Differentiation of the British late Middle Pleistocene interglacials: the evidence from mammalian biostratigraphy

Quaternary Science Reviews 20 (2001) 1693–1705

Differentiation of the British late Middle Pleistocene interglacials: the evidence from mammalian biostratigraphy Danielle C. Schreve* Department of Geography, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK

Abstract Acknowledgement of the climatic complexities of the marine oxygen isotope (d18O) record has inspired fresh investigations into methods of identifying and separating interglacial deposits on land. Biostratigraphical analysis of fossil mammalian assemblages, particularly those from long fluvial sequences, has proved to be a uniquely valuable technique in the differentiation of the various temperate episodes of the late Middle Pleistocene in Britain, thereby enabling a sequence of diagnostic mammalian assemblagezones to be established. The scheme has provided an important means of correlation between geographically-distant localities and between different types of depositional environment. Using mammalian biostratigraphical information, this paper explores the evidence for three post-Anglian interglacials prior to the Last (Ipswichian) Interglacial, correlated with Stages 11, 9 and 7 of the oxygen isotope record, and presents the case for the recognition of much smaller-scale environmental and climatic oscillations within these interglacials, possibly corresponding to marine isotopic substages. r 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Recent investigations have demonstrated that the biostratigraphical evidence from fossil mammal assemblages is a particularly effective tool for establishing the number and nature of the different climatic episodes in the late Middle Pleistocene in Britain (Schreve, 1997). The suitability of mammals for this purpose stems from several factors (see Lister, 1992), for example the rapid turnover of many mammalian lineages throughout the Pleistocene through origination and extinction of species, the quantifiable evolutionary trends shown in many of these lineages and the ability of mammals to track the complex climatic and environmental fluctuations of the Pleistocene through means of migration. The intensive and repeated environmental changes that affected Europe during this period led to major disruptions in the geographical distributions of mammalian species, thereby providing the potential for each successive climatic cycle during the late Middle Pleistocene to give rise to a different suite of mammals. This ‘reshuffling’ of the biological pack is of critical importance, since the specific combination of past geographical, environmental and biological factors which leads to a particular group of very diverse *Tel.: +44-1784-443569; fax: +44-1874-472836. E-mail address: [email protected] (D.C. Schreve).

mammals coexisting in Britain is most unlikely ever to be repeated. Earlier biostratigraphical models based upon mammalian evidence, such as those proposed by Sutcliffe (1976) and Currant (1989), acknowledged a greater degree of climatic complexity than had been allowed for in the original stratigraphic scheme of the Geological Society (Mitchell et al., 1973), but still recognised only three interglacials in the post-Anglian period in Britain. In recent years, however, there has been increasing acceptance of evidence for as many as four post-Anglian interglacials in the British Quaternary terrestrial sequence, as acknowledged in the recently-revised scheme of the Geological Society (Bowen, 1999). An important contribution has been the information from long fluvial sequences, generally in the form of alluvial terraces, since these can provide a stratigraphical framework for parts of the Middle and Late Pleistocene into which temporal markers such as glaciations or high sea-level events can be keyed. Successful attempts have been made in relating these long sequences to the marine oxygen isotope record (e.g. Bridgland et al., 1989; Maddy et al., 1991, 1995; Bridgland, 1994), thereby providing a detailed archive of climatic change through the Pleistocene. For the present purpose, the Thames valley has been selected as a testable framework for the relative dating of the various climatic fluctuations of the late Middle Pleistocene, since it has been claimed to

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have the most reliably-dated long terrestrial sequence in Britain (Bridgland, 1994; Fig. 1) and furthermore has yielded rich assemblages of fossil mammals. The results presented here identify three distinctive temperateclimate mammal assemblage-zones (MAZ) that are believed to correspond with three discrete climatic cycles between the Anglian glaciation and the Last (Ipswichian) Interglacial. With reference to the Anglian and Ipswichian as pinning points for Stage 12 and Substage 5e of the marine oxygen isotope record respectively (see later), it is suggested that these MAZs may correspond with marine isotope stages (MIS) 11, 9 and 7. However, it must be emphasized that the model presented here is based purely on the evidence from mammalian biostratigraphy and that it therefore stands alone as an independent means of relative dating, irrespective of actual chronology. It is hoped that future investigation of long fluvial sequences in conjunction with improved absolute dating will ultimately permit much firmer correlations to be made with the marine isotope record. Comparison with other relative dating schemes has demonstrated that the mammalian biostratigraphical model presented here is in complete support of the Thames terrace stratigraphy proposed by Bridgland (1994) and in partial support of the aminostratigraphic scheme of Bowen et al. (1989).

2. The Swanscombe mammal assemblage-zone: faunas of the first post-Anglian interglacial The Hoxnian Interglacial has traditionally been regarded as the first temperate episode after the Anglian glaciation in Britain (Mitchell et al., 1973). At the stratotype at Hoxne in Suffolk, a sequence of interglacial lacustrine deposits directly overlies Anglian Lowestoft Till (West, 1956) and a combination of lithological and high-resolution palaeobotanical evidence records a seemingly unbroken transition at the site from the end of the Anglian into the succeeding interglacial (West, 1956). A robust correlation has also been made with localities in the Lower Thames valley, such as Swanscombe in Kent and Clacton-on-Sea in Essex, on the basis of pollen and molluscan biostratigraphy (Kerney, 1971; Turner and Kerney, 1971). It is clear from the topographical and stratigraphical positions of these latter sites within the terrace ‘staircase’ of the Lower Thames valley that the interglacial episode represented there also immediately post-dates the Anglian (Bridgland, 1994; Fig. 1). The Swanscombe-Clacton-Hoxne inter-correlation would thus appear to uphold the view that the Hoxnian Interglacial is also of directly postAnglian age.

