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January 30, 2011

Modern shells of Littorina obtusata from Polochar, South Uist

Yesterday I was lucky enough to be able to attend a Conchological Society workshop at the Natural History Museum in London on the family Littorinidae, the winkles, which includes some species that are tricky to identify (often impossible to identify to species level from the shell alone, which is all that survives in archaeological contexts). Dr David Reid, who has worked on the family all over the world, gave a talk about the evolution, diversity and changing classification of the winkles, and a number of society members had shells on display.

There are a number of biologically interesting things about winkles that are of relevance to archaeologists. One is that many species develop genetically distinct ecotypes, which to over-simplify means that the same species can look different in different types of habitat. There are four broad ecotypes occuring within European species within the genus Littorina.

1) a moderate ecotype, which is large, thick-walled and has a moderately tall spire. The thickness of the shell is a defence against crab predation

2) a wave-exposed ecotype, which is usually smaller, thinner walled, has a lower spire and a larger aperture. Crabs are less common on very exposed beaches, so the winkle doesn’t need to develop such a thick shell, however the aperture is bigger because the winkle needs a bigger foot for better attachment to rocks.

3) barnacle ecotype, which is very small, thin-walled with a low spire and is often black and/or white; and occurs on exposed shores where there are lots of barnacles. The small shell allows the winkles to shelter in empty barnacle tests, and the colouring camouflages the shells.

4) sheltered/brackish ecotype, which is small, thin-walled and has a tall spire.

Winkles are edible and can be found in shell middens and other food-waste deposits in archaeological sites. Often in Britain these are Littorina littorea, which is a relatively large species, and quite easy to extract from its shell. I found several of these among the shells a Beaker period limpet shell dump at Sligeanach on South Uist, although they were nothing like as numerous as the limpets.

Often I find much smaller shells among my samples, of species such as the rough periwinkles Littorina saxatalis (or a closely related species such as Littorina arcana – it is not usually possible to tell the shells apart), or the flat periwinkle Littorina obtusata. Often these are very tiny juveniles which cannot possibly have been eaten (incidentally, Littorina saxatalis doesn’t make great eats because, unlike the other winkles, it gives birth to live young, and the embryos inside the females would have a very gritty texture if you ate them). Sometimes – as at the Viking sites of Bornais on South Uist and Quoygrew on Westray in the Orkney Isles, I think these are evidence of seaweed being brought onto a site – perhaps as a fertilizer, or as food for animals like sheep.

I’ve also found shells of Littorina saxatalis type and Littorina obtusata quite far inland in Somerset, preserved in sands which were laid down during the last interglacial, a warmer period when sea levels were much higher than they are today. The same deposit elsewhere in Somerset has yielded bones of aurochsen, a large species of cattle that became extinct in Britain during the Bronze Age, but survived in Europe until the last one was killed in Poland in 1627; as well as hippopotamus, wolf, spotted hyaena, rhinoceros and elephant – none of which live wild in Britain today.

9 Comments leave one →
  1. February 6, 2011 10:30 am

    Another interesting insight into your molluscular world.

    I think scales are very important, and I like ones you can relate to – like coins, however, as archaeologists we should be aware of the nature of change; I have had the strange experience of showing my childrens’ generation [20s] pre-decimal coinage, the existence of which was completed unknown to them.

    I started using a Ford Mondeo as a scale for archaeological buildings, as an international scale, [another disadvantage of coins].

    Yes, I know, you don’t always have a scale, but you always have a coin!

    • matthewlaw permalink*
      February 7, 2011 9:13 am

      Hi Geoff,

      You’re right. Whenever I read books or papers by geologists, I often wonder why they can’t just use a real scale in their field photos. It’s always lens caps or spades or trowels, which seems really shoddy to me. I don’t have a lot to say in my defence about this photo. Im afraid it was just whatever was to hand (I seem to have mislaid my bag of small scales). I certainly wouldn’t take a photo like that for an archive.

      • February 14, 2011 3:21 pm

        I’ve taken to carrying one in my bag constantly, I’m not sure what that says about me.

      • matthewlaw permalink*
        February 15, 2011 7:30 am

        Is it a nice colour?

      • February 16, 2011 5:41 pm

        The bag or the scale?

      • matthewlaw permalink*
        February 16, 2011 6:04 pm

        the winkle! It seems may have misunderstood.

  2. February 14, 2011 7:32 pm

    Lee – Makes you a professional!

    Matthew – Are there any snails you would expect in buildings?

    or is absence of snails indicative of a building interior?

    I am thinking about Type Ei buildings -[aka timber circles], those big postholes may contain collapsed floor surfaces; I would expect reeds/ rushes on the floor.

    • matthewlaw permalink*
      February 15, 2011 7:53 am

      Hi Geoff,

      Funnily enough Ive also thinking about snails in buildings. There certainly would have been snails, probably not many – depending I suppose on what the housekeeping was like. A humid, undisturbed environment with decaying plant matter and maybe even some decaying meat would be ideal for snails like Oxychilus cellarius or Zonitoides nitidus, so there may be a small population of them – so they might be some help (although both will flourish in any stable, well-vegetated reasonably damp environment). Other snails like Balea perversa, which lives on trees and walls, or Cochlicopa lubrica, a grassland snail which can often be seen dormant on walls, may well take advantage of those big posts. Perhaps a more interesting category in this case are the snails that can’t be accounted for by local environmental conditions. At sites near the coast, there might be small intertidal snails – such as winkles – which seem to indicate seaweed being brought onto site for use as fertilser/ fuel/ kindling or fodder. Similarly, snails associated with reeds and rushes in freshwater environments such as Succinea putris/ Oxyloma pfeifferi (you can’t tell them apart from the shell) could be useful if they don’t fit with the rest of the assemblage. An isolated snail wouldn’t be enough – it may have been carried onto site by a bird for example.

  3. February 16, 2011 6:01 pm

    Thanks, this is good info; one day I will get my view across that timber circles are architecture till then Phds will be done on things like ‘structured deposition’ in the postholes of large buildings; mad as a box of frogs.

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