One of the results of the comparison of the marine and terrestrial sequences was ... cold periods, the range of temperat
Eline van Asperen, Erik Becker, Beatrice Demarchi, Flora Gröning, Olga Panagiotopoulou
University of York, UK
ISBN 978-0-901931-05-4 Copyright © 2008 Eline van Asperen, Erik Becker, Beatrice Demarchi, Flora Gröning, Olga Panagiotopoulou All rights reserved Published by The University of York Heslington York YO10 5DD, UK
On the importance of mammalian biostratigraphy for the subdivision of the late
Middle
Pleistocene Eline N. van Asperen The late Middle Pleistocene in Europe was an important period of change. The climatic extremes led to strong fluctuations in the composition of the flora and fauna. In humans, adaptations to the sometimes harsh climate led to the evolution of the Neanderthals. This was accompanied by innovations in material culture and by changes in the way the landscape and its natural resources was used by humans. It is currently very difficult to place these processes in an appropriate time frame, as absolute dating methods for this period cannot provide sufficient precision. Therefore, other ways of addressing diachronic questions have been sought. One of the methods employed, the method of biostratigraphy, is investigated here, with specific attention to the role that horse remains could have for this method.
1. Characterisation of the late Middle Pleistocene in northwest-central Europe The late Middle Pleistocene (400,000 – 125,000 years BP, figure 1) in northwest-central Europe begins with the first major cooling of the Elsterian glaciation and ends at the beginning of the Eemian interglacial (Turner 1996: 295, Gibbard & Van Kolfschoten 2004: 442-443). In the British Isles, the late Middle Pleistocene is defined as the period from the Anglian glaciation to the beginning of the Ipswichian interglacial. The period was characterised by an alternation of cold, glacial periods and warm, interglacial periods with intervening periods of intermediate climate. The glacial periods are characterised by low temperatures, a continental climate with low sea level, the expansion of polar and mountain glaciers, and a treeless, herb-dominated tundra vegetation or polar desert in northwest-central Europe. During the temperate periods, this region of Europe was covered by deciduous forests and generally had an oceanic climate with a high sea level. The character of the intervening periods is unclear. Some researchers postulate an open, boreal coniferous forest, whereas other scientists reconstruct a ‘mammoth steppe’, a highly productive biome that has no modern counterpart (Van Kolfschoten 1995). The alternation of glacial and interglacial periods led to the deposition of a variety of sediments in northwest-central Europe. In the areas that were covered with ice, the landscape was altered by the scouring activity of the ice cap. Sediments of earlier periods were eroded, rivers were forced to change their course and ground moraines were deposited. When the glaciers retreated, the increased water and sediment discharge led to incision of the rivers and the deposition of glaciolacustrine and meltwater deposits in glacial basins and dead ice hollows. In areas with a periglacial climate, existing sediments were deformed by frost action and solifluction. Aeolian loess was deposited on the barren land surfaces. During periods with an
Middle Pleistocene Biostratigraphy interglacial climate, sedimentation was dominated by fluvial and lacustrine deposition. Most organic deposits were laid down during interglacials. Our knowledge of the Pleistocene mammal fauna is coloured by the conservation potential of these deposits. Glacial sediments are mostly sterile, except where older sediments have been reworked. The periglacial environments supported only few mammal species. Loess deposits are often decalcified with hardly any bone surviving. Therefore most of our knowledge is based on interglacial (and especially lacustrine and organic deposits with their high preservation potential), fluvial and cave deposits. Of these deposits, fluvial sediments often contain transported materials, producing assemblages with a low integrity. Lacustrine, organic and cave deposits can contain assemblages with a high integrity, deposited in situ. The accessibility of the deposits also influences our knowledge. Many sites have been found during commercial mining for brickearth, gravel, lignite or other deposits. Although mining activities provide important exposures of Pleistocene sediments, a large amount of disturbance is caused by these processes, leading to many unprovenanced finds. This is especially apparent in old collections, with collectors often taking interest only in large, well-preserved remains. In other areas, Pleistocene deposits occur at such a depth that they are inaccessible. Pleistocene sedimentation is interrupted by periods of erosion or non-deposition. At the end of the early Middle Pleistocene, a faunal turnover took place, characterised by the extinction of animals of Early Pleistocene affinities, the evolution of animals of modern affinities (most importantly the evolutionary transition from Mimomys to Arvicola) and an impoverishment of the small mammal fauna (Von Koenigswald 1973, Van Kolfschoten 1990). The temperate periods of the late Middle Pleistocene saw the emergence of an essentially modern mammal fauna. The earliest human presence in northwest-central Europe dates, according to current knowledge, back to the latter part of the early Middle Pleistocene (Parfitt et al. 2005, Roebroeks 2006). During the late Middle Pleistocene, a human population became established in Europe, though the periglacial areas were abandoned during the glacial maxima. Acheulian and Clactonian technologies were replaced with Levallois technologies and the Neanderthals evolved as an adaptation to the cool-temperate climate of Europe (Gamble 1999). Changes in mobility, settlement patterns and hunting techniques can be traced over this period, leaving a distinct material trace, with important consequences for human culture (White & Ashton 2003).