Fig. 1. Transverse section of the Lower Thames terraces (modified from Bridgland, 1994). Proposed correlations with the oxygen isotope record and biostratigraphically diagnostic features of the mammalian assemblages from each interglacial are shown.

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Using aminostratigraphic analyses of non-marine Mollusca, Bowen et al. (1989) correlated Swanscombe and Clacton with Stage 11 of the marine record but, controversially, referred the Hoxne lake beds to a younger temperate episode, equated with MIS 9, a correlation that is in direct contradiction of the wellestablished pollen and molluscan biostratigraphies noted above. The consequent implication is that distinctions should be apparent between Hoxne and Swanscombe in terms of their mammalian assemblages, since they potentially belong to different interglacials. In view of the doubts over the continuity of the record from sediments in closed depressions on the surface of glacial deposits, it seems prudent to consider first the evidence from Swanscombe, where an immediately postAnglian age can be unequivocally demonstrated on stratigraphic and lithostratigraphic grounds. The deposits at Swanscombe preserve a complex sequence of fluvial sediments (Smith and Dewey, 1913, 1914; Bridgland, 1994; Conway et al., 1996), comprising a series of gravels, sands and loams attributed to the Boyn Hill Gravel by the Geological Survey (=Orsett Heath Gravel Formation of Bridgland, 1983, 1994). The Orsett Heath Gravel Formation forms the highest (and therefore the oldest) terrace of the Lower Thames, and the Lower Thames sequence is, in its entirety, Anglian and post-Anglian in age, since the river was only diverted into this part of the valley by the Anglian glaciation. At Hornchurch, Essex, the Orsett Heath gravels directly overlie Anglian till or ‘Chalky Boulder Clay’ (Holmes, 1894; Dines and Edmunds, 1925). The temperate-climate sediments that form a part of the Boyn Hill/Orsett Heath terrace may therefore be considered as a representative of the first immediately post-Anglian interglacial. The Anglian has been widely correlated with MIS 12 on account of the apparent severity of this episode within the marine isotopic record, as estimated by high global ice volume (Shackleton, 1987). Additional support for an MIS 12 correlation came from various sources, including stratigraphy (Ehlers et al., 1991), terrace stratigraphy (Bridgland, 1994), aminostratigraphy (Bowen et al., 1989; Bowen, 1992; Scourse et al., 1999) and Uraniumseries dating (Rowe et al., 1999). The interglacial represented at Swanscombe may thus reasonably be attributed to MIS 11. Recent investigations by Schreve (1997) have characterized in detail the mammalian fauna of the first postAnglian interglacial in Britain, as typified at Swanscombe (see Table 1). The Swanscombe MAZ may be differentiated from faunas of the immediately preAnglian early Middle Pleistocene on the basis of several important characters, for example the absence of small mammal taxa such as the giant shrew, Sorex (Drepanosorex) savini (Hinton), and the vole, Pliomys episcopalis M!ehely. Large mammals of the late Cromerian Com-

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plex, such as the giant deer, Megaloceros verticornis (Dawkins), are also absent, whereas in certain lineages such as the cave bears, direct replacement has occurred, with the early Middle Pleistocene form Ursus deningeri Reichenau superseded by the more advanced form Ursus spelaeus (Rosenmu. ller and Heinroth). The interglacial represented at Swanscombe also witnesses the first appearance in Britain of four taxa that are not found in the preceding Cromerian Complex: Merck’s rhinoceros, Stephanorhinus kirchbergensis (J.ager), the narrow-nosed rhinoceros, Stephanorhinus hemitoechus (Falconer), the giant deer, Megaloceros giganteus (Blumenbach), and the aurochs, Bos primigenius Bojanus, in addition to the first record in Britain of the extinct stenonid horse, Equus hydruntinus Regalia. Furthermore, the occurrence of a large-bodied form of fallow deer, Dama dama clactoniana (Falconer), appears to be unique to this interglacial. Other biostratigraphically significant elements of the Swanscombe MAZ include the small mole, Talpa minor (Freudenberg), the giant beaver, Trogontherium cuvieri Fischer, the rabbit, Oryctolagus cuniculus (Linn!e), and the European pine vole, Microtus (Terricola) subterraneus (de Selys Longchamps) [=Pitymys arvaloides Hinton]. In combination with U. spelaeus and D. d. clactoniana, these taxa form a highly distinctive group that can be consistently observed in a range of

Table 1 Occurrence of biostratigraphically important mammalian taxa from British late Middle and Late Pleistocene interglacial deposits, with their suggested mammal assemblage-zone (MAZ) grouping and proposed correlation with the marine isotope record (MIS). Data from (1) Schreve (1996), (2) Schreve et al. (in press), (3) Schreve (1997), (4) Currant and Jacobi (this volume)

