The Bony Fish Fossils of the Western Limfjord: Evolution and Climate Adaptation in the Earliest Eocene

Description

Hanklit: N 56,895649° E 8,751962° (ETRS89)
Knudeklint: N 56,838216° E 8,964653° (ETRS89)

The Eocene fossiliferous sediments of the Western Limfjord Area constitute a natural site of universal value. The deposits are exposed in various coastal cliffs along the fjord, of which the two most important and complete cliff sections have been selected for the serial locality The Bony Fish Fossils of the Western Limfjord: Evolution and Climate Adaptation in the Earliest Eocene. The serial site is composed of the two picturesque coastal cliffs, Hanklit and Knudeklint, which are located on the islands of Mors and Fur, respectively. Hanklit covers 3.79 hectares and Knudeklint 3.19 hectares, in total 7 hectares. The altitude reaches 60 m at Hanklit and just above 30 m at Knudeklint. In addition, the deposits are visible in several clay pits on Fur and Mors where it has been excavated for commercial purposes during more than 100 years.

The serial site area is characterised by beautifully exposed sediments that formed the seabed in the Eocene some 56–55 million years ago. At that time, the site represented an offshore, marine environment, which was connected to the Atlantic Ocean and covered the entire Denmark. Most of the sedimentary succession is very rich in microscopic, siliceous algae named diatoms. Locally, this sediment is named “moler” (mo-clay in English). In geological terms, the two units in question are named the Fur Formation and the Stolleklint Clay of the Ølst Formation. Another special sedimentary feature is the characteristic occurrence of more than 200 ash layers originating from immense volcanic eruptions in the area near East Greenland and the Faeroe Islands. The ash layers have proven very effective for both absolute and relative dating of the succession.

The deposits contain very rich fossil assemblages, which lived during the significant global earliest Eocene temperature rise and the cooler period, which followed immediately after. The Early Eocene was a very important period with regard to the origin and evolution of several groups of organisms as well as the study of climatic changes. With regard to the latter, it is well established that the Early Eocene provide one of the best analogues for near-future climates.

The serial site contains exceedingly well-preserved, abundant and diverse fossil faunas and floras from different palaeoenvironments, which all together offer an outstanding view into the earliest Eocene ecosystems:

• Marine vertebrates
• Terrestrial vertebrates and invertebrates
• Marine algae
• Terrestrial plants including petrified trunks and branches, leaves and spores and pollen
• Marine invertebrates

Fossils from the lowermost Eocene deposits of the Western Limfjord Area are known since the first simple drawing of a fossil fish from Fur in 1763, but it took another 100 years before the first scientific description of fossils (diatoms) was published. The first insects were described in 1922, but it was not before 1941 that the first fossil fish was properly described. Since then, the Eocene deposits and fossils from the Western Limfjord site have played an increasingly important role in taxonomic and evolutionary studies of various fossil groups, especially fish, birds and insects, due to the extremely well-preserved fossils. As a result of the growing scientific interest, the number of scientific publications including data from the site has constantly increased.

Justification of Outstanding Universal Value

The serial fossil site allows a unique view into the earliest Eocene offshore marine and terrestrial biosphere. The Eocene period was a time of significant speciation and the spread of new animal groups, which can be considered ancestors of the present-day fauna. The rapid turnover of species was particularly associated with the massive mass extinction that occurred at the end of the Cretaceous Period 66 million years ago, but also the dramatic global warming that began abruptly at the boundary between the Paleocene and Eocene epochs 56 million years ago and continued for up to 170,000 years played a role. This climate event is internationally abbreviated as PETM (Paleocene-Eocene Thermal Maximum).

The Stolleklint Clay and the Fur Formation constitute together an impressive treasure trove with abundant fossils from the ancient sea, as well as flora and fauna from the adjacent land areas. In particular, the fossilized fish provide a unique insight into the origin and early diversification of the modern bony fish. The mo-clay deposits contain incredibly well-preserved fossils of various organisms, including complete skeletons of fish, birds, and insects, providing a unique insight into the appearance of the earliest ancestors of many modern organisms and the early stages of their evolution.

