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Mollusc Analysis

Mike Allen

Introduction
Climate, vegetation and soil type are fundamental in controlling economy, use of land and place of settlement. Moreover these are not static but ever changing – thus the environment we see today is not the one that existed 100 years ago let alone 1000 or 10,000 years ago!

Originally environmental reconstruction relied on macroscopic plant matter in peat bogs (eg, Geike), then latter the analysis of pollen was introduced. Pollen provides general environment and environmental change over wide area, and the waterlogged plant remains provide information about the of local vegetation habitats. But the chalklands - some of the prime archaeological landscapes - were devoid of peat bogs and pollen.  Some pollen studies that are on the edge of the chalk landscapes (from incised through the chalk) do provide some reference to the chalk, they do not explicitly provide information of the high dry chalklands utilised by prehistoric communities.

Why are land snails useful in environmental archaeology?
Snails have general habitat (and food) preferences. No snail is so specific to be exclusive to a precise habitat type, but by examining a sample (a representative proportion) of the whole former community, we can indicate the nature of the local snail habitat. The major controlling factor is moisture, and the ability to compete or survive is less-favourable habitats. Some species of snail can tolerate open ground better than others, while some are more shade and moisture dependant. As both individuals and as a community they are sensitive to changes in land use. Thus, habitats such as shady woodland, deciduous woodland (leaf litter) open ground, grassland, grazed and trampled grassland (Chappel et al. 1971), arable and rock rubble habitats (Evans & Jones 1973) can be discerned.

By examining a series of samples, changes in the former habitats can be identified. Changes during the archaeological timescale are usually anthropogenic and land-use, rather than climatic, related. Events such as woodland clearance, tillage and vegetation regeneration can be discerned; all of which relate to human activities.

Sampling, identification and quantification
There are 118 species of snail and slug in the British fauna. There have been a few extinctions and a few introductions as early as Roman and medieval times. The snails vary in size from Helix pomatia (apple snail – edible snail at 5cm), while some of the smallest eg, Punctum pygmaeum is only 1.2-1.5mm when adult! Snail shells are usually smashed into fragments in archaeological deposits – so representative collections cannot be recovered by hand, they have to be recovered from processed soil samples.

The shells can, on the most part, be identified to species by the examination of the apex, shell sculpture, shape, size and aperture. Even some of the smallest fragments we recover (0.5mm) can be identified to species.

The shells are quantifies by counting apices (or apertures)> Species identified by fragments alone are only recorded as present (+).

Recovering snails from the soil sample
Sample of between at least 1 - 2kg of soil are processed to recover, ideally, a minimum of 100+ quantifiable snails. Samples of this size are, in the main, usually large enough to recover sufficient numbers of shells, but in snail poor areas, samples as large as 5-10kg may be required. Apices are recovered from the 0.5mm flots and sorted from the various residue sizes down to 0.5mm, the identified and quantified.

Snail histograms
Histograms of relative or absolute (or both) are often generated to help with interpreting the assemblages. Usually shade-loving species are presented on the left and open country species on the right, enabling the reader to see changes through time. The grouping of snails, and use of absolute or relative abundances relies on the understanding of the communities represented and their ecology.

Habitat preference and interpretation
Ecologists and malocologists (largely pre 1940) have defined and listed the habitats of recorded species. They have been categories in to three main habitat groups (shade-loving, catholic/intermediate and open country), originally by Boycott (1934), Sparks (1961; 1963), Kerney et al. (1964) and latterly Evans (1972 & 1984). By using these groupings (synecology – association) we can start to indicate the nature of the vegetation character. However, full interpretation is much more complex. By examining the catholic group and re-assigning them according to the nature, composition and abundance of other species and groups, and with the use of indicator species (autecology) we can arrive at better models (Thomas 1985). These need to be modified and revised in the light of the taphonomy and still further information can be obtained via the appropriate use of species diversities.

Snails in archaeology
Using this data the great late John Evans was able to define a general model of the existence of Neolithic woodland on the chalk downs, and clearings followed by widerscale clearance in the Neolithic, leading to largely cleared, open and farmed landscapes by the later Bronze Age (Evans 1972).

The nature of clearances in association with hilltop causewayed enclosures indicated that clearances might only have been large enough to enable the occupant to view over the tree line (Thomas 1982). More recent analyses have concentrated on higher-level resolution, in attempting to map land-use over prehistoric landscapes such as Dorchester (Allen 1997), Stonehenge (Allen 1997), Cranborne Chase (Allen 2002; in press) and Avebury (Allen 2005). Also detailed statistical analysis allows the distance for, and speed of, woodland regeneration to be postulated (Davies 1997; Davies & Wolski 2001).

Indeed changes in land-use as defined by land snail evidence have been used to interpret human activities on sites (Allen et al. 1995) and social economy (Allen 1997).

MJA 2006

 

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