2. The subdivision of the late Middle Pleistocene Since the first recognition, early in the 20th century, of the fact that multiple ice ages shaped the landscape of northwest-central Europe, the number of ice ages has been based upon the presence of different ground moraines, whereas evidence for interglacial periods was mainly found in pollen assemblages and data on marine transgressions (Kukla 2005). Based on the study of these sediments, three glacials (the Elsterian, the Saalian and the Weichselian in the northwest European nomenclature) and three interglacials (the Holsteinian, the Eemian and the current interglacial, the Holocene) were recognised (Gibbard & Van Kolfschoten 2005, see figure 1). However, this forced mammal researchers to assume that the faunal assemblages dated to each of the warm stages were heterogeneous and that considerable changes
Eline van Asperen took place between and within the warm stages (Currant 1989). The study of the relatively complete sedimentary sequence recorded in deepsea cores and ice cores radically changed the outlook of the Quaternary. The oxygen isotope composition of the deep-sea sediments and long ice cores documents cyclical variations in temperature and global ice volume (Emiliani 1955, Shackleton & Opdyke 1973). It became apparent that the oxygen isotope (OI) record provides evidence of a large number of glacial-interglacial cycles, including cycles that seemed to be missing in the terrestrial sedimentary sequences (Shackleton & Opdyke 1973, Kukla 2005). Attempts to correlate the continental glacials and interglacials with marine oxygen isotope stages led to much research and discussion. Correlation of the Eemian with oxygen isotope stage (OIS) 5e, based on direct correlation of oxygen isotope values of pollen-containing cores and absolute dating (e.g. Shackleton 1969, Gascoyne et al. 1981, Turon 1984, Sánchez Goñi et al. 2000), is generally accepted. Correlation of the Saalian, Holsteinian and Elsterian with the OI record, however, is still a matter of debate (for a recent overview of the debate see Geyh & Müller 2005). The evidence seems to point to a correlation of the Holsteinian with the British Hoxnian and with OIS 11. One of the results of the comparison of the marine and terrestrial sequences was the recognition that not all interglacials produced a distinct pollen succession, a conclusion that is further corroborated by the analysis of long pollen cores (Turner & West 1968, De Beaulieu et al. 2001, Tzedakis et al. 2001) and long Chinese loess sequences (Kukla 1987, Vandenberghe 2000). It can therefore be expected that some of the sites that had previously been assigned to the Holsteinian and Eemian in reality date from one of the intervening warm periods. At present, hardly any sites are known from OIS 9 and 7 (often referred to as intra-Saalian temperate phases), whereas many sites are ascribed to the Holsteinian and Eemian (OIS 11 and 5e, respectively). In the UK, research on the terrace stratigraphy of major rivers (especially the River Thames) and of mammalian faunas led to the recognition of specific depositional and faunal
Middle Pleistocene Biostratigraphy complexes for each interglacial (Bridgland 1994, Schreve 1997). A group of sites is now known for each of the warm stages between the Anglian and the Devensian. Every interglacial period was shown to have a characteristic mammal fauna, and the biostratigraphic scheme that was built on the mammalian data is further corroborated by geological and floral data, by the composition of the molluscan and coleopteran faunas and by aminostratigraphy (Bridgland et al. 2004). The composition of cold-stage faunas remains equivocal, since hardly any faunal assemblages are known from cold-period deposits. On the European mainland, the German site of Weimar-Ehringsdorf is re-dated to an intra-Saalian warm phase (Heinrich 1994, Schreve & Bridgland 2002). During the last two decades, various sites have been found in northwest-central Europe that are tentatively dated to the warm phases between the Holsteinian and the Eemian; examples include MaastrichtBelvédère in The Netherlands (Van Kolfschoten & Roebroeks 1985) and Schöningen (Thieme 1999), Stuttgart-Bad Cannstatt (Wagner 1995) and Neumark-Nord (Mania & Altermann 1990, Mania 2000) in Germany. However, the dating of these sites remains highly controversial and the relative dating of these and many other sites is still hotly debated.
3. The role of biostratigraphy As a consequence of the ongoing controversy concerning the age of many archaeologically and palaeontologically important sites, it is difficult to examine questions that have a diachronic character, addressing developments in human behaviour and culture and changes in flora and fauna. Currently no absolute dating technique is available with which sediments or organic remains from the late Middle Pleistocene can be dated with sufficient precision. The faunal remains provide an alternative way through which temporal relationships between sites can be investigated. In biostratigraphy, the changes that occur in faunas over time are used as a means for placing sites in a relative order, thus providing a relative date for the sites. It is hypothesised that, due to factors detailed below, each glacial and interglacial period had a characteristic fauna that differed from the faunas of other glacials and interglacials. Both the faunal composition and evolutionary aspects of the faunas are studied. Presence, absence and the evolutionary characteristics of specific species provide clues to the age of the faunas and thus the sites that are studied. Various factors can be detailed that contribute to changes in faunas over time. During cold periods, the range of temperate species contracts to refugia in southern and southeastern Europe, whereas the range of cold-adapted species expands (Dalén et al. 2007). In the following warm period, temperate species expand northwards to recolonise the areas that were covered by ice or had a periglacial climate during the glacial period, whereas coldadapted species retreat. The number, size and location of refugia, combined with migration rates and the influence of mountain and water barriers on the dispersal of species influence the rapidity of the expansion and indeed the chance that a given species will reach northwestcentral Europe (Stewart & Lister 2001). Since each species responds individually to changes in the environment, species communities were sometimes established that have no modern equivalent (Graham et al. 1996).
