Читать книгу Mammals in the British Isles - L. Harrison Matthews - Страница 11
ОглавлениеTHE small number of mammalian species now living in the British Isles is sometimes spoken of as an impoverished fauna. This is not strictly correct; it is a small fauna compared with those of some other lands, but of the forty-six indigenous species only five have been exterminated in historic times, whereas fourteen that are not indigenous have been introduced and now permanently enrich it. The causes of the present composition and distribution of our indigenous mammalian fauna must be sought in the geological history of the islands.
The basic geological structure of the country has evolved through enormous periods of time during which the rocks were laid down as deposits on the floors of successive seas, or extruded through the earth’s crust by volcanic activity. If we could see from a satellite the events forming the present topography of the earth as in a time-lapse film, so that many millions of years were concentrated into an hour, the tortured crust would appear to be in constant movement, writhing and squirming as immense forces distorted it. The tectonic plates jostling each other or drawing apart to form the oceans, pushed asunder by the material rising between them from below, were sometimes sunk far beneath the sea from which they received deposits of enormous thickness, or thrust up into mountains and lands from which erosion carried their substance back to the oceans – everything was, and still is, in constant flux. During all these upheavals plants and animals were evolving ever since life first appeared on the earth some time in the Precambrian epoch, perhaps as much as three thousand million years ago.
The successive epochs into which palaeontologists divide geological history each had their characteristic faunas and floras which left their remains as fossils in the rocks, representing a biomass, or aggregate of living matter, so great that it is almost beyond comprehension to the human mind. Among this teeming swarm the mammal-like creatures first appeared in the Triassic epoch, which began some 225 million years ago; but ten million years were to pass before the Eutheria, the placental mammals, evolved towards the end of the Cretaceous epoch. In the succeeding Eocene epoch, which began about seventy million years ago, the orders of mammals that we know as living animals were already differentiated together with others that are now extinct.
During the following epochs, the Oligocene and the Miocene, in which great crustal disturbance took place, including the upraising of the great mountain ranges, the evolution of the mammals produced a vast variety of forms which reached a peak in numbers before the end of the Miocene some twelve million years ago. In the succeeding Pliocene, which lasted about ten million years, the land masses gradually took on their present shapes, and mammalian species began to decline in number, a decline that continues to the present day. Throughout these epochs the climate varied from time to time, sometimes temperate, at others cool and wet, or warm and arid, but it was not until the Pleistocene that the greatest climatic change in later geological history took place.
The Pleistocene epoch was comparatively short; it has been deeply studied using modern techniques during the last fifty years so that our knowledge of it increases every day. It was formerly thought to have lasted about a million years, but is now known to have been probably twice as long – some authorities consider it to have lasted as much as three million years. It is popularly called the ‘Ice Age’, a name that over-simplifies the matter, for the ice ebbed and flowed so that mild periods of sometimes almost tropical warmth, separated successive glaciations. At its height ice sheets covered most of Europe, North America and northern Asia, while another covered Antarctica, as it still does.
It was in 1837 that Louis Agassiz, the Swiss and later American geologist and zoologist, first drew attention to the evidence that glaciers had once covered much of the land, evidence which he had discovered in 1836 on his field excursions in search of fossil fishes.4 His views were adopted by the Reverend Dr William Buckland F.R.S., Canon of Christ Church and Professor of Geology at Oxford, later Dean of Westminster, the English pioneer geologist and palaeontologist. He found similar evidence in the British Isles, especially the grooves and scratches scored in rock surfaces of the north, over which glacial ice had flowed engraving the substrate with the burins of its entrapped stones.28
Buckland was the first President of the British Association for the Advancement of Science, and when he addressed the ‘British Ass’ on the subject of glaciation one of his waggish friends drew a caricature of the great man standing on a surface covered with glacial scoriations while at his feet lay two pebbles, one of them labelled ‘specimen no. 1, scratched by a glacier thirty-three thousand three hundred and thirty-three years before the creation’; the other, ‘no. 2, scratched by a cart wheel on Waterloo bridge the day before yesterday.’
Fig. 2. Position of the ice edge at maximum cover of Eurasian glaciation during the Anglian glaciation of the British Isles.
