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Introduction and methods

Shells from the flots of five samples taken through the ditch of Winterbourne Stoke 71 long barrow (WS71), were submitted for analysis. The palaeoenvironmental potential of the shells had previously been assessed by Professor Paul Davies, Bath Spa University. Samples had been processed by Historic England staff in a Siraf-style flotation tank, with the residue (not analysed here) caught on a 500µm mesh, and the flot on a 250µm mesh. All shells were identified to species level where possible under a low-power binocular microscope with comparison to a reference collection. Notes were made about the preservation of the shells. Ecological information is derived from Evans (1972), Kerney and Cameron (1979), and Davies (2008). Nomenclature follows Anderson (2008). For each gastropod taxon within a sample, the most commonly represented non-repetitive element (usually the shell apex, umbilicus, or body whorl with mouth) was counted to determine the minimum number of individuals (MNI) present. This avoids the underestimation reported when only shell apices are counted (Giovas 2009). In the case of Pomatias elegans, the opercula (calcareous plates used to seal the shell aperture when the snail is dormant) were also counted. As an aid to interpretation, taxa were arranged into groups, broadly following those of Evans (1972). These are:

• 1a. Oxychilidae. 'Glass snails', taxonomically related species of shaded places, represented here by Aegopinella nitidula and Oxychilus cellarius
• 1c. Discus rotundatus. A shade-demanding species
• 1d. Other shade-loving species. Represented here by Acanthinula aculeata, Clausilia bidentata, and Punctum pygmaeum.
• 2. Pomatias elegans. A burrowing snail found in shaded places, but where the surface is bare and rubbly.
• 3. Intermediate/ catholic. Molluscs with a broad range of ecological tolerances. Represented here by Cepaea sp., Cochlicopa cf. lubrica, Cochlicopa cf. lubricella, and Trochulus hispidus.
• 4a. Commonly open country. Snails associated with open habitats such as short grassland. Represented here by Helicella itala, Pupilla muscorum, Vallonia costata, Vallonia cf. excentrica and Vertigo pygmaea.
• 4b. 'Allied' open country. Open country snails that are local or rare in their occurrence. Represented here by Truncatellina cylindrica.

The groupings broadly represent a progression from woodland conditions through more open environments. Note that not all taxa within a group are present in all samples. Although useful as a broad guide, the use of ecological groups may mask fine details; therefore consideration is also made of individual species counts.

Results

Minimum numbers of individuals (MNI) for individual snail taxa are presented in Table 5. Overall, preservation was good. A small number of snails had a remarkably 'fresh' appearance, and were judged to be intrusive. These are discussed below.

Discussion

The molluscan sequence

Sample 52311, from the primary fill of the primary barrow ditch context 92309, contains both low numbers of individuals (n=23) and a relatively low number of taxa, denoted by s (s=6). These are likely to indicate instability and rapid deposition (greater ecological stability allows more species to become established, while slower rates of deposition allow more shells to accumulate). Some of the Pupilla muscorum and Vallonia spp. shells bore heavy calcareous accretion, which may suggest that they were residual in this context, perhaps derived from the material that the ditch had been dug through. Numbers were too low to carry much interpretative value in this sample; however, all of the taxa were Group 4 (open country) taxa, except for a single shell of Punctum pygmaeum, a Group 1 species usually associated with shady places, but also found in chalk grassland (Evans 1972, 183).

Two samples were taken from the secondary fill of the primary barrow ditch. Sample 52310 (context 92310) was taken from the outer side of the ditch, sample 52309 (context 92306) from the inner side. Sample 52310 contains a higher number of individuals (n=59) and species (s=11) than the primary fill. This implies slower deposition (as may be expected for a ditch silting up) in a regime of greater environmental stability. Group 1 taxa, suggestive of shaded conditions, and Group 3 taxa, tolerant of a wide range of ecological conditions, appear for the first time in the sequence, as does Pomatias elegans, a Group 2 species usually associated with broken ground that allows the snail to burrow. This may indicate cultivation or clearance of scrub or woodland. It may be that scrub or woodland had developed around the edge of the monument while the ditch was silting up. Overall, however, open country snails are dominant. Two shells of Cochlicopa lubricella and one Vallonia excentrica were translucent and glossy, suggesting that they may be modern intrusions. This may result from the action of worms bringing snail shells down their burrows. Sample 52309 contained a similar number of individuals (n=56) and species (s=9), reinforcing the suggestion of stability. Group 1 taxa are scarce in this sample, however. This is somewhat perplexing, as this context is on the north-west facing side of the ditch, side (although of course the shade cast by the monument itself may well have negated the effect of the south-east aspect). It is possible that the vegetation was taller on the outer side of the ditch, perhaps suggesting continuing maintenance of the monument while surrounding land had become less managed. Such sharply defined resolution in land snail palaeoecology seems uncomfortably optimistic, however, as snail species tend to encroach across ecological boundaries.