Eline van Asperen A second factor of influence is the climate change itself. Stochastic processes lead to minor differences between warm periods in the details of climate change. For example, the beginning of the Eemian / Ipswichian was characterised by rapid warming and a strong rise in sea-level. Various species, including humans and horses, failed to reach the British Isles before these became isolated from the European mainland (Ashton & Lewis 2002). Interglacials also differed in the degree of oceanity or continentality of the climate, which influences the size and location of habitats available to certain species. For example, late OIS 7 saw a dispersal of steppe species into western Europe due to the continental character of the climate (Schreve 1997). Lastly, evolutionary processes can change faunas over time. Phylogenetic changes can alter the appearance of an animal considerably. An example is the transition of the water vole Mimomys, which has rooted molars, to Arvicola, which has rootless, indeterminate growing molars (Van Kolfschoten 1990, Von Koenigswald & Van Kolfschoten 1996). Changes in behaviour, the ecological tolerances of species, the evolution of new adaptations and both intra- and inter-specific competition affect a species’ response to environmental change. Local extinctions and renewed colonisation can produce differences in the evolutionary signature of a species over its geographic and temporal range. These processes work together to produce variations in the composition and evolutionary imprint of faunal assemblages. An analysis of faunal assemblages can potentially reveal these differences, which can be utilised to create a biostratigraphic scheme with distinct faunal zones. Newly found mammal remains can then be dated relative to other assemblages by comparing their characteristics with those of the faunal zones. It should be kept in mind that there are important variations in the composition of mammalian faunas across regions, and that accordingly biostratigraphic schemes should be regional and cannot be applied indiscriminately to sites in other regions than their region of formulation.
4. The method of biostratigraphy Mammalian species are particularly suitable for formulating a biostratigraphic scheme, because of their rapid dispersal, their broad distribution, their rapid evolution, and accordingly their restricted stratigraphic ranges (Savage 1977, Lister 1992). A biostratigraphic scheme should ideally be based on more than one species, to increase its applicability to new sites. Additional data from e.g. geology, absolute dating techniques, flora, mollusc and insect research should be included to strengthen the inferences based upon the biostratigraphic sequence. The data used to establish a biostratigraphic scheme include the presence or absence of a species in fossil assemblages, relative abundances of species and the evolutionary stadium of the fossils (Lister 1992). The presence of a species is indicative of an age between the point of origination and extinction of the species and an age at which the geographic range of the species extended to the area of study (op. cit.). The absence of a species at a site can be the result of the small size of the fossil assemblage or of taphonomic factors. The absence of a species can therefore only be used as a biostratigraphic indicator after these possibilities have been ruled out. Presence or absence of a species can also be a function of migrations of the
Middle Pleistocene Biostratigraphy species, the pattern of presence and absence being complicated by the fact that a species can migrate in and out of a region repetitively. Evolution within a species or lineage greatly complicates, but also augments, the formulation of a biostratigraphic scheme. The presence of an evolutionarily advanced species or a derived character in a species can be of great value in biostratigraphic dating. However, the complexity of the process of evolution should caution us against drawing simplistic conclusions. Because the rate of evolution fluctuates through time, morphological similarity may be due to stasis over a period of time rather than to equivalent age. Conversely, morphological differences can arise in a geological instant by rapid change. The degree of morphological difference between samples is therefore not a reliable indication of the amount of time that has passed between them (Lister 1992). Fluctuations and even larger-scale reversals in morphology may occur (Levinton 2001), and evolution may not follow the same pattern in different regions. A species may evolve or become extinct in one part of its geographic range, and still show archaic features or persist in another part (Lister 1992). If an evolutionarily advanced population exists in one region, then the replacement of an evolutionarily primitive population by this population can mimic rapid evolution (Joysey 1972). Conversely, immigration of less-advanced populations could give the appearance of a return to an evolutionarily less advanced stadium, as is documented in Late Pleistocene Arvicola (Von Koenigswald & Van Kolfschoten 1996). Evolutionary changes that are extremely useful for biostratigraphic purposes are changes that are easily observable and quantifiable, genetically based, changing gradually and unidirectionally or appearing suddenly and synchronously over the geographic range of the species (Lister 1992). The more complex a characteristic is, the less likely will it show ecophenotypic changes, reversals, or be caused by different factors at different moments (Joysey 1972). In analyzing a specific trait, intra-population variation, including age and sex variation, should be taken into account, especially when samples are small. In formulating a biostratigraphic scheme, the danger of circular argument looms. A species should only be used as a biostratigraphic indicator to date sites if its geographic and stratigraphic range is well-known and well-dated, including knowledge of the complicating factors of e.g. evolutionary processes and migration. When information on the evolutionary stadium of a species is included, it is important to base biostratigraphic reasoning on the morphological characteristics of assemblages and not just on the chronospecies recorded in these assemblages, as the formulation of chronospecies is surrounded by difficulties and the inclusion of remains in a specific species is often somewhat arbitrary (Levinton 2001). Unfortunately, few of the large mammal species that are abundant in late Middle Pleistocene deposits show clear evolution during this period. Many species have a large temporal, spatial and environmental range, complicating faunal dating efforts. The first part of the late Middle Pleistocene is characterised by the sparse occurrence of some species of early Middle Pleistocene affinities, and some combinations of species are only present in certain periods (e.g. the presence of hippopotamus in Britain during OIS 5e). However, in the absence of positive evidence of the presence of these species, the correlation of sites with the oxygen isotope
Eline van Asperen record is difficult. Small mammals are more variable, but on many of the older sites that are rich in important large mammal and archaeological finds, small mammal remains have not or only sparsely been collected. The caballoid horse lineage is one of the few large mammal lineages that show a clear evolutionary trend during the Pleistocene. Remains of horses occur in abundance on many important late Middle Pleistocene archaeological and palaeontological sites. They are wideranging geographically and occurred both in warm and in cold periods. More detailed research of the horse remains may thus be critical in the dating of some of these sites.
