles’ to produce the Milankovitch effect.
Whatever the causes of glaciation may be, we may take it as certain that the present mammalian fauna of the British Isles originated after the end of the great Anglian glaciation nearly half a million years ago. Furthermore it seems probable that few species of mammal survived the Devensian glaciation, during which ice covered the northern part of the islands, and the southern parts were subjected to a severe periglacial climate with permafrost producing frost-tundra having little plant cover. At the end of the Devensian the succeeding Flandrian post-glacial stage saw the establishment of the mammalian fauna as we see it today, although it is now reduced by the loss of several species that have been exterminated by man.
During the last hundred and fifty years a host of geologists and palaeontologists, amateur and professional, has collected great quantities of mammalian fossils from the Pleistocene deposits, and has worked on the difficult problems of deducing the composition of the faunas of the various stages. The earlier workers did not appreciate the importance of recording the exact horizons at which they found the fossils, and consequently their specimens give less information than those collected by later workers who adopted a stricter discipline. In addition, much material collected on sea beaches came from strata exposed in the cliffs above, and cannot be accurately assigned to the horizon from which it is derived. Similarly fossil and subfossil bones found in caves have frequently been excavated without recording the precise horizon from which they came. The stratigraphy of cave deposits is complicated by the way in which the fossils were included. The remains from which the fossils are derived were often washed in by floods, or carried in by predators, so that specimens of different provenance are confusingly mixed.
The researches of many workers nevertheless combine to give a picture of the succession of faunas that can be accepted with confidence as reasonably accurate. The results are widely scattered throughout a vast literature, but the Monographs of the Palaeontographical Society120 in which large numbers of mammalian fossils from the Pleistocene have been described and illustrated during the last hundred and thirty years, deserve special notice. Many authorities, too, have gathered the available information together to give an account of the Pleistocene faunas, one of the earliest being Buckland’s ‘Reliquiae Diluvianae’27 published in 1823, which described fossils from caves and ‘diluvial gravel’ as evidence of ‘the action of a Universal Deluge’. A later classic is Owen’s ‘History of British Fossil Mammals and Birds’ published in 1846,118 and from the nature of the material available necessarily dealing mainly with Pleistocene faunas. In contrast a modern synthesis based on the results of researches supported by the latest technologies such as Stuart’s review,136 shows the complexity of the succession of faunas, and the differences in fauna with the alternation of cold and warm, glacial and interglacial stages. The following summary of events is based in part on this important work.
The deposits of the lower Pleistocene are the strata of the Red Crag, with the Norwich Crag lying above them, that cover much of East Anglia. The oldest part of the Red Crag is the Nodule Bed found at its base in several places. All are marine deposits laid down when the sea level was considerably higher than at present, sometimes as much as forty feet. The fossil bones of land mammals found in them must therefore represent animal carcases that were washed into the sea, especially by rivers in flood, and consequently may not be a fair sample of the contemporary fauna. The Red Crag Nodule Bed, however, is derived partly from the breakdown of older rocks and contains the remains of their fossils in addition to its own contemporary ones; some are derived from Pliocene or older formations and are much worn and polished by wave action.
The alternating temperate and cold stages of the pre-glacial Lower Pleistocene occupied about the first three-quarters of the epoch, some one and a half million years, leaving only half or at most three quarters of a million years for the more spectacular events of the Middle and Upper Pleistocene. The flora of the different stages, and consequently the nature of the contemporary climate, are inferred from a study of pollen analyses and the invertebrate and vertebrate faunas. Throughout this immense period of time the fauna appears to have changed little in composition. The mammalian fossils known from the deposits laid down in the British Isles during the Lower Pleistocene include giant beavers, voles, bears, a panda, hyaenas, sabre-toothed and other cats, elephants and mastodons, horses and zebras, a tapir, rhinoceros, deer and oxen, all of extinct species, together with the still existing beaver and red fox.
This list does not represent a large fauna for so long a period of time but when we remember that, with the exception of a few species known from the cave deposits in Dove Holes, Derbyshire, all are from marine deposits, it is not surprising that it is short. The carcases of animals washed into the sea soon decay and disintegrate so that the bones are scattered and the most durable parts, the teeth, are those more likely to be preserved in marine deposits. The Nodule Bed of the Red Crag, as mentioned above, contains a mixture of fossils. We can well imagine the sea eroding the cliffs of Pliocene or earlier epochs, and then rolling and polishing the released fossils on the beach until they were again buried in new deposits, just as today the fossils of the Crag can be found lying loose on the beach. Some of the fossils thus represent animals that were not members of the Lower Pleistocene fauna, for example the tapir, three-toed horse, and the panda.
The Middle Pleistocene began with a temperate stage, the Pastonian, which was followed by a cold subarctic stage, the Beestonian; this gave way to another temperate stage, the Cromerian, which preceeded the onset of widespread glaciation. The deposits of the Pastonian are marine sands and gravels known as the Weybourne Crag, the lower part of which was laid down in the Baventian stage of the Lower Pleistocene. The stages that follow are represented by the Cromer Forest Bed series which includes both marine and freshwater sediments and contains many mammalian fossils. A comparatively large mammalian fauna has been recorded from these beds; some species can be assigned to the cooler or to the temperate stages, but the exact position of many remains doubtful.
The fauna of the temperate Pastonian stage included extinct species of ground squirrel, beaver, voles, mammoth, horse, rhinoceros, deer and bison, as well as the still existing wolf, otter, wild boar and hippopotamus. Some of these species may belong to the succeeding cold Beestonian stage when the ground was frozen with permafrost in places, but it has not been possible to reconstruct the mammalian fauna of the stage; it was probably reduced in variety and confined to arctic species.
The rich fauna of the temperate Cromerian stage has yielded a great quantity of fossils that have been collected and studied for nearly two hundred years. Many of them, however, cannot be assigned to the various zones into which modern research has divided the stage because, as already mentioned, the early collectors did not appreciate the importance of recording the exact horizons from which they took their specimens. The mammals living during this stage included a monkey, many different species of rodent large and small, many carnivores from wolf and red fox to hyaenas, lion and sabre-tooth. The ‘big game’ were well represented with elephants and mammoth, horses and zebras, rhinoceros, wild boar and hippopotamus, giant and smaller deer, bison, aurochs, musk ox and sheep.
Some