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).
Dutch fishermen often catch remains of these Pleistocene mammals in their nets, sometimes even complete mammoth skulls, as has been previously described in this magazine by Dick Mol (see Two days of trawling in the Eurogully: Did trawler OD7 find a mammoth graveyard in Issue 30). However, smaller remains often literally escape the nets, and are therefore much rarer and quite under-represented in most collections of late Pleistocene mammal remains from the North Sea. On beaches reinforced with sediments from the Eurogeul area, these small remains can be manually collected, providing a very valuable addition to the extensive collections of remains that have been dredged up.
Mandibles of pike (Esox lucius) and dermal plates of the European sturgeon (Acipenser sturio) can be found (Fig. 9). Most likely, these date from the late Pleistocene or the Holocene. Fossil fish remains from Hoek van Holland have not yet been scientifically studied nor radiocarbon dated (see box, Radiocarbon dating), so we cannot be completely sure. Older fossil fish remains can also be found, including sharks’ teeth and dermal stings from rays. These older remains are most likely Miocene or Pliocene in age and have been reworked.
Fossil reptiles from Hoek van Holland are extremely rare, but remains of turtles have been found. Two fragments of turtle shell are shown in Fig. 10. These fossils have not yet been studied and are still unidentified. However, it is quite obvious that, since turtles are coldblooded animals, they could not have lived on the North Sea Plain during the Weichselian. These fossils must either be younger or older, dating from the warm Eemian period (the period before the Weichselian, also part of the late Pleistocene) or from the warm, early Holocene period. None of these fossils from Hoek van Holland have been radiocarbon dated yet, so we cannot say for sure.
Bird bones are very fragile and are therefore quite rare as fossils. Some damaged specimens have been collected at Hoek van Holland, but they have not yet been indentified or dated.
Shells are obviously very common finds on many beaches. However, some of the shells that are found at Hoek van Holland are fossil shells, sometimes much older than the mammoth fossils. The majority of fossil shells from Hoek van Holland are from the Eemian period. Three common species are shown in Fig. 11 – Acanthocardia tuberculata, Venerupis senescens (now extinct) and Tridonta borealis. During at least a part of the Eemian, there was a sea on the North Sea Plain (due to higher sea levels than during the Weichselian), in which a mollusc fauna lived that was quite similar to the North Sea fauna of today, but also showed some differences. In a small study of some of the bigger molluscs found at Hoek van Holland, I identified 33 species, most of which were from the Eemian, but some of them were from older periods, including the early or middle Pleistocene, the Pliocene (2.5 to 5myrs old) and the Eocene (about 48myrs old). The presence of all these fossils in Weichselian/Holocene sediments can, yet again, be explained by reworking.
Artefacts are pretty rare at Hoek van Holland. Those that have been found are most likely Mesolithic in age, that is from the early Holocene period, about 8,000 to 10,000 years old and include barbed points and carved antler of red deer (Cervus elaphus), of which some specimens are shown in Figs. 12 and 13. All artefacts were crafted by modern humans (Homo sapiens). The barbed points were made from the bones of animals and the carved antler was a waste product. Humans used almost the complete antler to make tools, except for the base, which was cut off and thrown away.
These people did not live on the mammoth steppe, since about 10,000 years ago the Last Ice Age had already ended and the climate had warmed. They lived in a forest environment and hunted for red deer C. elaphus and wild boar Sus scrofa, species of which remains have also been found at Hoek van Holland. The artefacts cannot be much younger than 8,000 years, since, from about 8,000 years ago, the North Sea Plain flooded once again and started to become the North Sea as we know it today.
Amber (fossil tree resin) is also quite rare. Where exactly the amber from the Dutch beaches comes from is unsure, as is its age. What we do know is that it is found along the entire coastline of The Netherlands, but is most common on the Frisian Islands. Some pieces near Hoek van Holland have also been collected.
The beach of Hoek van Holland has a wealth of fossils – mammals, big and small, fish remains, reptile remains, bird remains, fossil shells, artefacts and amber can all be found along a quite small stretch of beach. The smaller remains are especially scientifically valuable, since they are not dredged up by fishermen very often, if at all. Therefore, the process of reworking has considerably added to the fossil wealth of the beach of Hoek van Holland.
Thanks to Hans Langeveld for taking most of the photographs for this text.
Reworking of fossils
When plant or animal remains get quickly buried by fine sediments after death, decomposition may stop before the complete organism is lost and fossilisation of the harder parts of the organism may occur. However, the many geological processes on earth, like erosion by rivers, may uncover fossils and transport them, finally redepositing them in another place in other, younger sedimentary layers. Therefore, a fossil from the early Pleistocene, over one million years old, can end up in the same sediments as a fossil from the late Pleistocene, which may be as young as 30,000 years.
Radiocarbon dating is a method of dating organic remains that are up to about 50,000 years old, sometimes with very great accuracy. In short, it comes down to measuring the amount of naturally occurring radioactive carbon (14C) in an organic sample and comparing that to what is expected to be in the atmosphere at the time an organism died. As the amount of 14C in a dead organic sample halves every 5,730 years, the amount of it left compared with the amount expected at the time of death of the organism indicates the age of the specimen.