5. The evolution of the caballoid horses Caballoid or true horses are thought to have evolved from the more primitive stenonid horses during the Early Pleistocene (Forstén 1988). During the early Middle Pleistocene, caballoid horses became more numerous and gradually replaced the various stenonid species in Europe. In general, only a single caballoid species is present at any one time because of their ecological flexibility (Forstén 1988, Boyd & Keiper 2005). Their social behaviour is remarkably invariable across different environments (Linklater 2000). Over the course of the Pleistocene, the caballoid equids show a reduction in size. According to Eisenmann (1991), this size reduction becomes apparent in dental elements from 200,000 years ago, whereas a clear difference in limb bone size is traceable from 100,000 years ago. Morphological changes took place in the metapodials and the first phalanges. It is expected that earlier horses also show differences in size (Eisenmann 1991) as well as in shape, as an ecophenotypic response to the rapidly changing environments of the Pleistocene. According to Forstén (1993), horse size fluctuated around a mean with no net trend, implying evolutionary stasis. Up to date, no large-scale study has been carried out to map these fluctuations and to investigate what causes them. It has been noted that, although the caballoid horses are relatively homogeneous in their morphology, there is variation between local populations (Forstén 1993, Bignon 2003). This might be related to local adaptations, but random factors could come into play as well, especially in small populations. Teeth tend to be more conservative and therefore less affected by environmental factors than bones (Hillson 2005: 284). Alberdi et al. (1995) found that in equid species, there is only a weak correlation between body size and dental measurements. During the Pleistocene, instances of disproportion between teeth and metapodials have been documented, with the teeth being either much larger or much smaller than expected from the size of the metapodials (Eisenmann 1988). Dental elements and post-cranial elements should always be studied in conjunction, as their relative size can be a useful characteristic in biostratigraphy. In the evolution of the Equidae during the Tertiary, major changes took place in the postcranial skeleton, as the horses adapted to a cursorial (i.e. running) way of life in the expanding grassland biome (MacFadden 1992). The polydactyl horses became monodactyl, through the reduction of the lateral toes, a loss of flexibility in the foot and the development of the automatic spring mechanism (Sondaar 1968). These evolutionary trends still had an effect during the Middle Pleistocene. The proximal metapodials flatten antero-posteriorly. In the metacarpal, the articular surface for the magnum shows a relative increase in size, mirrored
Middle Pleistocene Biostratigraphy in the metatarsal by an increase in the size of the articular facet for the ectocuneiform (Sondaar 1968, Eisenmann 1979, 1988). The sagittal keel of the distal epiphysis of the metapodials is more strongly developed in more recent equids, but the distal epiphysis becomes flattened antero-posteriorly (Forstén 1973). Since the development of the supra-articular tuberosities is related to polydactyly, the breadth of the distal epiphysis increases relative to the breadth over the supra-articular tuberosities in more recent horses (Eisenmann 1979). Although the minimum and maximum size to which an animal can grow is under genetic control, many factors can influence the size and shape of an animal’s bones. These factors include age, sex, genotype, genetic exchange, migration, physical condition, habitat quality, temperature, humidity, population size, inter- and intra-specific competition and predation. Horses show very little sexual dimorphism or adult age variation (MacFadden 1992). Dental elements are expected to be less variable than limb bones, as the former are under genetic control, whereas the latter are influenced more strongly by environmental factors. As horses are highly mobile animals, that disperse from the harem group in which they are born, gene flow between populations is expected to be relatively large, leading to a high degree of uniformity in morphology over larger regions. If food is scarce, body size is limited by the availability of resources (Rosenzweig 1968). Differences in body mass are reflected in the weight-bearing elements of the limbs. The lengths of the leg bones indicate the height of the animal and the length of the extremities. Bone lengths and breadths together reflect slenderness. Slenderness and stature can be related to phenotypic or environmental differences (Meadow 1999: 293, Pöllath & Peters 2005: 226). Bone breadths and bone depths together denote animal weight, which is closely related to the quality of the habitat in which the animal lives (Peters 1983: 182, Meadow 1999: 293). According to Bergmann’s rule, endothermic animals in higher latitudes are of larger body size compared to closely related animals in lower latitudes, as an adaptive response to lower temperatures. The explanation for this relationship is often sought in the fact that animals of larger body size have a smaller ratio of surface area to body mass, thus reducing heat loss. A related concept, known as Allen’s rule, states that endothermic animals from colder climates tend to have shorter appendages compared to their close relatives in warmer climates, thus reducing the ratio of surface area to body mass even more. In addition to temperature, seasonality may exert a large influence on body size. Large body size increases starvation resistance because larger animals have proportionally larger fat stores (Lindstedt & Boyce 1985, Collinge 2001). Seasonal resource scarcity leads to higher winter mortality, with more resources available to the survivors during the growth season, and seasonal environments are usually exploited by a small number of species, which as a consequence can utilise a larger proportion of the available resources (Blackburn et al. 1999). Combined with temperature, humidity also plays a role. Smaller size may be correlated with a hot humid climate, as a relative increase of surface area will enhance evaporative cooling, whereas endotherms in a cold humid climate are expected to be larger to minimise heat loss (James 1970). Endotherms in a hot dry climate are also expected to be larger to minimise water loss (op. cit.). Broad-legged horses live in humid environments, whereas horses with slender legs are found in dry environments (Foronova 2006). The more distal segments of the leg (radius, tibia