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depositional environments and over a relatively widespread geographical area. Most critically, not one of these ‘indicator species’ has ever been recovered from any British Pleistocene deposit younger than those of the Orsett Heath Formation, either in the Thames valley or in the rest of Britain, although O. cuniculus and M. (T.) subterraneus are still extant. Noteworthy absences from the Swanscombe MAZ are the spotted hyaena, Crocuta crocuta Erxleben, and the hippopotamus, Hippopotamus amphibius Linn!e. The remains of the water vole, Arvicola terrestris cantiana (Hinton), from Swanscombe are also of biostratigraphic significance. It has long been recognised that the fossil A. t. cantiana possesses small and permanently-growing rootless molars that display a ‘Mimomys differentiation’, i.e. one in which the enamel of the trailing (convex) edges is markedly thicker than that of the leading (concave) edges). The extant European Arvicola terrestris terrestris (Linn!e), on the other hand, has relatively larger molars in which the enamel differentiation is reversed (‘Microtus differentiation’), with the thicker enamel on the leading edges of the salient angles. Throughout the Middle and Late Pleistocene, several evolutionary trends are apparent in the water vole lineage as the size of the molars and height of the crown increase, the anteroconid complex of the M1 lengthens (Meulen, 1973) and the thickened enamel on the convex sides of the molars decreases (Heinrich, 1982, 1987; Kolfschoten, 1990; Koenigswald and Kolfschoten, 1996; Maul et al., 1998). A quotient (SDQ) is employed to express the relative thickness of the enamel on the trailing edges of the molars as a percentage of the corresponding leading edges. The SDQ values for Swanscombe Arvicola sample (n ¼ 4) fluctuate around 140, compared to progressively declining values for younger Arvicola material (see later). Correlation of other British mammalian assemblages with the Swanscombe MAZ may be achieved purely upon the presence of either U. spelaeus or D. d. clactoniana, or where mean SDQ values for Arvicola are in the order of 140. However, the admission of lithostratigraphic evidence allows the ‘last appearance’ data of other species to be taken into consideration. Thus, where a post-Anglian age for a site can be unequivocally established on lithostratigraphic grounds, the presence of T. minor, T. cuvieri, O. cuniculus or M. (T.) subterraneus is sufficient to imply correlation with the Swanscombe MAZ. Detailed examination of the mammalian assemblages from both Hoxne and Swanscombe has demonstrated beyond reasonable doubt that the faunas from these two localities must be contemporary, on the basis of these highly significant occurrences and absences and other biostratigraphical criteria, such as size change and particular stages of morphological evolution (Schreve,

1997, 2000). Of paramount significance is the presence at Hoxne in both the Lower and Upper sequences of T. minor, T. cuvieri and M. (T.) subterraneus, taxa that were apparently restricted to the first post-Anglian interglacial in Britain. Although the subspecific identity of the Hoxne fallow deer is unknown, the body size of the Hoxne sample compares most closely with the large form from Swanscombe, thereby suggesting that the Hoxne fallow deer may also be the clactoniana subspecies (Lister, in Stuart et al., 1993), known only from the first post-Anglian interglacial. The close correspondence of the Hoxne mammalian fauna to the Swanscombe MAZ strongly implies that the two are of equivalent age. Correlation of the Hoxnian Interglacial, as represented at the type locality, with the interglacial episode represented in the Orsett Heath Formation of the Lower Thames is accordingly suggested. Since the interglacial represented at Swanscombe may be demonstrated to immediately post-date the Anglian glaciation and is correlated here with MIS 11, an age within MIS 11 is also proposed for the interglacial deposits at Hoxne (and hence for the Hoxnian Interglacial), contra Bowen et al. (1989). This attribution has recently received support from the re-calculation of previously-published Uranium-series and ESR age estimates on teeth from the upper part of the interglacial sequence at Hoxne (Stratum C). The revised estimates resulted in a mean age of 404+33 @42 ka BP, indicating deposition within MIS 11 (Gru. n and Schwarcz, 2000). Using information from the biostratigraphically diagnostic mammalian fauna established at Swanscombe and Hoxne, other assemblages attributable to the Swanscombe MAZ have been identified in the Boyn Hill/Orsett Heath Formation of the Lower Thames (and its equivalents) from Ingress Vale (Kent) and Clactonon-Sea. The recognition of a diagnostic Swanscombe MAZ mammal fauna has further proved invaluable in demonstrating the age of sites outside long fluvial sequences, in particular those which rest immediately on top of Anglian till but which otherwise may have little or no chrono-stratigraphic control (Schreve, 1997). The majority of these are developed within shallow depressions or basins on top of Lowestoft Till, such as Barnham, Suffolk (Ashton et al., 1998), Copford, Essex (Brown, 1852) and the Hitchin lake beds, Hertfordshire (Reid, 1897). Others are of fluvial origin, such as Woodston, Cambridgeshire (Horton et al., 1992), or the product of calcareous (tufa) deposition, for example West Stow (Beeches Pit), Suffolk (Preece et al., 2000) and Hitchin (Oughtonhead Lane), Hertfordshire (Kerney, 1959) (Fig. 2). The evidence from mammalian biostratigraphy is further reinforced by molluscan biostratigraphy at some sites, for example, the presence of the distinctive ‘Rhenish’ fauna at Swanscombe, Ingress Vale and Clacton-on-Sea (see Keen, this volume; Roe, this volume), and the presence of Retinella

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Fig. 2. Locality map of late Middle Pleistocene fossiliferous sites mentioned in the text.

(‘Lyrodiscus’) skertchyli Kerney in the tufas of West Stow and Hitchin (see Keen, 2001).

