Fossil treasures of Denmark, 55 million years ago

When God had finished creating the Earth, He had some sand and gravel in excess. To get rid of it, the story goes, He dumped it in the southern North Sea, and that became Denmark. On closer inspection, this gravel heap reflects a varied range of deposition environments from the Lower Cretaceous through the Pliocene. Denmark is slightly tilted to the SW, with the oldest rocks exposed on North Jutland, becoming progressively younger southwestwards. Seismic data and drills for ground water, petroleum and salt have proven even older rocks below: Salt is produced from Zechstein diapirs, and plans are underway for testing even Cambrian hot shales at 4 km depth for potential shale gas.

But we digress. Most of the Mesozoic and Cenozoic sediments in Denmark are marine, and fossils are abundant. One interval is especially rich: The Lower Eocene ”Moler” is the goal for our visit. It was deposited from the Paleocene-Eoene boundary at 55,8 ma., through three million years that brought Denmark from a tropical climate with deposition in an isolated sea, to a temperate climate, with contact to the open ocean.

In this “proto-North sea” was deposited a sedimentary rock consisting of around 60% silica-skeletons of diatomé microfossils, 30% clay and 10% volcanic ash. Diatomites have a very high porosity, and are therefore dried and used to fabricate thermal isolation plates for buildings, and, because it can absorb water, as cat litter (!). Moler is found only around Limfjorden, because diatomites were only deposited in a narrow belt from Limfjorden into the North Sea. The main excavation pits on the island of Fur and on northern Mors.

Moler has a characteristic expression, with beige-greyish diatomite constituting the main part, interbedded with thin bands of dark volcanic ash, carried from eruptions around the incipient Atlantic mid-oceanic ridge.

During Quaternary glaciations, glaciers from the North compressed the Moler into folds, and pushed it up to the surface. It therefore exhibits sharp folds and faults, beautifully enhanced by the interchanging light and dark layers.

The Moler with interbedded light grey diatomites and darker volcanic ash layers. The whole sediment pile has been sharply folded by glacial push.

A part of a large Moler pit next to the museum on Mors.

Bispehuen – “the Bishop’s hat” – a remaining, strongly folded, pillar of Moler in an abandoned pit on Fur.

Fossils have made the Moler famous among palaeontologists, as it provides spectacular preservation even of very delicate fish, insects and bird remains. Insects can have thin wings and colours preserved, and several birds have feathers. In addition come turtles, sea stars, crabs, crustaceans… Altogether, they provide a detailed window into Denmark 55 million years ago.

Denmark was at around 56 north latitude, only one degree south of today. But climate was considerably warmer. In the Earliest Eocene, global sea levels were low, and an inland sea comprised what are today the North and Norwegian seas. The Atlantic spreading ridge had started separating the continents, and the volcanic Greenland-Faroe ridge blocked the connection to the southern Atlantic. Lack of circulation led to anoxic conditions, and through a few hundred thousand years thick clay was deposited. This earliest clay, called the Stolleklint Ler (ler=clay) contains only marine fossils. Many of the fish found are equivalent to tropical fish today. Water depth was estimated to around two hundred meters.

Into the Lower Eocene, sea levels rose and provided a connection to the opening Atlantic ocean, which ended the anoxic sea floor conditions. Climate became somewhat colder, but still subtropical. The shifting tectonics then directed currents towards the central North Sea and Northern Denmark, which is believed to have created an upwelling and blooming of planctonic diatomées. These in turn created so large amounts of organic matter that their decay on the bottom created anoxic conditions. The diatomé layer, interbedded with ash beds, is the Moler proper, with the stratigraphic name the Fur formation.

Fish are, naturally, the most prominent of the fossils, and more than 75 species are known. The most common in the lower, Stolleklint ler, is a small herring fish, while the Moler is dominated by a small argentine. Sharks are represented only by teeth and a few calcified vertebrae, the remaining are bone fishes. These comprise both modern and more primitive ray finned fish, many of which today are found in tropical seas. Among the fossils are eels, tarpons, perches, mackerels, pipefish, sandeels, morays, and blowfish (spiked tropical fish that can blow up their body to a ball – in Danish accurately named hedgehog-fish!). One fossil shows a redfish swallowing an argentine its own size – which aleo became its fate. Flatfish are, however, absent, probably due to the anoxic bottom conditions.

The most beloved and spectacular fossil from the Moler is a complete baby sea turtle. Even the soft tissue on the paddles and along the shell is preserved. Nicknamed “Luffe”, it has become a mascot for both the Moler museum and fossil loving kids.

Luffe, the baby sea turtle.