Eline van Asperen and metapodials) are relatively longer in animals that live in more open environments and have a more cursorial way of life (Eisenmann 1984). Equids that walk on heavy, soft ground have relatively wide hooves, while equids that frequent hard grounds have relatively narrow hooves (op. cit.). A high population size will lead to a larger pressure on the resources and can lead to a decrease in body size (Peters 1983). Strong competition for the same resource between species can exert a similar pressure on body size. There is disagreement on the effect that predation has on prey populations. The influence of predation on prey populations can vary according to the number and individual size of predators, hunting behaviour of the predators, the degree to which the predators consume their prey, the size and spread of the prey populations, the extent of their movement, their fecundity, other sources of mortality and the quality and quantity of the resources available to the prey species (Schaller 1972, Taylor 1984, Mills & Shenk 1992). Predators take roughly 10% of the prey biomass each year (Schaller 1972). Animals in a prey population that is heavily preyed upon can grow faster because intraspecific competition is lower, more available resources per individual allowing them to grow to a larger body size (Taylor 1984: 67). During the Middle Pleistocene in northwest and central Europe, predators that regularly take prey that weigh more than 250 kg. included Canis lupus (wolf, Mech et al. 1998), Panthera leo (lion, Schaller 1972) and Crocuta crocuta (spotted hyena, Kruuk 1972, Diedrich & Žàk 2006). Alberdi et al. (1995) and Collinge (2001) estimated the body mass of various fossil equid species to have been between 360 and 670 kg. Skeletal remains of wolf, hyena and lion are relatively numerous in Middle Pleistocene deposits. Because horses were relatively variable throughout the Pleistocene, they are potentially useful in biostratigraphy. However, the phenotypic plasticity displayed in the size and shape of the leg bones greatly complicates the pattern. The following section explores a case study which serves as an example of the way in which horse remains can be utilised in biostratigraphical dating.
6. Late Middle Pleistocene horses in the British Isles As mentioned in section 2, a robust biostratigraphic scheme has been formulated for the late Middle Pleistocene in the British Isles, based on terrace stratigraphy, pollen, mammalian faunas, molluscs and amino acid racemisation (Bridgland 1994, Schreve 1997, Penkman 2005). From each interglacial and from two glacial stages of the late Middle Pleistocene, faunal assemblages are now known, and each stage yielded sites with horse remains. The British Isles oscillated between being a desolate peninsula of northwest Europe during the glacials and being isolated from the mainland during at least some of the interglacials concerned. As the region was deserted during the glacial maxima, the fauna of the interglacials and the warmer parts of the glacials is thought to have repetitively migrated to the British Isles, most probably from northwest Europe across the southern North Sea, which was dry land during the low sea-level phases (Keen 1995). The potential for observable differences between the faunas of the various interglacial stages is therefore high. Because of the existence of a good biostratigraphic scheme and consequently the availability of well-dated faunal assemblages, and because of its recurring insular status, the region
Middle Pleistocene Biostratigraphy provides an excellent opportunity to study the evolutionary trends in the caballoid equids. Here research on caballoid metapodials from late Middle Pleistocene sites in the British Isles is reported.
6.1 Material and measurements The
material
analysed
comes from various regions of
OIS
Site
the British Isles, with some of
11
mc
Swanscombe
2
the remains coming from old
Clacton
1
collections of amateur palae-
Hoxne
2
Grays Thurrock
1
Pershore
5
ontologists and other remains
9
being recovered in controlled excavations (see table 1 for a list of sites with numbers of analysed remains and figure 2 for the location of the sites).
Wolvercote 8
Barling
7
Aveley
2
4 4
5
12 1
Stanton Harcourt
The provenance of some of
mt
1
Ilford
5
6
Crayford
9
15
Brundon
9
22
attributed to the interglacial or
Stoke Tunnel
3
1
glacial concerned was included.
Marsworth OIS 7
9
6
The interglacial material is
Pontnewydd
correlated to OIS 11 (Hoxnian),
Oreston
1
OIS 9 (Purfleet interglacial)
Hindlow
4
1
Marsworth OIS 6
3
3
Horses are noticeably absent
Balderton
5
14
from
Brighton
3
8
the older material is unclear, and therefore only material that could with certainty be
and OIS 7 (Aveley interglacial). OIS
5e
6
(Ipswichian)
faunas in the British Isles. Two glacial periods, OIS 8 and OIS 6, also produced sites with horse
3
Table 1. Sites and numbers of analysed remains; mc = metacarpals, mt = metatarsals
remains. These sites probably date from slightly warmer periods within the glacial stage. In this study, horse metapodials from the British sites are analysed. Due to the compact nature of the bone and the fact that they are not a good source of meat, metapodials are often relatively numerous in fossil assemblages. Furthermore, the metapodials are suitable elements to study with regard to horse evolution. The foreleg of a horse carries a larger part of its weight, whereas the hindleg provides propulsion. The adaptations of the metacarpals and the metatarsals consequently differ. In addition to long-term evolutionary changes, horse metapodials show both size and shape changes related to the environment throughout the Pleistocene. These morphological changes are caused by changes in height, body weight, slenderness or robustness and locomotion. As discussed in section 5, various factors could be responsible for these changes. Only adult material, characterised by fully fused epiphyses, 10
Eline van Asperen
was used in this study. However, it is possible that slender metapodials with fused epiphyses belong to a young adult, since the cortex of the bone continues to thicken after the epiphyses become fused, resulting in an increased robusticity with increasing age (Silver 1969: 284). Measurements were taken according to the method of Eisenmann (1979) with vernier callipers and recorded to the tenth of a millimetre. In the following sections, measurements will be abbreviated with ‘M’, e.g. M1 = measurement 1.