3. The Purfleet mammal assemblage-zone: mammalian biostratigraphic evidence for a post-Hoxnian, pre-Ipswichian (MIS 9) temperate episode The model for terrace succession in the Lower Thames valley proposed by Bridgland (1994) recognizes interglacial sediments of putative MIS 9 age within the second post-Anglian gravel terrace, the Corbets Tey Gravel Formation, equivalent to the Lynch Hill Gravel Formation of the Middle Thames and the BarlingDammer Wick Gravel of eastern Essex (Bridgland et al.,

1993; Bridgland, 1994). According to Bridgland’s model, these interglacial deposits occupy an intermediate stratigraphic position in the Thames terrace ‘staircase’ (Fig. 1), lying between the MIS 11/Hoxnian interglacial deposits of the Orsett Heath terrace and the interglacial deposits present within the lower Mucking terrace. The deposits at Swanscombe in the higher Orsett Heath Formation are separated from the lower-level (and therefore younger) Corbets Tey terrace deposits by sediments indicative of a period of cold conditions. This episode is also represented in the Corbets Tey terrace by basal, pre-interglacial deposits, and in the valley as a whole by the erosion event that caused the river to cut down to the level of the Corbets Tey Gravel. The interglacial deposits present within the Corbets Tey

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terrace may be similarly differentiated from those of the next youngest terrace, that formed by the Mucking Formation. It would accordingly be expected that faunal differences should be apparent between mammalian assemblages from the Swanscombe interglacial deposits and those from the Corbets Tey terrace, and between those from the Corbets Tey terrace and the younger Mucking terrace. However, this has been challenged by Gibbard (1994, 1995), who has assigned all of the Corbets Tey interglacial deposits to the Ipswichian Interglacial. Once again, the mammalian biostratigraphical evidence has proved critical in testing the merits of each lithostratigraphic scheme and in determining the relative age of the Corbets Tey interglacial deposits. A significant development has been the recovery of a highly diagnostic suite of mammals from the site of Greenlands Pit, Purfeet, analysis of which has confirmed the hypothesis that interglacial deposits are present in the Corbets Tey Formation that cannot be related to any previously recognized temperate stage (Schreve, 1997; Schreve et al., in press). The mammalian evidence from Purfleet is considered here to form the characteristic assemblagezone for this interglacial. Similar mammalian assemblages, also attributable to the Purfleet MAZ, were recognized by Schreve (1997) in the existing collections from other interglacial deposits within the Corbets Tey Formation at Grays Thurrock (Morris, 1836) and Belhus Park (Bridgland, 1994), Essex, and from Thames tributary deposits at Cudmore Grove on Mersea Island, Essex (Roe, 1995) (Fig. 2). These assemblages enhance and extend the evidence from Purfleet, particularly in the case of Grays Thurrock, from where there is an extremely rich assemblage of over 1500 specimens collected mostly during the 19th century. However, for the present purpose, Purfleet is deemed the more appropriate ‘type locality’ on account of the continued exposure of the Pleistocene deposits there. The Purfleet MAZ differs from the Swanscombe MAZ in a number of significant ways. Of particular importance is the observation that the mammalian assemblages from Purfleet, Grays Thurrock, Belhus Park and Cudmore Grove do not contain a single example of the ‘indicator species’ that have been found to characterise the Swanscombe MAZ. Thus, in the case of the small mammals, Talpa minor, Trogontherium cuvieri, Oryctolagus cuniculus and Microtus (Terricola) subterraneus are all absent (see Table 1). This negative observation is based on extensive sieving for microvertebrate remains at both Purfleet and Cudmore Grove, the recovery of large and diverse faunal assemblages (over 3000 specimens combined, including large quantities of small mammal remains of other species) and the physical proximity and similar environmental conditions of these sites to the Swanscombe MAZ localities in the Thames valley.

In addition, Grays Thurrock and Cudmore Grove contain the remains of a water shrew, attributed on the basis of size to Neomys cf. browni Hinton. N. browni was first described by Hinton (1911) from Grays Thurrock and was considered by him to represent a transitional form between the small early Middle Pleistocene water shrew, Neomys newtoni Hinton, and the larger, modern species Neomys fodiens (Pennant). Whether this animal represents a legitimate, distinct species remains open to question, but the size of the Neomys material may nevertheless serve as a useful marker for this MAZ. The lion, Panthera leo (Linn!e) is apparently very poorly represented during this interglacial, with only a single specimen from putative Corbets Tey deposits at the Cauliflower Pit, Ilford, noted. This contrasts markedly with the abundance of other carnivore remains from deposits of this age, in particular the brown bear Ursus arctos Linn!e. The appearance of this latter species in the British fossil record at this time is especially significant, since it appears to have exclusively replaced the cave bear U. spelaeus, known to characterize the preceding Swanscombe MAZ (Fig. 1). Again, the remains of water vole (A.t. cantiana) from the Purfleet MAZ are of biostratigraphical signficance, since they show a more derived morphology than those of the Swanscombe MAZ. In contrast to the higher SDQ values for the Swanscombe MAZ samples, those from Cudmore Grove and Grays Thurrock have a mean value of 130, thereby implying a younger age. The Purfleet MAZ is further differentiated from the Swanscombe MAZ by the presence of C. crocuta, which has never been recovered from any supposed Hoxnian site, and by the presence of elk (Alces sp.), which is equally unknown from the deposits assigned to the first postAnglian interglacial, although this latter species is a rare component of the British fossil record. Unfortunately, although remains of fallow deer are present in some abundance in assemblages of the Purfleet MAZ, no antlers have yet been recovered. In the absence of these crucial diagnostic elements, the subspecific identity of the Purfleet MAZ Dama dama remains unknown. However, the smaller size of the postcranial material from this group of localities suggests that it does not belong to the D. d. clactoniana of the Swanscombe MAZ. The Purfleet MAZ may be differentiated from younger interglacial assemblages on a number of counts. The macaque monkey Macaca sylvanus (Linn!e) is a regular component of Purfleet MAZ sites but is unknown from all later British Pleistocene deposits, despite the occurrence of suitable habitats. In addition, as outlined above, the particular evolutionary stage of the water vole is also significant, since younger British temperate stage deposits are (to date) characterized by a more derived form of Arvicola in which the M1 s display lower mean SDQ values than those encountered in the Purfleet