This sea was, however, not at a large distance from shore, probably the coast of Norway and a band across the North sea to Sweden. Fossils of birds, insects and plants provide a fascinating look at the life on that land.

Over 30 different species of birds have been found, and they are solely terrestrial. No marine birds have been found, which is something of a mystery, but may stem from marine birds not drowning even if they are carried far offshore by the wind. Among the birds are those similar to today’s tropical and subtropical wrens, trogons, turaco and treeswifts. The most complete skeleton is similar to a kingfisher, and has the belly full with small fish remains. Among larger birds are resemblants to flying ostriches or cranes. The bird fauna of the Moler is one of the largest after the K-T boundary, and therefore provide a glimpse into the early evolution of modern birds, suggesting that their main shapes evolved quickly after the K-T catastrophe.

Beautifully preserved fossil of a small shorebird.

Insects are even more varied, with over 200 species: butterflies, cicades, flies, grasshoppers, wasps, dragonflies, crane flies, mosquitoes, cockroaches… Notably, all are winged, suggesting that only flying insects had been caught by the wind and brought to sea. Large winged ants in the lower Stolleklint ler suggest that the seashore was closer during the lowstand.

Plant remains, mainly twigs and wood pieces have also drifted to sea. Among them is an eight meter long trunk, which probably belong to the sequoia family. Among the plants are also remnants of pines, maple seeds, ginkos and water ferns.

Altogether, the fossils paint a vivid picture of the landscape onshore: Butterflies and cranes suggest a savannah-like lowland, while trunks of trees and the tree-dwelling birds show that it was draped with woods, presumably concentrated around the dragonflies and kingfisher’s wetlands and rivers.

The fossil treasures from the Moler have been known for more than a hundred years. Two local museums exhibit the highlighs: The local museum on Fur has a large display of fossils in addition to local history, while the Moler museum north on Mors is entirely dedicated. Both museums also tell the history of the geology and palaeogeography in the Eocene. Displays are somewhat old-fashioned, but informative and gives a good impression of life in Eocene Denmark.

Denmark has a unique approach to preserve its fossil heritage: In 1985, a German tourist sent a postcard to the Fur museum, stating that he had found a large fish fossil, brought one slab of the fossil home and dug the other slab down at a location, which, like a pirate’s treasury map, he had marked on the photo on the postcard. The slab was soon found and prepared, and it was realised that this was a remarkable fossil.

It was realised that it was necessary to secure such scientifically important fossils for the future, and in 1989 a law was passed, demanding that all important fossils must be handed over to the state. If examined and found scientifically significant, they would get the status of Danekræ – loosely meaning “Danish animal” – and the founder would be paid the value, and be recognised as finder. The law also comprises meteorites and important mineral specimens.

The Danekræ law has had the fortunate effect that many collectors compete to get the honour of finding such specimens. Around 600 fossils have been accepted as Danekræ, whereof half from the Moler. And, since the honour only is given to the most significant fossils, many more reside in collections. As several pits still are in operation, and the Moler also outcrops along the Limfjorden waters, new ones are regularly unearthed. Searching is encouraged, and the museum on Mors even rents out geologist hammers to those who want to try their luck! The best preservation, including feathers and bones, are, however, in concretions, cemented by bacteria preciptitated calcite. These are very hard – you will need to bring a sledge to crack them.

How to get there:

The closest major airport is Aalborg, with flights directly from Copenhagen, Amsterdam, London, Berlin and Oslo. Driving distance to Fur is ca 80 km. Bus services exists, but renting a car is the most efficient way to get there – or do as I did: go by bike. Denmark is a perfect country for biking, with its gentle topography and well developed bike road net. Camping sites have high standard, or stay at a “Kro”, a country side hotel with a restaurant.

Do not forget to also visit the Rainforest in Randers; three large cupolas of glass are filled with rainforests, buzzing with the sounds of parrots, bats and monkeys.

I strongly recommend the book Danekræ – Danmarks bedste fossiler (“Danekræ – Denmark’s best fossils”) which shows many beautiful fossils from both the Moler and elsewhere. (Bonde, N. et al. 2008, Gyldendal fakta, Copenhagen, 225 pp).

2 responses to “Fossil treasures of Denmark, 55 million years ago

  1. Pingback: Where the dinosaurs died: The white cliffs of Stevns Klint in Denmark | Adventures in geology - Karsten Eig·

  2. People (hominins) lived in Poland 500.000 years ago and onthe British Islands 700.000 years ago.
    So far no traces of hominin have been found in Denmark… Is it pssible the last ice age destroyed all traces… of hominins that may have lived in Denmark around the 700-500.000 ?….

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