Dean Lomax (UK)
A Lagerstätte is a sedimentary deposit that exhibits exquisite fossil richness, detail and/or completeness, often preserving fine details, including soft parts, which wouldn’t normally be found as fossils. There are two main types of fossil Lagerstätten: concentration Lagerstätten, which simply consists of large concentrations of fossils found together in deposits such as bone beds; and conservation Lagerstätten, where the defining feature is the preservation of quality rather than the quantity of fossils.
A few examples of famous Lagerstätten include the Eocene Green River Formation, which is primarily known from Wyoming, but can also be found in Colorado and Utah. Famous European Lagerstätten include the Solnhofen Formation of Bavaria, Germany. This has produced some spectacularly preserved fossils, including Archaeopteryx, which is considered to be a transitional fossil between dinosaur and bird evolution. Another famous Lagerstätte, situated in central Germany, is the Messel Pit (Grube Messel). This quarry contains Eocene-aged strata and has produced specimens such as Darwinius masillae, identified as a basal primate and described in 2009.
Geological setting and location
Menat is a small village located within the department of Puy-de-Dôme, Auvergne in central France, near the town of Gannat, a town famous for Oligocene and Miocene-aged fossil deposits. The geology of Menat consists of sedimentary rock that includes soft shale layers (including bituminous, pyritious and oil shales) and hard layers consisting of diatomite. The preservation of the fossils in any of the layers is quite fantastic and fossils can be found quite literally everywhere throughout the village, where there are exposed outcrops.
However, there are two private dig sites where the main fossil-bearing strata are specifically and scientifically excavated for fossils. The area represents a fossil lake, which was created through volcanic activity and was, therefore, also a ‘maar’ lake. Such a lake is generally formed by an explosion caused by groundwater coming into contact with lava or magma, leaving behind a volcanic crater, which is eventually filled with water creating a lake. Since 1825, the area around Menat has been worked for the extraction of mineral pigments and it is within these layers that the numerous fossils are found.
The rocks of Menat were originally identified as belonging to the Eocene or even Miocene ages. However, as a result of a radio-chronology K-Ar study, the area was found to be of Palaeocene age – the first Epoch of the Palaeogene Period. The exact age is still undetermined, although the K-Ar study suggests that it is around 56Ma, towards the end of the Palaeocene Epoch. The area is perhaps one of only a few, true Palaeocene Lagerstätten in Europe; whereas, many others can be found in North and South America, such as the Almont-Beicegel Creek Lagerstätten of North Dakota. As a result, this most impressive site is exceptionally important, as it is one of the few places in Europe that allows researchers to understand something about the palaeoecology of this part of the world.
A variety of fossils have been found throughout the area of Menat, which are of exceptional preservational quality, including the preservation of soft tissues, feathers, hair and so on, making Menat a conservation Lagerstätte. In fact, it can be compared with the Green River Formation in respect of the types of fossils found. However, the Green River Formation covers an area far greater than the fossil lake of Menat.
The lake of Menat is estimated to cover an area of between 800m2 and 1,000m2. The geology and fossils discovered in the area indicate a warm palaeoclimate, with a forest environment surrounding the lake, the inhabitants of which were quite diverse. They include three species of fish from the relatively common, small-to-medium-sized Properca angusta, (a type of primitive perch), the uncommon Thaumaturus brongniarti (a small species related to the salmon), to the rarest and largest of the three species, the predatory Cyclurus valenciennesi (a type of primitive amia, similar to the extant American bowfin).
The range of fossil flora at Menat is immense. It includes aquatic plants, branches, fragments of wood, seeds, fruits and a large array of flowering plants, with some specimens preserving plant-insect interactions, with bite marks preserved on the leaves. The insects themselves are quite common, with over 200 species recognised. Usually, only isolated body segments are found. However, complete remains, including detailed wings, antennae and legs, can sometimes be discovered, and such finds have included beetles, wasps, flies and some of the earliest known bees in the fossil record (after those documented from the Cretaceous).
The much rarer aquatic animals found at Menat include amphibians and reptiles, such as crocodilians, choristoderes and turtles. Very rarely, terrestrial taxa have been found in the shales of Menat, including a small number of beautifully preserved birds and a few mammals. In addition, Menat has yielded one of the oldest known, early primate-like (proprimate) mammals – ‘Menatotherium’ – now considered by most palaeontologists to be a synonym of the relatively well-known Plesiadapis, from Europe and North America.
There is a small palaeontology museum situated within the town, which contains a moderately-sized collection of the local fossils.
I would like to thank Christian Falipou and the museum, Rhinopolis, for supplying information. A big thanks also goes to Dr Andrea Valli for translating the information supplied and for reviewing this article. Finally, thanks go to Dr Burkhard Pohl for allowing access to the quarry, and François Escuillié and Jean-Marc Pouillon for advice and general information.
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