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MAZ. The unusual paucity of P. leo from the Purfleet MAZ may also be a significant feature, since remains of this species are very commonly encountered in all other late Middle and Late Pleistocene interglacial episodes. The combined presence of horse (Equus ferus Boddaert), humans (Homo sp.) (attested by artefacts and butchery marks on bones) and S. kirchbergensis clearly differentiates the fauna of the Purfleet MAZ from those of the Ipswichian interglacial (see Currant and Jacobi, this volume), since none of these species are recorded from that period (Currant, 1989; Sutcliffe, 1995). A preIpswichian age is further supported by the abundant presence of the bivalve Corbicula fluminalis Mu. ller at Purfleet, a species that was also apparently absent from the British Isles during the Last Interglacial (Keen, 1990 and this volume; Bridgland, 1994; Meijer and Preece, 1995). This information, in conjunction with terrace stratigraphy and new mammalian biostratigraphic evidence, makes it clear that the Ipswichian age proposed for Purfleet can no longer be substantiated (contra Hollin, 1977, and Gibbard, 1994, 1995). Assemblages from the four localities of Purfleet, Grays Thurrock, Belhus Park and Cudmore Grove together make up a highly distinctive, diagnostic group that cannot be attributed by any previously defined interglacial sensu Mitchell et al. (1973). Correlation of the interglacial represented by the Purfleet MAZ with MIS 9 is therefore proposed. The rejection of the MIS 9 age for the Hoxnian Interglacial and the correlation of that episode with MIS 11 has thus enabled a genuine MIS 9 interglacial mammal fauna to be identified and described for the first time in the British Isles. The mammalian evidence presented here supports the terrace model proposed by Bridgland (1994), providing a clear indication that the interglacial represented at Purfleet is not merely preIpswichian but that it is older than MIS 7. In eastern Essex, the Thames-Medway equivalent of the Corbets Tey Formation at Barling has also yielded interglacial sediments of putative MIS 9 age (Bridgland et al., in press). Although the mammalian assemblage recovered from these deposits is as yet small, the absence of any Swanscombe MAZ indicator species tentatively reinforces the ‘post-Swanscombe’ age inferred from other lines of evidence such as terrace stratigraphy and molluscan biostratigraphy. Further afield, temperateclimate deposits underlying Terrace No. 5 of the River Avon at Pershore, Worcestershire, have also been correlated with MIS 9 (Maddy et al., 1991; Maddy, 1997). Evidence for the age of the Pershore interglacial deposits is based on the aminostratigraphic model for the Severn and Avon basins and correlation of the Pershore Member with its equivalent, the Bushley Green Member of the Severn Valley Formation (Maddy et al., 1991; Maddy, 1999). The mammalian remains from Pershore are too limited to be of any assistance in

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pinpointing the precise age of the deposits, although the presence of horse indicates that the deposits must predate the Ipswichian Interglacial. However, the existence of Hippopotamus in the Last Interglacial age in Terrace No. 3 of the River Avon, and of interglacial deposits attributed to the penultimate MIS 7 interglacial underlying Terrace No. 4 at Ailstone, Warwickshire (Bridgland et al., 1989; Maddy, 1999), support an older age for the interglacial sediments at Pershore. It is to be anticipated that future collection will improve our knowledge of these faunas in areas beyond the Lower Thames valley. The recognition of a diagnostic mammalian fauna attributable to MIS 9 is considered here to be a very significant step towards the understanding of the British Quaternary succession, since no other biostratigraphical scheme has previously been able to differentiate between the two temperate-climate late Middle Pleistocene temperate episodes that have been assigned to the ‘Hoxnian sensu lato’. Separation of those sites with ‘Hoxnian-type’ pollen signatures into two distinct interglacials, correlated with MIS 11 and 9 respectively, has confirmed the presence of high counts of silver fir (Abies) pollen, the unidentified palynomorph ‘Type X’ and the water fern Azolla filiculoides in both episodes (Thomas, this volume), thereby casting doubt upon the use of these taxa as indicators of only one post-Anglian interglacial. Certainly, all three taxa appear in conjunction with mammalian assemblages that are clearly referable either to the Swanscombe MAZ or to the Purfleet MAZ. Other relative dating methods, such as aminostratigraphy, have produced inconsistent and widely-differing results on the Purfleet/Grays Thurrock/Belhus Park/Cudmore Grove group of sites. It is therefore concluded that the mammalian biostratigraphic evidence established here provides a reliable means of separating the MIS 11 (and by extension, the Hoxnian Interglacial) from MIS 9.

4. The Ponds Farm mammal assemblage-zone and the Sandy Lane mammal assemblage-zone: mammalian biostratigraphical evidence at Aveley for a complex penultimate (MIS 7) temperate episode During the 1970s, discrepancies between the palynological record and other lines of evidence, particularly those from fossil mammals, revealed the possibility that sites of more than one age had been conflated into a single Ipswichian Interglacial (Sutcliffe, 1975, 1976; Sutcliffe and Kowalski, 1976). The recognition in the British terrestrial record of a post-Hoxnian, preIpswichian interglacial, correlated with MIS 7, originally centred on the evidence from three key localities in the Thames valley, Aveley and Ilford (both in Essex) and Trafalgar Square in central London. At all three