Three important factors contribute to make the serial site outstanding in a global perspective:

A. The site contains the oldest (c. 55.9 – 55 million years) known occurrence of well-preserved, diverse and oceanic marine, ray-finned fish lineages succeeding the Cretaceous-Paleogene mass-extinction. The fish fauna is very well-preserved and highly diverse as it contains between 25 and 29 fish orders and more than 80 species. Hence, it offers outstanding information on the origin and early evolution of several lineages of the modern marine, bony fish.

B. The site gives a unique insight in the earliest Eocene biosphere because it contains abundant and diverse, extremely well-preserved fossils from both offshore marine and terrestrial environments. It contains a mixture of marine organisms including fish, sea-turtles, diatoms, silicoflagellates and dinoflagellates, together with several, but less abundant, invertebrate groups in addition to terrestrial organisms. The latter includes several, nearly complete bird skeletons preserved in three dimensions, abundant insects, spores and pollen together with more rare leaves and petrified trunks from trees. Not only fish but also fossil birds and insects from the site offers exceptionally valuable information on the origin and early evolution of several lineages.

C. The deposits of the Western Limfjord Area represent the period including the very warm climatic conditions during the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago to the cooler period that started immediately after. Thus, the site constitutes an outstanding geochemical and biological climate archive, which provides insight into how the dramatic climate changes influenced the composition of marine and terrestrial ecosystems. The bony fish fauna underwent significant changes in response to the climate fluctuations. This is of great importance in terms of clarifying how climate changes affected the marine fish fauna in the Early Eocene, but also as a tool to interpret how future climate changes may affect the marine life that exists today.

The finest fossils are found in scattered levels where the pores and cavities in the mo-clay were filled by calcareous fluids that solidified and hardened just below the seafloor. As a result, hard, lens-shaped structures known as calcareous concretions or cement stones were formed. The concretions protected the fossils from compression, which explains why most of the finest and most complete fossils, often in three dimensions, are found within the concretions of the Fur Formation. The high quality of preservation has even led to the discovery of original molecules in some of the fossils. For example, molecular remnants of haemoglobin from red blood cells have been found in the carbonaceous film representing the original skin between the fingers of a sea turtle hatchling. Another example is the conservation of microscopic melanosomes related to colouration of feathers situated in situ on bird skeletons from the Fur Formation and to pigmentation related to camouflage in fish larvae from the Stolleklint Clay.

Together, the numerous findings provide a detailed and vivid picture of the fascinating animal life that existed in the deeper marine environments some 55 million years ago.

Criterion (viii): The c. 56 to 55 million years old strata of the Western Limfjord site represent a unique view into the marine fish fauna, the general biosphere including the birds and insects, and the significant climatic changes, which occurred in the earliest part of the Eocene. The site makes an exceptional window into the initial phase of the Eocene rapid rise in diversity among the bony fish. Moreover, it represents the highest known Early Eocene diversity among the marine ray-finned fish orders excluding the “true” spiny-rayed fishes. In addition, The Western Limfjord site is the only known substantial fish-bearing site where the abrupt climate changes at the Paleocene-Eocene boundary, the PETM, is recorded with confidence. In summary, the Western Limfjord site meets the requirements for criterion (viii) because it contains a unique record of life in a major stage of the Earth’s history as exemplified by: 1) The origin and diversification of several modern lineages of marine bony fish just after the last and fifth major extinction event on Earth 66 million years ago, 2) The content of unique biotic and geochemical evidence through one of the most significant temperature fluctuations in the Earth’s history, the Paleocene-Eocene Thermal Maximum.

Statements of authenticity and/or integrity

Hanklit and Knudeklint of the Western Limfjord Site display a complete succession of Early Eocene deposits clearly demarcated by the ash-layers which effectively dates the deposits and the fossil assemblages found within. Due to the poor oxygen content on the earliest Eocene seabed, bottom-dwelling fauna was very rare. Hence, deposited flora and fauna were left largely undisturbed. The Fur Formation comprises approximately 1 million years of intact fossiliferous deposits.

The spectacular landscapes on northern Fur and Mors were formed by two advances of the Scandinavian Ice Sheet between 30.000 and 20,000 years ago. The ice sheet pushed and folded the mo-clay sediments. Impressive fold structures and large thrust displacements are clearly visible in cross-sections of layers of the mo-clay at the cliffs of Knudeklint and Hanklit. Knudeklint contains the most complete time series in the area, while Hanklit, being the largest of the cliffs, displays the thickest intact sequence.