6.2 Statistical methods Since a fundamental aim of this study is to investigate whether assemblages from different oxygen isotope stages can be distinguished from each other based on their size and shape, the data were divided into groups based on the attribution of the remains to specific oxygen isotope stages. The sample of each oxygen isotope stage was tested for normality with the Shapiro-Wilk test for normality when n ≤ 50 and the Kolmogorov-Smirnov test for normality when n > 50. Hereinafter, it can be assumed that all data are normally distributed unless specified otherwise. When appropriate, non-parametric tests were used. Differences between the morphology of horses of the different oxygen isotope stages were tested using ANOVA for data that were normally distributed, and the Kruskal-Wallis test for non-normally distributed data. Where a posteriori tests were appropriate (i.e. for those analyses in which the ANOVA showed a significant difference), Tukey’s test was used for data in which variances
11
Middle Pleistocene Biostratigraphy were homogeneous, and Tamhane’s T2 multiple comparison test was used for samples with unequal variances. Results for all the tests were considered to be significant if p ≤ 0.05. The data were analysed using log ratio diagrams. This technique was introduced for palaeontological material by Simpson (1941) and first applied to archaeological material by Meadow (1981). On a logarithmic scale, equal vertical distances represent equal ratios, because log x – log y = log x/y (Simpson et al. 1960: 341). Log ratio diagrams represent various measurements on the same anatomical element in such a way that the vertical distance between the different measurements expresses their relative sizes (the ratios of their dimensions). Another result of converting absolute measurements to logarithms is an exaggeration of small values and a minimisation of large values, making it easier to compare the ratios of different specimens (Simpson et al. 1960). In order to create a log ratio diagram, all measurements are converted to their logarithms. One specimen or group of specimens is taken as the standard of comparison, representing the base or reference line of the diagram. In this study, the standard chosen is a sample of Equus hemionus, as this is the species most commonly used as a standard for log ratio diagrams of horse remains (e.g. Eisenmann 1979, Dive & Eisenmann 1991). For the other specimens or groups of specimens, the difference between their logarithmic values and the logarithmic values of the standard is calculated and plotted on a graph. A line is drawn to connect the values of the different measurements for each specimen or group of specimens, and the more similar the lines connecting two different sets of measurements, the more similar their proportions. The order in which the measurements appear in the diagrams is chosen according to their usefulness in comparing different samples and the differential survival of different parts of the bone, such that measurements that often cannot be taken on incomplete specimens are at both ends of the axis (Eisenmann 1979).
6.3 Results 6.3.1 Metacarpals (table 2) The data for M3 on the OIS 6 material are not normally distributed (n=10, Shapiro-Wilk test statistic=0.819, df=10, p=0.025). The Kruskal-Wallis test is significant for this measurement (n=63, chi-square=15.815, df=4, p=0.003). The ANOVA indicates that there is a significant difference in all measurements between metacarpals from different oxygen isotope stages (table 3). Two clusters of data emerge from the post-hoc tests: there are no significant differences between OIS 11, 8 and 7 on the one hand and few between OIS 9 and 6 on the other hand. The differences of OIS 11 and 7 with OIS 9 concern length and both the proximal and distal epiphyses. The differences of OIS 11 and 7 with OIS 6 concentrate on the distal epiphysis. The differences of OIS 8 with OIS 9 and 6 are relatively small and can be found in the proximal epiphysis. The results of the statistical analysis are reflected in the log ratio diagram (figure 3). The first aspect to note is that the two clusters of data that stand out in the statistical analysis also stand out in the diagram: the material of OIS 11, 8 and 7 is larger than that of OIS 9 or 6 on an absolute scale for all measurements. The material of OIS 11, 8 and 7 is similar in shape. The distal keel is relatively more developed in the OIS 11 material (M12, 13 and 14), and much less so in the OIS 8 material. 12
Eline van Asperen n
M1
M3
M4
M5
M6
M10
Equus hemionus
22
214.1
25.7
21.1
43.2
27.0
38.9
OIS 11
5
241.8
39.6
29.3
55.4
37.6
53.1
OIS 9
6
221.9
36.6
27.1
48.7
32.1
47.8
OIS 8
5
246.3
39.9
30.2
56.2
37.4
51.9
OIS 7
40
237.4
40.1
29.8
56.7
37.9
53.8
OIS 6
11
219.8
36.3
27.0
53.0
35.9
47.7
n
M11
M12
M13
M14
M7
M8
Equus hemionus
22
38.7
29.3
24.3
26.1
34.0
12.8
OIS 11
5
54.2
40.6
32.4
34.0
46.4
16.5
OIS 9
6
46.8
36.7
28.7
30.9
39.5
14.7
OIS 8
5
51.5
38.0
31.8
34.2
45.6
18.0
OIS 7
40
55.0
41.0
32.3
34.0
46.5
17.9
OIS 6
11
49.1
34.7
28.8
29.3
44.9
16.0
Table 2. Mean values of measurements on the metacarpals of Equus hemionus and Middle Pleistocene horses from the British Isles; measurements according to Eisenmann (1979)
Measurement
n
F
p
M1
48
6.210
0.000
M4
63
7.094
0.000
M5
48
4.564
0.004
M6
49
9.218
0.000
M10
53
12.747
0.000
M11
49
19.014
0.000
M12
39
9.123
0.000
M13
50
8.253
0.000
M14
42
10.188
0.000
M7
49
7.772
0.000
M8
43
3.201
0.023
Table 3. Results of ANOVA of measurements on the metacarpals
The metacarpals of OIS 9 are relatively narrow proximally (M5, 7). The sagittal keel of the distal epiphysis is relatively well-developed (M11, 12, 13 and 14). The material has some primitive evolutionary features, with a large breadth over the supra-articular tuberosities relative to the breadth of the distal epiphysis (M10 and 11). This seems to be a reversal of the long-term evolutionary trend of reduction in breadth over the supra-articular tuberosities. In the OIS 11 material, the breadth of the epiphysis is relatively large, and in the OIS 8 material, this breadth is increasing again, in OIS 7 leading to horses with a distal epiphysis of similar shape to those of OIS 11. The relatively narrow articular surface for the magnum (M7) is another primitive feature found in the OIS 9 material.
