Drs. F.E. (Francien) Dieleman, research associate

Research interests

The Arvicolids are the most common rodents of the northern hemisphere since the Pliocene. They inhabit our world in large numbers. Predators like owls are mainly responsible for accumulation of bones and teeth in pellets. Bones and teeth of small mammals disintegrate rapidly within a few month on soil surface, but when pellets are deposited in rivers or lakes, this is in favour of their preservation in sediments. In the Netherlands many sediments are reworked during regressions and transgressions of the North Sea. As a result fossil Arvicolid remains can be found almost anywhere in our country, especially in boreholes and on beaches where sand dredged from the North Sea is deposited.

Usually all that is left of such creatures is their teeth: incisors and molars, only a few mm in size, but still visible by the naked eye. Besides being beautiful objects with intriguing shapes, when viewed under a microscope (see link http://www.fossiel.net/forums/verzameling.php?verzameling=1818), molars appear to be very useful too.

The rapid evolution of some vole taxa, which can be seen by their teeth, makes them very interesting as biostratigraphic markers for the Pliocene and Pleistocene. At the moment I'm mainly working on Late Pleistocene and Late Pliocene voles, taking a special interest in the molar morphology of molars of members of the Mimomys polonicus- Mimomys pliocaenicus lineage (watervoles of the Late Pliocene/ Early Pleistocene), and the Mimomys savini- Arvicola terrestris lineage (leading to the present watervole), because these are the most common voles that can be found on the shores of Zeeland and Zuid-Holland.

Searching for fossils at the Kaloot near Borssele. Eight species of rodents were found at this locality, mainly cold and steppe favouring species: Lemmus lemmus (norway lemming), Dicrostonyx torquatus (arctic lemming), Spermophilus sp. (ground squirrel), Microtus oeconomus (root vole). The presence of Arvicola terrestris (water vole) with undifferentiated enamel indicates an Early Weichselian and/or Saalien age.

Another  project is making an inventory of all small mammals that can be found in deposits from the Meuse/ Scheldt delta. This often concerns material from reworked sediments found on beaches. Although this material is not anymore in stratigraphical context, it can help us date with more accuracy fossils of big mammals found on the same beaches. Surprisingly, shell and shark teeth collectors are very likely to encounter vole teeth in their sieving samples. Preliminary reports are frequently published (Dieleman, 2005; 2006a; 2006b; 2008; 2010a; 2010b; van den Hoek Ostende & Dieleman, 2010). Material from sand dredged from the North Sea contains minimally twenty five small mammal species (voles, other rodents and insectivores), from faunas of very different ages, varying from Holocene/ Late Pleistocene to Early Pleistocene/ Late Pliocene.

I therefore invite the public to come forward with any finds of voles or other small mammals from beaches and boreholes.

  “Be surprised by a lemming!” 

Dicrostonyx torquatus (arctic lemming, halsbandlemming) from Nieuwvlietbad (beach) found by mr. Jos Billekens  

Cooperation between professional palaeontologists and active amateurs can lead to spectacular finds, as was the case when two devoted men, Joop Boele and Louis Verhaard, independently screened boreholes for fossils, and came up with the most primitive vole species of Mimomys ever encountered in the Netherlands. Research then crosses the borders of the Netherlands to compare with material from sites elsewhere in Europe. In this project I worked together with Naturalis microvertebrate specialist dr. Lars van den Hoek Ostende, and dr. David Mayhew, Naturalis research associate, specialised in Late Pliocene/ Early Pleistocene voles. We published a paper on the oldest Dutch vole up till now: Mimomys hajnackensis (Mayhew et al, 2008). Mr. Boele and Mr. Verhaard pulled the record for fossil voles down with another million years to approximately 3 Ma by finding two lower first molars of M. hajnackensis in boreholes!