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.