Sample 52307, from the primary fill of the recut, context 92305, yielded higher numbers of snails (n=385) and species (s=15), suggesting even more stability and a slow rate of sediment deposition. The sample contained high numbers of Group 4 taxa, reflecting open conditions, but also the highest numbers of Group 1 taxa, especially Punctum pygmaeum. These may suggest long grassland rather than wooded conditions. Truncatellina cylindrica appears for the first time in the sequence, suggesting dry conditions or rocky rubble. This species recurs in several prehistoric assemblages in Wiltshire, although was believed to be locally extinct at the time that Evans wrote Land Snails in Archaeology (Evans 1972, 140). Nonetheless, the Conchological Society of Great Britain and Ireland record it presently living in two locales either side of the A303 near to Bulford and Durrington (National Biodiversity Network 2017). One shell of Helicella and one Vertigo had a fresh appearance, suggesting that they were intrusive. The sample also contained segments of a millipede, which are likely to be recent intrusions.

The greatest numbers of species and individuals were recorded in sample 52308, from the secondary fill of the recut ditch, context 92304 (n=544, s=18). Again, open country taxa are predominant; however, there are relatively high numbers of Group 1 taxa, especially Carychium tridentatum, which can thrive in grassland providing conditions do not become too dry. Clausilia bidentata, a Group 1d species, is usually rupestral, living off ground on trees, logs and rocks. Its presence here may indicate shrubs or trees close to the monument, or it may have been living among chalk rubble or even tussocky grass.

Prevailing environmental conditions

Although the snail assemblage from the primary ditch fill is rather poor, it would appear to indicate that the ditch was dug in an environment of open grassland. Pupilla muscorum in particular is generally indicative of short sward grass (Davies 2008, 175). Shade-demanding taxa begin to appear during the time that the secondary fill of the original ditch is accumulating. This appears to follow a trend observable at many long barrows, which were constructed in open conditions, but where the snail assemblage indicates increased shade over the time the ditch was infilling. The primary fill of the ditch at South Street Long Barrow, for example, is dominated by the Group 4 taxa Vallonia costata, Vallonia excentrica and Helicella itala, while the secondary fill shows increases in Trochulus hispidus and Zonitidae, suggesting a more shaded (perhaps overgrown) environment (Evans 1972, 328–332). Long barrow W483, south of Stonehenge was similarly constructed in pre-existing open conditions (Hazell and Allen 2013, 23).

It is worth noting that in the Druids Farm ditch, Group 4 taxa always remain dominant (this is also the case at South Street). Although generally associated with open conditions, many of these species may be present in low numbers even in open woods, especially Vallonia costata (Davies 2008, 176). The scenario at Easton Farm long barrow, where there is secondary woodland regeneration in the Bronze Age, does not seem likely here given the mixed signals (Whittle et al. 1993, 215). It is also worth noting that there is a complete absence of species such as Vertigo substriata and Ena obscura, which are typical of woodland in dry places (Davies 2008, 83).

A likely scenario would seem to be that broadly open conditions remained throughout the entire sequence; however, rank or tussocky grassland developed within and perhaps around the ditch. True woodland conditions do not appear to have been attained during the times that the fill was accumulating, although this is not to rule out the possibility that they did develop near to the monument. Certainly such a scenario could provide a source population for the adventive Group 1 taxa taking advantage of shaded/ more rank conditions in the ditch (shade does not only come from trees!). It may be the case that the snail assemblage shows the temporally mixed result of phases of overgrowth and management of the monument and its environs. Unfortunately, the resolution of sampling here is too poor to attempt to assess this; indeed owing to factors such as earthworm action it may never be possible to achieve such clear resolution.

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