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Small is beautiful: fossil voles as stratigraphic aids

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David Mayhew (The  Netherlands)

When you walk through the countryside,youwill not often come across a vole. However, they are present in most habitats and are one of the most successful groups of small mammals, widely distributed in both Eurasia and North America. Broadly speaking, Voles are blunt- nosed, short-eared, mouse-like rodents and many of them are specialised for burrowing. They can eat hard vegetation such as grasses that are very abrasive due to the presence of silica spicules. Therefore, many species of voles haveevolvedcontinuously growing cheek teeth (that consist of molar teeth: three upper and three lower) as well as the continuously- growing incisors that are typical of rodents.

Finding fossil remains of voles

This evolution took place largely in the last three million years.For this reason, fossil remains of voles are very useful for helping us unravel the stratigraphy of deposits from the Pliocene and Pleistocene periods. And, as you can see from the photographs, they are beautiful objects in their ownright.

We are talking here of quite small fossils, for example, the molar teeth are between 1 and 3mm in size. So, where and how are they found?

Many, even thousands of specimens, can be found in cave and fissure deposits, such as Foxholes at High Wheeldon in Derbyshire. Often, such localities have no stratigraphic context other than the fauna contained in the sediments. However, the material may be very complete (skulls, lower jaws and limb bones).

Fig. 1. Remains of vole Microtus sp. from Foxholes cave, High Wheeldon, Derbyshire.

Teeth can also be recovered from open sediments of various types. Most promising are poorly-sorted, freshwater sediments (such as the base of channel fillings). Theseare a grey colour and contain mollusc shells and have not been not oxidised or decalcified. For example, the Upper Freshwater Bed at West Runton has long been known for the relatively abundant remains of small mammals.

Fig. 2. Shelly sands: a channel filling in the Upper Freshwater Bed exposed at West Runton in 1974 yielding abundant vole remains.

It is also extraordinary and of great value that fossil voleremains are found in marine sediments,for example, the crags of East Anglia (Weybourne Crag and Norwich Crag, for instance, shelly sands exposed on the foreshore during tidal scours at Covehithe).

Fig. 3. Norwich crag exposed in pit at Wangford.
Fig. 4. Weybourne crag on contorted chalk masses at Sidestrand.
Fig. 5. Norwich crag and clay intercalations exposed on Covehithe beach.

Crag localities are of Early Pleistocene and Pliocene age. Material collected from them over the course of many years has proved very useful, even though consisting largely of isolated teeth. Many scientifically significant specimens have been collected in the past by local collectors. However, as far as I am aware, no vole remains are known from surface exposures of the Red Crag (Pliocene).

Stratigraphic value

The stratigraphic contextof teeth from open sites consists of lithostratigraphy, other animal and plant remains found with them in the sediment (for example, pollen, foraminifera and dinoflagellates) and, perhaps, palaeomagnetic information. This means that they are more useful than teeth from caves for correlation of deposits, both nationally and internationally.

Fig. 6. Arvicola terrestris: side view of lower first molar, unrooted (permanently growing): Foxholes Cave, Derbyshire.
Fig. 7. Arvicola terrestris: crown view of lower first molar: Foxholes Cave, Derbyshire.
Fig. 8. Mimomys pliocaenicus: close-up, crown view of lower first molar with characteristic enamel island: Norwich Crag Wangford.

The remains of voles from the Crags include the ubiquitous Mimomys pliocaenicus that can be seen as an early forerunner of the present water vole, Arvicola terrestris. An intermediate form, Mimomys savini, is present at West Runton. Various other extinct species are also known from the UK.

Fig. 9. Mimomys pliocaenicus: side view of lower first molar with roots: Norwich Crag, Wangford.
Fig. 10. Lower jaw of Mimomys savini from Cromerian deposits at West Runton.
Fig. 11. Eroded lower first molars of Mimomys savini : digestion marks of predators.

Fossil remains of voles are useful for relative dating of UK deposits, and for correlation with the deposits of the Netherlands.

Using voles, we can distinguish between the Pliocene Bramertonian deposits at Bramerton and Bulcamp and those of later age such as the “Weybourne Crag” sediments at Weybourne, Sidestrand and East Runton.

Recovering fossil voles from open sites

Exceptionally, whole jaws are found on the weathered surface of outcrops of sands, muds and clays. More usually, the teeth are isolated and invisible to the naked eye. Therefore, they can be only recovered by wet-sieving of sediments through various mesh sizes (as small as 1 or even 0.5mm), followed by drying and picking out the teeth using a low- powered binocular microscope. A trial sample of a couple of buckets of sediment may be sufficient to indicate the presence of bone. Finding fragments of vole incisors or molars is a certain clue that further sampling is worthwhile. The amount of sediment to be sieved depends on locality. However, several hundred kilograms should yield a few identifiable teeth together with several fragments.

How do voles become fossilised?

To understand this, we need to answer two questions: why are there so many vole teeth and how do they get into sediments?

There are so many teeth because voles are the most abundant of mammals and they reproduce extremely quickly. There can be hundreds of individuals in an acre of land, and they are able to produce several generations (not broods!) every year.Therefore,the absolute number of individuals is very high.

The study of the processes by which fossil remains accumulate in sediments is called “taphonomy”. An understanding of the taphonomy helps us to interpret the fossil remains correctly. Voles get into sediments and become fossilised largely because they are the prey of a wide range of predators including owls, hawks, herons, stoats, weasels and foxes. Several of these produce pellets or “scats” (animal droppings) containing concentrated, undigested (owls) or partly digested (hawks and foxes) bone material.

Fig. 12. Lower jaw of Mimomys pliocaenicus from Norwich Crag of Bramerton.

The fossil remains found in caves are often derived from the pellets of roosting owls: the proportions of the skeletal element and their position to each other (lower jaws and skulls) may confirm this.

Fig. 13. Mimomys savini: upper first molar with roots characteristic for Mimomys.

In open sites with disarticulated material, it may be more difficult to say what the process of fossilisation was. However, it is certain that predators were often involved and that the pellets or scats were transported into water, since this protects the bones. (Bones on the surface of the soil soon disintegrate or are eaten and destroyed by other animals.) In some cases, characteristic erosion marks can be seen on fossil teeth indicating that the animals were eaten by predators.

Fig. 14. Breccia of fossil small mammal bones originating from owl pellets, Derbyshire.

Once in the water and sediments, there is the question of transport and reworking of teeth. Some teeth in the marine crags appear rolled and mineralised (dark brown or black). Others are light, unrolled and very fragile. The question is: Do the teeth found together come from different times? Remarkably, it can be said that, up to now, there is no clear evidence from the marine crag material that voles of different geological ages are mixed up together.

Conclusions

The study of fossil voles has proved to be of great value in understanding the sequence of the crag deposits and Pleistocene sediments in Suffolk and Norfolk and further important discoveries continue to be made. Detailed information is available from many continental European localities for comparison with the UK. Further collecting can be expected to provide interesting and valuable information, as well as specimens that are, in their own small way, beautiful objects.

About the author

When this article was written, the author, David Mayhew (PhD Cambridge, 1975), was an honorary research worker at the Rotterdam Natural History Museum.

Acknowledgements

I am grateful to Ian and Alister Cruickshanks and to the Sedgwick Museum, Cambridge for access to material and permission to reproduce photographs.

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