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2000 series: Elveden, Suffolk

Andy Currant, of the Natural History Museum, writes on the Elveden excavation and the 'vole clock'

When we are processing large quantities of sediment for microvertebrate extraction we dry the material before attempting to sieve it. Most clays split down quite easily once they have been dried, though marl – clay with a high calcareous component – is notoriously difficult stuff to deal with and often requires several drying and wetting cycles to break it down enough to sort for small finds. On a three-day television shoot all you can do is act the part – there really isn't time to carry out many of the detailed investigative processes – and you always have to bear in mind that those poor guys and gals have got a programme to put together.

For some strange reason, Time Team wouldn't let me do my two-hour introduction to fossil voles – can't imagine why – but to give you the briefest summary these are the salient points. Most voles can be readily identified from the characteristic enamel patterns on the worn surfaces of their cheek teeth. Usually we go for the first lower molar, but sometimes the second and third upper molars can be brought into play as well.

Voles are often good general indicators of environmental conditions. For example, today in Britain we commonly find the short-tailed grass vole [Microtus agrestis] and the bank vole [Clethrionomys glareolus] in our woods and fields and both of these species turn up together under warm interglacial conditions in the Middle and Late Pleistocene. During the colder periods of the Pleistocene, species such as the narrow-skulled vole [Microtus gregalis] and the Norwegian and collared lemmings [Lemmus lemmus and Dicrostonyx torquatus] are often very abundant. Other species like the northern vole [Microtus oeconomus] are tolerant of quite a wide range of climatic conditions but are generally absent from the warmest periods and are good indicators of seasonally flooded grassland.

Several voles, notably the water vole [Arvicola terrestris] and its possible ancestors [Arvicola terrestris cantiana, Mimomys savini etc] show what are interpreted as evolutionary changes through time. At some stage in the early Middle Pleistocene the water voles loose the roots on their cheek teeth and thereafter show a progressive change in the distribution of enamel on the biting surfaces of their teeth. Such changes usually allow us to work out roughly where we are in the Pleistocene, providing we can find enough teeth to do the critical measurements on which such divination is founded.

Then we have voles that became extinct at known times in the past. Here, for example, we have a vole with rooted molar teeth called Pliomys episcopalis, which is not known to occur in deposits after the Anglian glaciation [Oxygen Isotope Stage 12 or about 450,000 years ago].

These are just a few examples of the kinds of evidence we can use to tell where we are in time using vole teeth. The process is not as well refined as say for fossil beetles, where there may be hundreds of species in a fauna, but vole teeth are durable, often quite common and are preserved in a wide range of different kinds of deposit, so they are pretty useful.

The deposits at Elveden were a bit of a nightmare to explain in Time Team language. The fossiliferous sequence was truncated by a very irregular decalcification horizon, which meant that the top part of the sequence, which should have been richest in terrestrial things like voles and shrews, had been dissolved away in most places. Only where occasional patches of chemically unaltered sediments had survived would we have stood much of a chance of getting a rich small mammal fauna, but it was worth trying for it. As things turned out we were not as high in the sequence as we thought at the time we took the samples, and later on we came across higher patches of undecalcified sediment that may have been richer – but we ran out of time to process them. Stuff of life really.

Anyone who is passionate about voles can send me an e-mail (A.Currant@nhm.ac.uk). There is a cure but it involves sorting many tons of samples over half a lifetime. If you have the time, we've got the samples!

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