As with all new theories before they become accepted as established truth, the glacial theory at first met with much opposition as well as ridicule – indeed Buckland himself at first strongly disagreed, and it was only during a tour of Scotland in company with Agassiz in 1840 that he was convinced. Thereafter he has strongly supported the theory and, through the evidence of glacial action on polished and scoriated rocks and the presence of morraines, most of the leading geologists of the day agreed with him. He was also the first to suggest that the famous parallel roads of Glenroy in Scotland were the former shore lines of a glacial lake formed by the damming of Glen Spean by two glaciers coming down the north and east sides of Ben Nevis.
The knowledge that the country, and later that all countries on both sides of the north Atlantic, had once been in the grip of an Ice Age stimulated geologists to more detailed research, and it soon became apparent that there had been not one Ice Age but several. The difficulties of identifying and dating them were enormous, because younger glaciations are bound to disturb, distort, and confuse the traces of older ones, as are denudation, erosion, and changes of sea level in the often long intervals between them. Local variations in the extent and intensity of glaciation further complicate the problem.
The basic pattern of the successive glaciations in Europe was appropriately worked out by investigating the glaciations of the Alps, where Agassiz had first discovered evidence of the ‘Ice Age’. About the beginning of this century Penck & Brückner122, after prolonged study of the gravel terraces laid down by rivers rushing forth from beneath the melting glaciers, concluded that there had been four main ice ages separated by long interglacial periods when the land was free from ice-cover and the climate was comparatively warm. They named the four glaciations after rivers flowing down from the Austrian alps to southern Germany, in the valleys of which they examined the fluvioglacial gravels and moraines; the oldest they named Günz, and the succeeding ones Mindel, Riss and Würm.
The last glaciation, the Würm, reached its peak about 20,000 years ago, but it was not so severe or long-lasting as some preceeding ones. During the Mindel glaciation the ice sheets reached their greatest size and covered an enormous area of Europe, much more extensive than that covered in the later Riss and Würm stages. The interglacial stage between the decline of the Mindel and the onset of the Riss lasted nearly a quarter of a million years, during which a contemporary intelligence might have thought that ice ages had gone never to return. Although the Günz was designated the oldest or first glaciation, there are now known to be indications of numerous glaciations older still, hence the differences of opinion between authorities on the probable length of the Pleistocene epoch. There cannot, in any case, have been any sharply defined boundary between the Pliocene and the Pleistocene, for the whole of geological and biological evolution is a continuous process. The boundaries between all the geological epochs are arbitrary, and are used merely as a convenience with the tacit admission that they cannot represent any specific moment in time.
The history of the Pleistocene is, however, by no means the simple and clear cut sequence as might appear from the basic pattern. During glaciations the edges of the ice sheets advance and retreat to different extents and in different places, and during interglacial periods they re-advance from place to place and retire again in an unending chain of fluctuations that bring variations in topography, climate, flora and fauna. Furthermore, the sequence worked out for the glaciations of the Alps may not correspond exactly with those found elsewhere.
Geologists of many lands studying the glaciations and alternating interglacial periods of the Pleistocene in their own countries have gone deeply into the problems of correlating their local findings with the basic alpine pattern. A general measure of success has been achieved in this though much detail remains obscure, and the sequences in Scandinavia and northern Europe and in North America are found to correspond reasonably closely. They are, as well, found to correspond with the pluvial sequences found in land further south which, though never covered with ice sheets, experienced periods of high rainfall when the ice held more northern latitudes in its grip.
Although the pattern of successive glaciations in the Alps corresponds roughly with that of other parts of Europe and elsewhere, it is in some ways a special case. Even at the maximum of glaciation when a continuous sheet of ice blanketed northern Europe and Asia and covered the British Isles and the site of the North Sea, the ice cap over the Alps was separate and not continuous with the great ice sheet. The causes of the glaciations were similar for both regions but the effects were subject to local variations; consequently the nomenclature for the Alpine glaciations is now applied less uniformly to those of regions further north, including the British Isles.
The difficulty of making exact correlations between Pleistocene events in different places has been resolved by classifying them according to local stratigraphy. Pleistocene deposits, both those of glacial and interglacial stages, are not continuous, and the geologists have to put together the history of the epoch from the examination of scattered and limited samples from many different places. The glacial and interglacial stages are named after the places where well-known deposits of each stage have been studied, and consequently the nomenclature for north western Europe differs from that for the Alps, and from that for the British Isles. Thus the last or Würm glaciation of the Alps corresponds to the Weichselian glaciation of north-western Europe, and the Devensian of the British Isles.