Slow but continuous erosion from the sea hinders overgrowth and keeps the cliffs in a pristine state.

The cliffs are protected in their natural state through comprehensive legislation and designations at both national and municipal level. The coastal zone is in general well protected in national legislation ensuring that the coastline is accessible and undisturbed. The areas from the coastline and 300 m inland is protected by the so-called beach-protection. In this zone no changes in the present state of the area are allowed which hinders excavations, artificial changes in terrain, erection of buildings etc. The sites are both designated as protected areas – Hanklit since 1937 and Knudeklint since 1973, which means that the cliffs are to be maintained in their natural state and that fossils cannot be extracted from the sites. Municipal plans furthermore designate both sites as landscapes of particular importance due to their geological values, meaning that the integrity of the sites is given utmost priority in municipal administration and that no activities will be allowed, which could deteriorate the values of the sites.

Fossil discoveries are most often made in open excavations, but also in the debris at the bottom of coastal cliffs. While new discoveries are most easily made in commercially active mo-clay pits, the serial site including Knudeklint and Hanklit stands as a pristine reference for the diatomite-rich Fur Formation and the upper part of the Stolleklint Clay with its characteristic layers of volcanic ashes and its content of fossils.

Fossil specimens of exceptional national value are according to national law preserved for scientific and exhibition purposes. Fossil trove belongs to the state, and any person who finds or gains possession of fossil trove after 1989, shall immediately deliver it to the Natural History Museum of Denmark, or a nationally recognized local natural history museum. Collection of fossils is possible for both researchers, amateur geologists and tourists from several abandoned mo-clay pits on Fur and Mors, which are not subject to the same protective regulation as the coastal zone and the nomination site.

The local museums Museum Mors and Museum Salling, and the Natural History Museum of Denmark (Copenhagen) house magnificent specimens of the most important finds from the mo-clay.

Comparison with other similar properties

Eocene localities with well-preserved fossils (so-called Konservat-Lagerstätten) are known from several parts of the globe, but only very few contain exceptionally well-preserved, rich and diverse fish faunas. These localities represent different ancient environments, which can be divided into three distinct main categories:

• Terrestrial sites. Land areas with lake or fluvial deposits, e.g. Grube Messel (D), Green River Formation (USA), Okanagan Highlands (Can, USA), Florissant Formation (USA)
• Nearshore marine sites, e.g. Bolca (I), London Basin (UK)
• Offshore marine sites including Western Limfjord Area (DK) and Uylya-Kushlyuk (Turkmenistan)

Among these, the celebrated locality Grube Messel is already on the UNESCO World Heritage list, partly because of its beautifully preserved early mammals. Apone Valley including the world-famous fish-rich Bolca localities is on the World Heritage Tentative List together with the Western Limfjord Area.

The fossil fish fauna of the Western Limfjord site differs significantly from other known fish-bearing localities in the world where complete fish skeletons are preserved. Firstly, the fauna is approximately five million and eight million years older than the faunas from the better-known localities of Bolca and Grube Messel, respectively. Therefore, the Danish locality represents an earlier evolutionary stage among the ray-finned bony fish, where diversity had not yet developed as extensively. As a result, the species diversity is lower than in some of the younger fish-bearing sites, for instance Bolca, but in return, the Western Limfjord area represents the first known species diversifications within a range of fish orders.

Secondly, the Western Limfjord area represents a more offshore oceanic environment than the other known localities with extraordinary well-preserved fish fossils. This means that the Western Limfjord Area site contains a unique fish fauna including some of the very earliest known deep-water fish, for instance complete specimens of a whalefish and a frogfish relative. The only other known locality, which possibly represents a similar, offshore palaeoenvironment, is Uylya-Kushlyuk of Turkmenistan. This site, however, is characterised by a lower diversity and is rather poorly dated.

Another fish-bearing UNESCO World Heritage Site is The Miguasha National Park (Escuminac Formation) of Canada became a UWH site in 1999, partly because of its spectacular Late Devonian fish fossils. However, this much older fish fauna containing e.g. 370 million years old placoderms and exceptional lobe-finned fish is very different from the advanced ray-finned fish of the Eocene.