13
Middle Pleistocene Biostratigraphy
The material of OIS 6 is small in size and relatively robust, characterised by large breadths (M3, 5, 7 and 11) relative to length (M1), except for the breadth over the supra-articular tuberosities (M10), which is relatively small as a result of the long-term evolutionary trend of reduction in breadth over the supra-articular tuberosities. In the same context, the large size of the articular surface for the magnum (M7) should be noted.
6.3.2 Metatarsals (table 4) There is insufficient material available for OIS 11 to merit a statistical analysis, therefore the OIS 11 sample is not included in the statistical analysis. The data for M3 on the OIS 7 material are not normally distributed (n=46, Shapiro-Wilk test statistic=0.895, df=46, p=0.001). The data for M3 are compared using the Kruskal-Wallis test, which yields a significant result (n=87, chi-square=26.541, df=3, p=0.000). The ANOVA indicates significant differences between the samples of the different oxygen isotope stages for all analysed measurements, except for M8 (table 5). The post-hoc tests again indicate that the data for OIS 8 and OIS 7 cluster, with a second cluster being formed by the data for OIS 9 and OIS 6. There are relatively few differences between OIS 9 and OIS 8. The differences between OIS 9 and OIS 7 can be found in both epiphyses, whereas OIS 8 and 7 differ from OIS 6 both in the epiphyses and in length. In the log ratio diagram (figure 4), it is apparent that the horse metatarsals of OIS 6 are again of small size. The length (M1) of the OIS 9 material is closer to the lengths of the other oxygen isotope stages, but most other dimensions are clearly smaller. There are more differences in shape than in the metacarpals, with the material of OIS 8 and OIS 7 being the most similar in shape. However, since the line of OIS 11 is based on only two specimens, one of which is incomplete, conclusions based on this material should be regarded as preliminary. The OIS 11 material has a relatively slender diaphysis (M3 and 4), a broad proximal epiphysis (M5) and a well-developed distal epiphysis (M10, 11, 12, 13 and 14). As a primitive evolutionary feature, the facet that articulates with the ectocuneiform (M7) is relatively weakly 14
Eline van Asperen
Equus hemionus OIS 11
n
M1
M3
M4
M5
M6
M10
22
250.6
25.2
25.3
40.5
35.1
38.0
2
289.3
35.4
34.8
60.1
48.8
59.4
OIS 9
8
274.0
34.5
32.1
54.0
43.7
49.0
OIS 8
12
287.6
38.4
36.6
56.0
48.0
55.3
OIS 7
56
281.0
38.8
36.0
58.0
48.1
57.0
OIS 6
25
258.9
34.7
31.9
53.2
42.9
50.0
n
M11
M12
M13
M14
M7
M8
22
37.5
30.0
23.9
26.5
35.9
9.0
Equus hemionus
2
57.8
45.1
34.8
38.4
53.8
15.4
OIS 9
OIS 11
8
49.0
34.7
26.6
29.9
48.4
13.0
OIS 8
12
55.3
41.5
31.8
34.1
51.8
12.4
OIS 7
56
55.7
42.5
32.3
35.3
51.3
13.0
OIS 6
25
49.0
38.4
30.4
32.0
47.7
13.1
Table 4. Mean values of measurements on the metatarsals of Equus hemionus and Middle Pleistocene horses from the British Isles; measurements according to Eisenmann (1979)
Measurement
n
M1
59
10.170
F
0.000
p
M4
85
11.092
0.000
M5
67
8.234
0.000
M6
73
12.443
0.000
M10
64
19.583
0.000
M11
61
14.599
0.000
M12
59
7.444
0.000
M13
67
12.425
0.000
M14
62
8.401
0.000
M7
67
6.467
0.001
M8
61
0.210
0.889
Table 5. Results of ANOVA of measurements on the metatarsals
developed. The metatarsals of OIS 9 have relatively small depths (M4 and 6). The distal keel is not well-developed (M12, 13 and 14), but the breadth over the supra-articular tuberosities is small relative to the breadth of the distal epiphysis (M10 and 11). Another evolutionarily advanced feature is the large facet for the ectocuneiform (M7). The diaphysis of the material of OIS 8 and 7 is relatively broad (M3), but both the proximal epiphysis and the distal sagittal keel are weakly developed (M5, 6, 12, 13 and 14). The facet that articulates with the ectocuneiform (M7) is fairly large. In the OIS 6 material, breadths are relatively large (M3, 5, 10 and 11) whereas depths are relatively small (M4, 6, 12, 13 and 14). The articular facet for the ectocuneiform is fairly large (M7).