In the British Isles many of the typical pleistocene sites are found in East Anglia and take their names from nearby towns and villages of Norfolk, Suffolk and Essex. The last glaciation however takes its name from the Devenses, the ancient British tribe that lived over 50,000 years later in the area including Four Ashes in Staffordshire, the typical site.128 The succession of deposits is not complete, so that information is lacking about the earliest Pleistocene, and for a period of about a million years in the middle Pleistocene. In spite of these gaps the deposits indicate alternating colder and warmer phases but give no unequivocal evidence of glaciation, with ice sheets covering much of the country, until comparatively late in the epoch when ice cover reached its maximum during the Anglian glaciation, corresponding with the Elster glaciation of northwest Europe and the Mindel of the Alps.
Conditions immediately after the end of the Pliocene, some two to two-and-a-half million years ago, are imperfectly known but there appears to have been a cold stage at first, represented by the Nodule Bed at the base of the Red Crag deposits of East Anglia. A gap in the record of nearly half a million years is then followed by an alternation of two warm and two cold stages represented by pre-glacial deposits of the lower Pleistocene. These are the Ludhamian (Ludham, near Norwich) warm, Thurnian (river Thurn, Norfolk), cold, Antian (river Ant, Norfolk) warm, and Baventian (Easton Bavents, near Southwold, Suffolk) cold. At the end of the Baventian stage another gap in the record lasting about a million years is followed by the warm Pastonian stage (Paston, near Cromer, Norfolk), the first stage of the middle Pleistocene, about half a million years ago.
The following Beestonian (Beeston, near Dereham, Norfolk) was the first cold stage of the middle Pleistocene and was succeeded by a warm stage, the Cromerian (Cromer, Norfolk), which lasted until the onset of the great glaciation over 450,000 years ago. This, the Anglian glacial stage (East Anglia), lasted between fifty and sixty thousand years and covered the whole of the British Isles as far south as the Thames with a sheet of ice that produced the greatest glaciation in the whole of the Pleistocene. When the Anglian stage came to an end the land was free of ice for about 185,000 years during the temperate Hoxnian stage (Hoxne, on the Suffolk–Norfolk border near Eye and Diss); in this stage the temperature was at times higher than that of the present day.
The next glaciation, the Wolstonian (Wolston, near Coventry, Warwickshire) lasted some 60,000 years from about 240,000 to 180,000 B.P. The ice cover did not extend as far south as in the Anglian stage; the ice edge ran south from northern Norfolk and then west across the midlands to the mid Welsh border, thence turning south to reach and follow the north coasts of Somerset, Devon and Cornwall. The succeeding Ipswichian (Ipswich, Suffolk) temperate stage lasted about 60,000 years until about 120,000 B.P. when the cold returned with the onset of the last, Devensian, glacial stage in which the ice covered Scotland, northern England, Wales, and most of Ireland. A large area of the midlands and east Yorkshire was thus free from ice cover, though the ice covering the North Sea encroached on the east coast as far south as Norfolk. The ice of the Devensian stage melted comparatively quickly some twelve thousand years ago so that before 10,000 B.P. the post glacial or Flandrian temperate stage was established, which extends to the present day; it takes its name from the transgression of the North Sea over the former dry land bordered by England and Flanders, when the sea level rose as the water from the ice returned to the sea.
Fig. 3. Limit of ice covering during (a) the Anglian, (b) the Wolstonian and (c) the Devensian glaciations.
In all the glacial stages there were at least two maxima of cold separated by less cold interstadial intervals, and similarly in the interglacial stages the climate fluctuated between cold, temperate, and warm. The beginnings and ends of the glacial stages were gradual, so that as the ice retreated after a glaciation the land was at first polar desert becoming steppe or tundra as the temperature rose; it was then invaded by open boreal forest with birch and pine dominant, which in turn was replaced by dense deciduous forest with alder, oak, ash and other broad-leaved trees. As a glacial stage approached the succession was reversed.
| Epoch | British Isles | Climate |
| Holocene | Flandrian | Temperate |
| Upper Pleistocene | Devensian Ipswichian Wolstonian | Glacial with permafrost Temperate Glacial with permafrost |
| Middle Pleistocene | Hoxnian Anglian Cromerian Cromerian Cromerian | Temperate Glacial with permafrost Temperate Cold with permafrost Warm |
| Lower Pleistocene | (Gap) (ca. 1 M. years) | Cold with permafrost Temperate Cold Temperate |
| Baventian Antian Thurnian Ludhamian | ||
| (Gap) (ca. 1/2 M. years) | ||
| Pliocene | Waltonian |
Fig. 4. Stages of the Pleistocene in the British Isles.