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sites, sediments of clearly temperate-climate aspect had been assigned to the Ipswichian (Last) Interglacial (West, 1969; Mitchell et al., 1973; Hollin, 1977), based upon comparison of their pollen records with that from the Ipswichian stratotype at Bobbitshole, Suffolk (Spencer, 1953; West, 1957). However, the temperateclimate deposits at Aveley and Ilford are situated at a higher terrace level than Trafalgar Square (c. 15 m O.D. compared to just below present-day sea level at Trafalgar Square) and had yielded different mammalian assemblages that, according to Sutcliffe (1975), were impossible to reconcile with the view that all three sites were contemporaneous. The presence of Hippopotamus amphibius in pollen subzone Ip IIb deposits was considered to be diagnostic of that part of the Ipswichian Interglacial, as was the absence of Equus ferus, according to Stuart (1976). The evidence from Trafalgar Square accords well with this interpretation, since the sediments of Ip IIb age have yielded an assemblage characterized by hippopotamus in association with straight-tusked elephant, fallow deer and narrow-nosed rhinoceros, but seemingly lacking in horse (Franks et al., 1958; Franks, 1960). Other sites outside the Thames valley, such as Barrington, Cambridgeshire (Gibbard and Stuart, 1975) and Maxey, Lincolnshire (French, 1982), have similarly produced remains of H. amphibius from Ip IIb deposits, although at Swanton Morley, Norfolk, hippopotamus was recovered from early Ip III sediments (Coxon et al., 1980), thereby suggesting that the chronological range of this species should be extended into the posttemperate part of this interglacial. No site in Britain yielding hippopotamus has also produced remains of horse and it has consequently been concluded that this latter animal was genuinely absent from the British fauna during the Ipswichian (Currant, 1989; Sutcliffe, 1995). The mammalian fauna of the Last Interglacial climatic optimum (the Joint Mitnor Cave MAZ of Currant and Jacobi, this volume) has been correlated with MIS 5e on the basis of consistent radiometric dating of deposits clearly associated with this fauna to around 120 000 BP. Uranium-series dating of speleothem sealing deposits containing a Hippopotamus assemblage at Victoria Cave, North Yorkshire has produced age-estimates of 12076 ka BP (Gascoyne et al., 1981) and these have since been corroborated at other Last Interglacial sites by further U-series ageestimates of 129–116 ka BP obtained from stalagmite fragments from Bacon Hole, Gower (Schwarcz, 1984a; Stringer et al., 1986), approximately 121 ka BP on travertine from the raised beach at Belle Hougue Cave, Jersey (Keen et al., 1981) and 127–107 ka BP on flowstone from Minchin Hole, Gower (Schwarcz, 1984b). In complete contrast, the assemblages from Aveley and Ilford are characterized by abundant horse, in

association with a primitive form of woolly mammoth, Mammuthus primigenius (Blumenbach), in a clearly temperate context. At Aveley, horse first occurs in deposits attributed to pollen zone Ip IIb, contra Stuart (1976), and persists into zone III at both Aveley and Ilford, where it is joined by M. primigenius. Hippopotamus is apparently absent from both localities (see Table 1) from a combined assemblage of over 2500 specimens. At Ilford, S. kirchbergensis has also been recorded, yet this species is unknown from the British Last Interglacial. Its absence has been explained by its failure to re-colonise from the continental mainland before a rise in sea level during the early Ipswichian cut off access to Britain (Koenigswald, 1992). The differences in the mammalian assemblages from Aveley, Ilford and Trafalgar Square make it unlikely that all three deposits could be contemporaneous and it was therefore suggested that they must either represent different parts of the same interglacial, or two separate interglacials (Sutcliffe, 1964, 1976; Sutcliffe and Kowalski, 1976). The palynological succession would oblige the mammoth-horse fauna of zones Ip III–Ip IV to be later in time than the hippopotamus fauna of zone IIbFbeginning of zone III; but in reality, a mammalian assemblage of Aveley/Ilford type has never been found to overlie a hippopotamus assemblage in any Pleistocene deposit (Stringer et al., 1986). The combined evidence from mammalian biostratigraphy and differences in the heights of the interglacial deposits led Sutcliffe to conclude that the Trafalgar Square deposits were laid down during the Ipswichian Interglacial but that the Aveley and Ilford deposits accumulated during an earlier, post-Hoxnian, pre-Ipswichian temperate interval, now correlated with Stage 7 of the oxygen isotope record (Shotton, 1983). Corroborative evidence has come from molluscan biostratigraphy (Keen, 1990, this volume), coleopteran biostratigraphy (Coope, this volume) and aminostratigraphy, with sites of Aveley-Ilford affinity consistently producing higher ratios than sites of Last Interglacial age (Bowen et al., 1989). The presence of deposits of MIS 7 age within the terrace succession of the Lower Thames has been upheld in recent years by the model proposed by Bridgland (1994; Fig. 1), although dating of these deposits continues to be a matter of controversy and the hypothesis of a single, complex Ipswichian stage is still favoured by certain workers (Gibbard, 1994, 1995). However, despite the increasing recognition of a separate MIS 7 interglacial, the details of mammalian succession within this interglacial remain relatively poorly known. Again, the sequence in the Lower Thames valley may be employed as a base model for investigating and correlating deposits of putative MIS 7 age. Further fieldwork at Aveley was undertaken by the author along the course of the newly-upgraded A13 dual-carriageway, resulting in the collection of important new mammalian

