Land Snail Practical Write Up
Land snails are used as an archaeological methodology of reconstructing the past and are an environmental method that can fill in the gaps of other methods. For example, pollen and macroscopic plant matter study show the general change of a large area and only survive when waterlogged. Chalk lands are prime archaeological landscapes useful for study and land snail evidence can survive in them where other evidence cannot. Also, as oppose to representing a large scale generalized area land snails indicate a local environment. This is because land snails, like many insects, do not move far and when the environment changes they do not adapt, but instead always require a specific habitat or a small number of suitable habitats. There are over 118 species of land snail and slug in the British fauna and there have been only a small number of extinctions. This along with their rarity in adaptation or change means the modern snails can be studied and each of their different habitat preferences understood. They are also very sensitive to change in land use. All of these points mean that the presence of certain types of snail show a very specific environment which can be narrowed down by identification of these and used to show a past environment. Snails are identified archaeologically by shell morphology because any soft tissue has been previously destroyed. Whilst most assemblages will contain a majority of broken shells each one is unlikely to hold over 30 types of snail species so identification of a small number of these can soon lead to a general pattern of the number and types that are within the sample being studied. Differences in the axis, apex, whorl, spire, columella, mouth, lip and the overall shape and colour lead to the morphology of a preserved snail shell. Whilst many look similar subtle differences in these features usually lead to a fairly certain conclusion. However, some such as Ceciliodes acicula can be extremely difficult to identify as it buries itself to over 2m under the ground so can get confused in stratigraphic layers. The methodologies for snail shell analysation start before identification however, and begin with extraction from the soil. The extracted shells are originally separated into groups that are useful and are not useful. The archaeologically valuable materials are floated then sieved into different sized mesh’s, the residue of which is dried and then taken from the flots left over. This is the point at which the samples were viewed in practical study. During the practical undertaken on the ditch in Wiltshire, Microscopes were used to view the snails which can be less than one millimetre. In the session a soft brush was used to first separate the shells from the invaluable residue then individually studied to search for defining features within the shell in order to indicate the species of snail. Once a species was identified the rest of the sample was looked at to find other of these species in order to determine the Minimum Number (MNI) present within the sample. This process was repeated for as many snail species as possible.
From the practical the data recorded was altered so that each land snail represented a percentage of the overall site assemblage for each given time period. This graph looks at each species of mollusc and gives the amount that this makes up of the total found. For example, the Trichia hispada species was found to make up 2.82% of the molluscs in the Early Neolithic assemblage, 4.34% of the Late Neolithic and 4.96% of the Bronze Age assemblage totalling at 113 of the species found across the three assemblages.
The graph also shows discreet reference to the habitat preferences of the mollusc with the Woodland species broadly on the left hand side leading into the interlineate snails (that is the more flexible) in the middle whilst the Snails who colonise in Open land have been confined to the left hand side. The reason I have specified...
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