Apart from the climatic changes correlated with the glaciations and producing their advances and retreats, there were during the Pleistocene great changes in the level of the sea in relation to the land. The enormous masses of water withdrawn from circulation and locked up in the form of ice caused a fall in sea level of many hundreds of feet – indeed, it is reckoned that if all the ice even now in the form of glaciers and ice-caps were to melt the level of the sea would rise about three hundred feet.155 On the other hand the land is depressed towards sea level during glaciation by the sheer weight of ice resting upon it. At the same time there has been throughout the Pleistocene from time to time a slow upraising or lowering of the land, the eustatic movements of the tectonic plates.
An important consequence of these changes in sea level, whether caused by withdrawal of liquid water or by movement of the land, was that the British Isles were periodically part of the continent of Europe so that they shared its fauna and flora. Thus the bed of the southern part of the North Sea has for long periods been dry land, and the final opening of the Straits of Dover did not come about until some seven thousand years ago. One cannot help wondering whether this was a sudden dramatic happening in some furious equinoctial gale when low atmospheric pressure and a high spring tide combined with a surge such as those that have brought disastrous floods to East Anglia in recent times, broke the crumbling barrier and sent the waters of the North Sea pouring over into the English Channel – or whether an unusually high tide crept over a low dune between the salt marshes on each side so that the waters met and mixed with so little fuss that no one would have noticed.
The connection with the continent facilitated the return of the flora and fauna after it had been exterminated by each glaciation. At the time of the greatest glaciation some 450,000 years ago an unbroken sheet of ice covered the whole of northern Europe, including the British Isles, except southern England south of a line joining the Thames to the mouth of the Severn.61 The part left free of ice was deeply covered with winter snow, and the sea was full of floating ice. It is doubtful if any of the flora or fauna was able to live there; certainly no mammals could survive, and consequently our present fauna must have arrived after the ice of the great glaciation retreated.
Subsequent glaciations were less extensive so that the midlands as far north as York and the southern part of Ireland were free of ice and provided a possible habitat for those species that could withstand the arctic or subarctic conditions. The changes in flora and fauna are sometimes spoken of as retreats to more congenial climates in the south during the glaciations – the distribution of the plants and animals retreated, but there was no physical movement of individuals, they were merely killed. The return during interglacials was different; the flora gradually spread in by the usual manner of seed dispersal, but the animals and especially the mammals did move in ‘on the hoof’, not as mass migrations but in the course of populations extending their ranges under pressure of numbers as new habitats became available.
The amount of extermination among the mammalian species even in the last glaciation, which did not blanket the whole of the British Isles and ended some twelve to ten thousand years ago, is shown by comparing the 167 species of land mammal now living in western Europe with the 41 of Great Britain and the 21 of Ireland.151 Our fauna is not so much ‘impoverished’ as incomplete; there was not a long enough period of time before the breaching of the Straits of Dover for more species to extend their range into the islands. As H. W. Bates, the naturalist of the Amazon and later for many years secretary of the Royal Geographical Society, said in 1878, the British Isles are ‘a half starved fragment of the Palaearctic’.18
Many methods are now used for dating the events of the Pleistocene: geological methods such as the study of varves, the annual variation in the composition of deposits giving laminated sediments in freshwater lakes; investigation of the palaeomagnetism of rocks; and chemical methods such as radio-carbon dating of organic material derived from living organisms, and potassium-argon dating for older rocks. But in tracing the changes in the composition of the flora and fauna the discovery and study of the fossil or subfossil remains of the plants and animals themselves provides the most important evidence. If the horizons in which mammalian remains are recovered are accurately recorded it is possible to know the composition of the fauna from time to time, and to infer much about the conditions of the environment – when, for example, hippopotamuses lived in the Thames before the Devensian glaciation the climate was, presumably, much warmer than at present.
