Holland is a small country that lies for the most part below sea level, which can be quite problematical. However, if you are a fossil collector hunting for the fossils of animals from the Weichselian (Last Ice Age) and early Holocene, it is not such a bad thing. That is because the Dutch government regularly has sand deposited on Dutch beaches, which is dredged up from the bottom of the North Sea to fight erosion of the beaches by the sea. Taking this one step further, Holland also has large scale land reclamation projects, where whole new parts of Holland are made by spraying sand from the bottom of the North Sea onto a location close to shore until it rises above sea level.
Much of this sand is dredged up by big, specially equipped vessels, called trailing suction hopper dredgers, from a location known as ‘Eurogeul’, which is the route for big vessels to reach the port of Rotterdam. Here, the sea is approximately 13m deep, but is deepened to 30m, by removing sand from the bottom. Much of this sand is used to reinforce beaches and for land reclamation projects.
However, it is not just sand that is dredged up …
The North Sea Plain
If we could travel back in time – approximately 30,000 to 100,000 years ago – we would find ourselves in the Last Ice Age, during the Weichselian period, which is part of the late Pleistocene. During this time, we would not find the North Sea between Holland and the UK. Rather, we would see a big, dry plain, which I will call North Sea Plain in the rest of this article. That is because, during an ice age, a lot of water is locked down as ice, so global sea levels are much lower than they would otherwise be. During the Weichselian, the North Sea Plain was inhabited by many mammals, including woolly mammoth (Mammuthus primigenius), woolly rhinoceros (Coloedonta anitquitatis) and giant deer (Megaloceros giganteus). However, it was not just big mammals that lived on the North Sea Plain. Small mammals also lived there, as is shown by the presence of their fossil molars in sediments from this area. Examples include the European water vole (Arvicola terrestris) and lemming (Dicrostonyx). All these lived on the so-called ‘mammoth steppe’, which had a climate that is not comparable to an arctic climate at all. It was dry, with very little snow and was cold to the extent that there were no or only very few trees. However, there was a lot of grass, as it was a fertile land. From the west, the Thames meandered into the North Sea Plain and, from the east, so did the rivers Meuse, Rhine and Scheldt. It must have been quite a paradise for the many different species that are known from the late Pleistocene of this area.
One of the beach locations, which is reinforced with sand from the Eurogeul, is the beach near Hoek van Holland, which lies close to Rotterdam in The Netherlands. I collect fossils from here quite often and have, over time, collected a small collection of fossils, not just including fossil mammals, but also fish, reptiles and birds, fossil shells, bone artefacts and amber (fossil tree resin). Some of these fossils are discussed below.
Obviously, the most impressive mammal remains are those of mammoth and rhinoceros, which are sadly often damaged by the process of dredging the sediments containing them from the bottom of the North Sea. Occasionally, good specimens, like the mandible fragment of Coelodonta antiquitatis in Fig. 3 or the two molars of juvenile Mammuthus primigenius in Fig. 4, are found. However, when paying close attention and getting down on your hands and knees on suitable parts of the beach, there are so many more Pleistocene treasures to find, including fossil molars and incisor teeth of rodents, just as old or even older than the mammoth remains.
The most impressive remains of these small animals are mandibles, which still include small molars, for example, of Arvicola terrestris (Fig. 5). At Hoek van Holland, there are fossils of several different species of rodents, the most common species of which is the European water vole (A. terrestris), which has molars that grow up to about 4mm in length. Also present are the molars of smaller mice of the genus Microtus. Fossils of a beaver (Castor fiber), which is quite a big rodent, have also been found. Shown is a proximally damaged humerus (upper arm bone) of C. fiber (Fig. 6). Remains of other groups of small mammals have also been found. Examples are the lower jaw fragment of hare Lepus sp. and the lower jaw fragment of otter (Lutra lutra) shown in Figs. 7 and 8. What is remarkable is the presence of now extinct, more primitive voles from the early or middle Pleistocene, including Mimomys pliocaenicus and M. savini. These fossils cannot be late Pleistocene in age, since the genus Mimomys went extinct before this time. However, they are still found at Hoek van Holland, which has predominantly late Pleistocene fossils from late Pleistocene/Holocene sediments. The occurrence of these fossils at Hoek van Holland can be explained by considering the process of reworking (see box, Reworking of fossils).