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assemblages (Schreve, 1997). These latest excavations augment investigations in the 1960s at Sandy Lane Quarry, Aveley, where stratigraphically-equivalent interglacial deposits yielded virtually complete skeletons of straight-tusked elephant, Palaeoloxodon antiquus (Falconer and Cautley), and woolly mammoth (Anon, 1966; Blezard, 1966). The mammalian biostratigraphical evidence thoroughly supports the presence in the terrestrial record of a third post-Anglian pre-Ipswichian temperate episode. However, it is apparent that the sequence at Aveley represents more than a single climatic event, thereby permitting still higher-resolution patterns to be discerned. Preliminary evidence from the most recent exposures suggest that two, and possibly three, temperate climatic episodes are present, separated by two breaks in deposition. The findings indicate that the mammalian fauna from Aveley may be subdivided into two discrete groups, considered to reflect the earlier part of the interglacial and the later part of the interglacial, respectively, possibly representing isotopic substages (Schreve, 2001). This is the first time that the mammalian remains from the penultimate interglacial have been interpreted in a way other than as part of a single temperate episode. To avoid confusion, the older of the two mammal assemblage-zones (representing the earlier part of the interglacial) is named here as the Ponds Farm MAZ, whereas the younger is referred to as the Sandy Lane MAZ. 4.1. The Ponds Farm MAZ At Aveley, the oldest temperate sediments in the Mucking Formation are represented by the Aveley Sands, which have yielded fully interglacial vertebrate assemblages, including common mole, Talpa cf. europaea Linn!e, bank vole, Clethrionomys glareolus (Schreber), wood mouse, Apodemus sylvaticus Linn!e, and the obligate thermophile, the European pond terrapin, Emys orbicularis Linn!e. Although the assemblage is as yet small, remains of Arvicola from this bed show a more derived morphology than those from the Purfleet MAZ, with a mean SDQ of 120 (n ¼ 4). In places, the overlying lower silts at Aveley cut across bedding structures in the sand; elsewhere the junction is marked by gravels or concentrations of winnowed shells. This non-conformity indicates erosion between the two phases of deposition, although the cause and duration of this hiatus are unknown. The lower silts themselves are equally characterized by a predominance of temperate and woodland indicators. Important diagnostic features of this period include the presence of a whitetoothed shrew (Crocidura sp.), which today has a predominantly southern European distribution (Churchfield, 1988), in association with P. antiquus. Horse, red deer (Cervus elaphus Linn!e), aurochs and

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bison (Bison priscus Bojanus) are also present, but woolly mammoth is apparently absent. Assemblages assigned to this early part of the penultimate interglacial include that from Stone Point, Hampshire (Reid, 1893; Brown et al., 1975), where estuarine deposits attributed to the pre-Ipswichian Lepe Gravel Member of the New Forest Formation of the Solent River have yielded remains of fallow deer and straight-tusked elephant. The fauna from the Otter Stratum at Tornewton Cave, Devon, (Roberts, 1996) may also belong to the Ponds Farm MAZ, although the unusual nature of this assemblage (almost exclusively insectivores and carnivores) renders biostratigraphical attribution problematic. The greater white-toothed shrew, Crocidura russula Hermann, is present in the Otter Stratum, as is the only British record of the clawless otter, Cyrnaonyx antiqua (Blainville). A pre-Ipswichian age for the Otter Stratum is upheld by the stratigraphic position of this bed below deposits now recognised to be of Ipswichian age (the Bear Stratum) and is further supported by recent Uranium-series dating of stalagmite deposits above and below the Otter Stratum that have yielded age-estimates of 224 ka and 298 ka BP, respectively (A.P. Currant, pers. comm.). 4.2. The Sandy Lane MAZ Following deposition of the lower silts at Aveley, a second break in deposition, suggested by preliminary evidence of the development of a soil (R. Kemp, pers. comm.), is considered to represent a landsurface, possibly exposed during a period of cooler conditions (Schreve, 1997, 2001). A return to temperate climatic conditions is indicated by a detritus mud, in turn overlain by the Aveley upper silts. In contrast to the Ponds Farm MAZ of the earlier part of the interglacial, the Sandy Lane MAZ is defined by a predominance of species favouring open grassland, in particular woolly mammoth and horse. M. primigenius is represented frequently (but not exclusively) by the ‘Ilford type’ mammoth, a morphologically less-derived form characterized by a combination of small size and low plate count in the molars. The occurrence of the ‘Ilford type’ mammoth is considered to be of biostratigraphical significance in demonstrating an age within the Sandy Lane MAZ and the percentage of molars attributed to this form reaches 100% at some localities. A significant element of the small mammal fauna of the Sandy Lane MAZ is a large form of northern vole, Microtus oeconomus (Pallas), the molars of which show a progressive increase in size throughout the late Middle Pleistocene, reaching their maximum in deposits attributed to the cold episode immediately prior to the Last Interglacial (Schreve, 1997). An important absentee from the Sandy Lane MAZ (from a combined assemblage of over 13 000 specimens) is the fallow deer, the

