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.
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