On the other hand the presence of various species of elephant need not of necessity imply that the climate was warm; the order Proboscidea, now reduced to only two species facing extermination in the not too distant future, was once numerous in species some of which were no doubt able to live in temperate or even cold climates provided there was sufficient vegetation for their food – it does not follow that all were warm climate creatures because their living relatives are. Indeed the mammoth, which was clothed in a warm coat of shaggy hair, was an inhabitant of cold regions. Similarly the woolly rhinoceros, which also had long hair, was present with the mammoth in the last cold interstadial of the Devensian glaciation.155 But hair on rhinoceroses does not imply that the animals live in cold climates for the hairiest of the living species, the Asiatic two-horned rhinoceros, lives in tropical south east Asia.
The hair sticking to the frozen remains of mammoths found in Siberia is red, but may have been darker or brown in life, for the pigments in long-preserved hair, especially when buried, undergo a change towards an auburn red – the hair of Egyptian mummies often has this tinge. The hair of Ben Jonson, who died in 1637, was found to be red when his skull with hair still attached was exposed in 1859 during the reburial in Westminster Abbey of the remains of John Hunter, the surgeon and anatomist.26 When the skull of Sir Thomas Browne, who died in 1682, was exhumed in 1840 the hair associated with it was ‘of a fine auburn colour’; before it was re-interred in the chancel of St Peter Mancroft, Norwich, in 1927 it was examined at the Royal College of Surgeons by H.L. Tildesley,144 who remarked that ‘hair of persons long buried is commonly found to have acquired a reddish tinge, whatever the original shade.’
Our most detailed knowledge of the composition of the flora and the nature of the climate, at different times during the Pleistocene is, however, derived from the study of fossil pollen, a technique now known as palynology. In addition, a study of peat, freshwater and marine molluscan shells, and of insects, especially the wing covers of beetles, has thrown much light on the changes in climate.
The outer layers of a pollen grain are made of the substance sporopollenin which, unlike the inner cellulose layers, is extremely resistant to the action of chemical changes so that pollen grains are almost indestructible by natural agencies. Vast quantities of pollen released by plants, and especially those species that are wind-pollenated, became mixed with the soil and waters and included in the deposits and sediments. The surface of pollen grains is thrown into a great range of shapes and patterns that are characteristic of, and identify, the different species; the presence and relative quantities of pollen in any sample of Pleistocene deposit therefore show the presence and relative abundance of the plants from which they were derived. So, for example, a predominance of conifer pollen indicates a cool climate, and a preponderance of oak pollen points to a milder climate. There are, naturally, difficulties in using pollen analysis; pollen can be carried great distances by the wind, and the indestructible nature of the pollen coat itself allows pollen from old deposits to be washed out and included in younger ones. Palynology has nevertheless proved to be one of the most valuable tools in reaching an understanding of the changes during the Pleistocene.
Palynology was born in Denmark and was developed with great success in the British Isles by Sir Harry Godwin and his pupils at Cambridge so that the history of the British flora, and with it that of the environmental ecology, is now better known than that of any other area of similar size.67 The earlier work of Zeuner155 on the climate, chronology, and faunal successions of the Pleistocene, not only in the British Isles but throughout the world, was extended in great detail by Charlesworth twelve years later.33 This immense work summarises and reviews world-wide research on the Pleistocene up to 1956, and discusses all the different theories that have been put forward to explain its occurrence and the fluctuations that took place during it.
Charlesworth points out that terrestrial causes such as deformations of the crust of the earth are not sufficient to have brought about glaciations. He favours the theory that long-term variations in the amount of solar radiation reaching the earth were the probable cause, though it may not be possible to prove their occurrence by direct observation. This hypothesis was first made by Simpson,130 who suggested that an increase in solar radiation by raising the world temperature intensified the atmospheric circulation, and brought about glaciation by augmenting the amount of cloud and precipitation. Charlesworth points out that glaciation was probably produced by a number of factors, of which variation in solar radiation was only one, and that meteorological, geological, and astronomical changes ‘all interacting and so delicately balanced that a slight change, such as would be undetected by less than a century of acute observation, might induce great effects.’