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large size of the collections lending considerable weight to such negative evidence. Woodland indicators such as P. antiquus and S. kirchbergensis persist during the Sandy Lane MAZ, albeit in greatly reduced numbers compared to open grassland taxa such as M. primigenius and S. hemitoechus. P. leo is extremely common with U. arctos and the wolf, Canis lupus Linn!e, also wellrepresented. In addition, smaller numbers of C. crocuta are consistently recorded and the leopard, Panthera pardus Linn!e, is present in upland cave sites. Other assemblages attributable to the Sandy Lane MAZ include those from the Uphall Pit at Ilford, Essex (Cotton, 1847), West Thurrock, Essex (Bridgland, 1994), Lexden, Essex (Fisher, 1868), Sible Heddingham, Essex (Whitaker et al., 1878), Great Yeldham, Essex (Whitaker et al., 1878), Brundon, Suffolk (Moir and Hopwood, 1939), Stoke Tunnel, Suffolk (Layard, 1912), Stutton, Suffolk (Whitaker et al., 1878), Harkstead, Suffolk (Spencer, 1962), Crayford, Kent (Kennard, 1944), Northfleet, Kent (Bridgland, 1994), the lower channel at Marsworth, Buckinghamshire (Green et al., 1984), Stoke Goldington, Buckinghamshire (Green et al., 1996), the Stanton Harcourt channel deposits, Oxfordshire (Bridgland, 1994), Selsey, West Sussex (West and Sparks, 1960), West Wittering, West Sussex (Reid, 1892), Upper Strensham, Worcestershire (de Rouffignac et al., 1995), Bleadon Cave, Somerset (Schreve, 1997), Hutton Cave, Somerset (Schreve, 1997), Oreston, Devon (Buckland, 1823), Bielsbeck, East Yorkshire (Schreve, 1999), Hindlow Cave, Derbyshire (Schreve, 1997) and Pontnewydd Cave, Clwyd (Green, 1984) (see Fig. 2). The dramatic change in the composition of the mammalian fauna between the Ponds Farm MAZ and the Sandy Lane MAZ, both present at Aveley, lends support to the idea of an intervening cooler episode during the penultimate interglacial. This would have permitted a lowering of sea level and allowed the immigration of taxa such as M. primigenius and the exotic felid, the jungle cat Felis chaus Gu. ldenstaedt, known only from the Sandy Lane MAZ at Aveley. At the correlative site of Crayford, other new species appear during the Sandy Lane MAZ, including the collared lemming, Dicrostonyx torquatus (Pallas), the Norway lemming, Lemmus lemmus (Linn!e), the ground squirrel or suslik, Citellus citellus (Linn!e), the woolly rhinoceros, Coelodonta antiquitatis (Blumenbach) and the musk ox, Ovibos moschatus Zimmerman. Without a period of terrestrial reconnection, it is difficult to envisage how a number of species that are characteristic of continental environments could have appeared in Britain at this point. The timing of the suggested reconnection has yet to be established, although it is possible that it occurred during substage 7d of the marine isotope record, when global ice volume appears to be sufficient to have lowered sea level enough to rejoin Britain to mainland Europe (Schreve, 2001). It is

clear that far more of the assemblages mentioned above relate to the later part of the interglacial, that characterized by the Sandy Lane MAZ. The sites concerned cover a wide range of depositional environments, including both cave and open sites, yet these assemblages are remarkably consistent in their composition. This might suggest that they are the result of very rapid (perhaps even catastrophic) depositional events occurring over a relatively short space of time, or even the consequence of one major flood episode (Schreve, 2001).

5. Conclusions The mammalian biostratigraphical evidence has identified consistent faunal differences between the various temperate episodes of the late Middle Pleistocene in Britain and has distinguished three characteristic mammal assemblage-zones, as typified by the sites of Swanscombe, Purfleet and Aveley, in descending relative age. The terrace succession in the Lower Thames valley provides an excellent framework against which the mammalian evidence may be tested, since the deposits have furnished rich palaeontological assemblages from similar depositional and climatic environments. The marked differences between these three mammalian groupings, and their clear separation from assemblages of Last Interglacial age, would lend support to the notion that three post-Anglian interglacials are represented in the terrestrial record, all of which predate the Ipswichian. The mammalian evidence has permitted the resolution of a long-standing debate over the age of the type Hoxnian Interglacial, through the comparison of assemblages from the stratotype with those from the Orsett Heath terrace of the Lower Thames at Swanscombe. The mammalian biostratigraphical evidence firmly supports correlation of the Hoxne and Swanscombe assemblages and consequently favours correlation of the Hoxnian Interglacial with MIS 11. The attribution of the Hoxne type-site to the younger MIS 9 interglacial, based upon aminostratigraphy, is accordingly rejected here. This has allowed the differentiation of many assemblages formerly attributed to a generalized ‘Hoxnian’ group into two separate interglacials, each one characterized by a distinctive mammalian assemblage-zone. The Purfleet MAZ has been described and species of biostratigraphic importance identified, again enabling the attribution of a number of localities (that had previously been erroneously assigned to either the traditional Hoxnian or Ipswichian Interglacials) to a more appropriate position within the British Quaternary sequence, namely MIS 9. The Ponds Farm and Sandy Lane MAZs of the penultimate (MIS 7) interglacial, have also been examined in detail and their separation from the Ipswichian Hippopotamus

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faunas of Substage 5e confirmed. In addition, the mammalian evidence has identified much smaller-scale environmental and climatic oscillations within the penultimate interglacial, possibly corresponding to isotopic substages. In conclusion, the mammalian biostratigraphical scheme has provided an effective framework for the dating and correlation of deposits both within the Thames valley and further afield. Highly significant differences exist between the identified mammalian groupings, differences that cannot be explained simply by taphonomic, environmental or sampling factors. Indeed, past patterns of species presence and absence in Britain are likely to be all the more significant precisely because of Britain’s periodic isolation from continental Europe. Thus, certain species may appear in a particular interglacial but never return, either because they failed to recolonise from the continent before a rise in sea level isolated the British Isles, or because of a host of chance factors. The results presented here confirm the potential of fossil mammals to differentiate between the various climatic episodes of the late Middle Pleistocene in Britain and have provided a new and challenging biostratigraphic framework that may be both tested and refined as new sites become available in Britain, and also compared with the evidence from continental north-west Europe.

Acknowledgements Danielle Schreve acknowledges the support of Dr. Adrian Lister and a Research Studentship from the Biotechnology and Biological Sciences Research Council (BBSRC) from 1993–1996. The financial support of the Leverhulme Trust is gratefully acknowledged for a 3 year research project entitled ‘Mammalian Biostratigraphy of North-West European Rivers’. I also wish to thank David Bridgland who read and commented on an earlier draft of this manuscript, as well as Simon Lewis and Andy Currant, who provided many useful comments. The figures were kindly produced by Chris Orton of the Department of Geography, University of Durham.

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