15
Middle Pleistocene Biostratigraphy
6.4 Discussion The changes in size and shape displayed by horse metapodials from the late Middle Pleistocene of the British Isles are complex. As the size and shape of these bones is influenced by a large number of factors, identifying the causes of a particular change in size or shape is not straightforward. Environmental conditions have a large effect on animal morphology, and the knowledge we have from other sources about the specific conditions during the interglacials can aid in the interpretation of the observed changes. The relative sizes of the material from the different oxygen isotope stages shows a similar pattern in the metacarpals and the metatarsals. The material of OIS 11, 8 and 7 is large, whereas that of OIS 9 and 6 is small. The metatarsals of OIS 9 are relatively larger than the metacarpals, and to a lesser extent, this also holds for the metapodials of OIS 6. The large size of the OIS 11 horses can be attributed to their age, as the caballoid horses were large when they first evolved. The size of the horses of the other periods will be discussed in conjunction with their shape differences. In the metacarpals, the morphological characteristics that are influenced by long-term evolutionary trends are relatively advanced in the earliest horses studied here (OIS 11). The horses of OIS 9 are characterised by a return to a more primitive morphology. This could signify the migration of a population characterised by retention of or return to more plesiomorphic traits into the British Isles after the intervening glacial period of OIS 10. From OIS 8 onwards, a gradual development of the derived condition can be observed, culminating in the evolutionarily advanced OIS 6 horses. In the metatarsals, horses from all periods are of fairly advanced morphology, with the oldest horses (OIS 11) being the most primitive. The horses of OIS 6 clearly show a flattening of the metapodials, which is an advanced evolutionary feature.
16
Eline van Asperen It is unknown if and to what extent changes in locomotion result in detailed changes in morphology in the metapodials (Bignon et al. 2005). The distal keel of the metacarpals of OIS 11 and 9 is well-developed, whereas in the OIS 8, 7 and 6 horses, the distal keel is weakly developed. It is known that broad-legged animals live in humid environments, which would correspond well with the oceanic climate of OIS 11 and 9 (Shackleton & Opdyke 1973). However, on the metacarpals of the OIS 11 and 9 horses only the distal epiphysis is strongly developed. On the metatarsals, both epiphyses are well-developed in the OIS 11 horses, but in the OIS 9 horses, the distal epiphysis is only weakly developed. The diaphysis is slender in both the OIS 11 and the OIS 9 horses. The small size of the OIS 9 horses may be related to a combination of high temperatures and a high humidity. Horse remains from the sites Orgnac and Lunel Viel in southern France, both tentatively dated to OIS 9, are small and relatively robust (N. Boulbes, pers. comm.). As an alternative, the scarcity of large carnivores in OIS 9 deposits (Schreve 1997) may indicate lower levels of predation, resulting in a larger population competing for the same resources and a reduced selection pressure for larger body size. However, it should be kept in mind that there are only a few sites known dating from OIS 9, and therefore our knowledge of the fauna of this period is limited. The weakly-developed sagittal keel and the relatively large breadths of the metapodials of the OIS 8, 7 and 6 horses may relate to the dry continental climate of all three phases (Ruddiman & McIntyre 1982, Shackleton 1987). The large body size of the horses of OIS 8 could be a corollary of the low temperatures, as an example of Bergmann’s rule, whereas the large body size of horses in OIS 7 could have been induced by the continental and thus seasonal climate that characterised this interglacial. The OIS 6 horses have short, robust metapodials as an adaptation to the severe climatic conditions of this period. As the British Isles were largely covered by ice during this period, the presence of the horses in a marginal environment can explain their small size when compared to the larger OIS 6 horse remains from Germany (author, unpublished data) and southern France (N. Boulbes, pers. comm.).
7. Conclusion Preliminary studies of horse remains from the late Middle Pleistocene of the British Isles show that the horses displayed differences in size and shape throughout this period. Although the trends in size and shape do not follow a unidirectional trend, they can be potentially useful in biostratigraphy. When the results of various skeletal elements are brought together, it is possible to date a site relative to other sites. However, as our knowledge of glacial faunas is very limited and as the causes for the size and shape differences are diverse and can affect the bones synchronously, the current results should be regarded as preliminary. The results of more skeletal elements need to be considered in conjunction and more research needs to be carried out into the factors that together produce the specific size and shape the bones take on. Once the pattern of horse evolution throughout the late Middle Pleistocene in the British Isles is firmly established, this knowledge can be applied to horse remains from mainland 17
Middle Pleistocene Biostratigraphy Europe, where the dating of sites is even more problematic. Knowledge of the effects of particular environmental factors will be crucial in this process. As the horse remains from the British Isles dating from various interglacials clearly differ although these interglacials are to a certain degree similar in environmental conditions, it is to be expected that horse remains from mainland Europe, where climatic conditions were different from those in the British Isles for the greater part of the Middle Pleistocene, will not follow exactly the same pattern. The research on the British material shows that horse remains can aid greatly in establishing a biostratigraphic scheme. Since horses are among the few large mammals that are numerous on archaeological and palaeontological sites and show clearly identifiable variation in size and shape throughout the late Middle Pleistocene, large-scale study of their remains from mainland Europe can potentially solve the dating controversies surrounding some of the most important sites dating from this period.
Acknowledgements I am particularly grateful to the curators of various museums who kindly provided access to material in their care. Dr. D. Schreve was of great help in accessing the British Middle Pleistocene collections and in providing information about the provenance of many specimens. I thank Prof. T.P. O’Connor and Dr. S. Elton for discussions and helpful advice.
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