As an outcome of recent studies it is now widely accepted87 that major glaciations are due primarily to the positions of the continents resulting from continental drift and the movements of the tectonic plates. Ice ages can only occur when there are land masses in high latitudes on which ice and snow can accumulate – the condition of the earth today, with an Antarctic continent and an Arctic sea surrounded by land. With the continents in these positions the ‘Milankovitch effect’ comes into operation, and small regular changes in the earth’s orbit and orientation towards the sun cause the rhythmic alternation of glaciations and mild interglacial stages through the changes in the amount of heat received by high latitudes. The ‘wobbles’ in the earth’s movements are astronomically predictable, and consequently the sequence of ice ages can be shown to have occurred many times, probably twenty or more, during the Pleistocene.19 Predictions warn that our present interglacial may not last more than another thousand years until it begins to decline into the next glaciation, which, at its peak after some 20,000 years will be more severe than the Devensian.
‘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 of the species of this extensive list are typical of colder climates such as the ground squirrel, pine vole, glutton, and musk ox; and others of warmer ones such as the monkey, spotted hyaena and hippopotamus. The majority, on the other hand, are species that might live under a temperate climate like that of the present day in the British Isles. When the Cromerian stage drew towards its end the climate became cooler, and mixed oak forest was replaced by boreal forest with pines and birch, and with open heaths, until the Anglian glacial stage wiped out most of the flora and probably all of the mammalian fauna. The history of the present mammalian fauna of the British Isles must therefore start at the end of the Anglian glaciation, which wiped the slate clean for a new start, leaving us only a few tons of fossil bones from which to infer what had gone before.
It is not surprising that hardly any mammalian fossils are known from the Anglian glaciation, for at its severest the southern part of the country, the only part that was not covered by the deep ice sheet, was an arctic desert. The few that have been found are assigned to the early or late parts of the stage when glaciation was developing or retreating – a ground squirrel to the former and the red deer to the latter. As, furthermore, no vertebrate fossils of other classes are known from the Anglian the conclusion that the glaciation exterminated the entire mammalian fauna is inescapable. The deposits of the Anglian stage are a complicated series of tills, including the Boulder Clay, produced by the ice moving in different directions at different times as the glaciation proceeded.
When the ice at last retreated the temperate flora and fauna of the Hoxnian stage gradually moved in from the continent as the desert gave way to tundra, then to boreal forest followed by mixed oak forest. The fossils of this interglacial stage are preserved mainly in freshwater deposits, though some marine and estuarine deposits exist from its later part. It was during this stage, too, that man first made his way into the British Isles, for his palaeolithic flint artifacts have been found in several places. The former claim that man had been present at a much earlier time is now discredited – the ‘eoliths’ from the Crag that were supposed to be primitive tools are no more than fortuitously broken stones. The only skeletal remains of alleged palaeolithic man living in the Hoxnian stage that have been found in the British Isles are some fragments of a skull from the Thames terrace gravel at Swanscombe, Kent. One bit of the skull was found in 1935, another in 1936, and a third in 1955. Although Oakley in 1969116 tabulated the Swanscombe skull, among the ‘early Neanderthaloids’ dating from about a quarter of a million years ago, the fragments, the occipital and two parietal bones, are indistinguishable from those of modern man. Since then there has been some controversy about the dating of the Hoxnian interglacial119’ 127; but a dating of material from ‘a few centimetres below’ the horizon of the Swanscombe skull gave ages of up to more than 272,000 years.139 This, unfortunately, does not give irrefutable proof of the age of the skull, and still leaves open the possibility advocated by some that the skull fragments may have become included in the gravel as intrusions at a later date. With the possible exception of the Swanscombe skull, the earliest remains of man in the British Isles, apart from his artifacts, date from the middle of the Devensian glaciation, at least some two hundred thousand years later.
The Hoxnian mammalian fauna that moved in from the continent differed from that of the Cromerian, although many species were the same, or similar, such as the beaver, some voles, the wolf, marten, lion, boar, the straight-tusked elephant Palaeoloxodon antiquus, and the red, fallow, and roe deer. New arrivals included the arctic lemming, several voles, the cave bear, two species of rhinoceros replacing Diceros etruscus, Megaceros giganteus replacing several other species of giant deer, and the aurochs. Those that did not return, or were by then extinct, included the vole genus Mimomys, the sabre-tooth, the southern elephant, etruscuan rhinoceros, hippopotamus, zebrine horses, several species of deer, giant deer, and elks, the bison and musk ox.
When the climate became colder with the onset of the Wolstonian glaciation which reached its peak about 140,000 years ago, the fauna became more typically arctic, and those parts of the country not covered with ice were inhabited by hamsters, the arctic, Norway and steppe lemmings, the woolly mammoth, Mammuthus primigeneus, the woolly rhinoceros, Coelodonta antiquitatis, and the reindeer Rangifer tarandus.
At the end of the Wolstonian stage, about 120,000 years ago, the temperate Ipswichian interglacial stage began and lasted about 50,000 years. The climate and flora followed the usual sequence of an interglacial stage, the temperature reaching a peak higher, however, than that of the present day, and the flora progressing from arctic tundra to boreal forest, mixed oak forest and then regressing by similar steps to the onset of the next glaciation. Mammalian remains of the Ipswichian occur in river and lake gravels, muds, and brick earths, and in the deposits of some caves. Many of the mammals are species that form part of our present day fauna, and include the bank, water and field voles, the wood mouse, the red fox, badger, and wild cat, the red, fallow and roe deer; and some extinct only in historic times such as the beaver, wolf, brown bear, wild boar and aurochs. The cooler parts of the stage were also marked by the presence of ground squirrels, the woolly mammoth and the musk ox, whereas the warmer parts supported the spotted hyaena, the lion, the straight-tusked elephant, two kinds of rhinoceros, the giant deer, and the hippopotamus, the last indicating a comparatively high temperature as does the presence of the European pond-tortoise Emys orbicularis. Palaeolithic man, as shown solely by his artifacts, was present throughout the stage.
The following Devensian glaciation began about 70,000 years ago and lasted nearly sixty thousand years until it came to an end rather quickly about 10,000 years ago. It was the least severe of the three great glaciations as it left southern England and the midlands free of ice and thus a possible habitat for many species that can withstand a cold climate. During this stage the sea fell some three hundred feet below its present level so that England was widely connected with the continent over the site of the southern part of the North Sea, and northern Ireland was narrowly connected with southern Scotland. Stuart136 remarks that the vast majority of Pleistocene vertebrate remains found in the British Isles, excluding post-glacial material, is probably of Devensian age. Most of the remains are found in river gravels and caves, some of the later ones in lake sediments. The flora of the ice-free regions was mostly tundra or open grassland, with some patches of boreal forest during short interstadial recessions of glaciation.
The fauna is typical of cold regions, though it includes some species of our present fauna such as the common shrew, the bank, water and field voles, the mountain hare, the fox, stoat, polecat, and red deer. Some of the species are not now associated with severely cold climates but nevertheless can withstand more cold than might be supposed; these are the leopard, lion, and spotted hyaena. On the other hand there are many species typical of colder habitats: a pika Ochotona, ground squirrel, the arctic lemming, several voles including the northern and tundra voles Microtus oeconomus and M. gregalis, the arctic fox Alopex, polar bear, glutton, woolly mammoth which became extinct at the peak of the glaciation about 18,000 years ago, woolly rhinoceros, reindeer, and musk ox. Several other large mammals left abundant remains in gravel and cave deposits; they include the wolf, the brown and cave bears Ursus arctos and U. spelaeus, a sabre-tooth Homotherium, the horse, giant deer, elk, a bison Bison priscus, and the aurochs.
Man returned after the peak of the Devensian glaciation as shown by his artifacts and by a few skeletal remains.30 Some human teeth of middle Devensian age from Picken’s Hole cave in Somerset are the earliest human remains known in the British Isles apart from the Swanscombe skull whose alleged age has been challenged by some people. The largest find of palaeolithic man belongs to the late Devensian deposits of Aveline’s Hole in the Mendip Hills of Somerset, where bones representing thirty-one skeletons were excavated.
As the ice melted during the late glacial stage of the Devensian the climate became milder and reached a peak after about a thousand years in the Allerød interstadial or ‘amelioration’ as it is sometimes called, though amelioration could have a different meaning for a reindeer than for a red deer. Thereafter the climate again became colder until about 10,000 years ago when the ice finally disappeared inaugurating the Flandrian interglacial which has lasted until the present. By the end of the late glacial many of the large mammals had become extinct, although there appears to be no reason why they should not have survived into the Flandrian. Perhaps the change of climate and the resulting changes in vegetation deprived them of their ecological niches, but it is also possible that improved hunting skills of upper palaeolithic man may have overcropped and thus exterminated them. The few species not part of our present mammalian fauna that survived from the late glacial disappeared in the early part of the Flandrian, which is discussed in the next chapter.
