The Messel Pit is a fossil preservation site in Hesse, central Germany, dated to the Eocene epoch (about 47 Ma). The fossil-bearing rock unit is a geologic formation named the Messel Formation; its geographic range is restricted to the Messel pit. There, the formation unconformably overlies crystalline Variscan basement and its Permian cover (Rotliegend) as well as Eocene volcanic breccias derived from the basement rocks. The formation mainly comprises lacustrine laminated bituminous shale ("oil shale") renowned for its content of fossils in exceptional preservation, particularly plants, arthropods and vertebrates (e.g. Darwinius masillae).
Messel pit
The Messel pit (German: Grube Messel) is a disused quarry near the village of Messel (Landkreis Darmstadt-Dieburg, Hesse) about 35 km (22 mi) southeast of Frankfurt am Main, Germany. Bituminous shale was mined there. Because of its abundance of well-preserved fossils of the Messel Formation dating from the middle of the Eocene, it has significant geological and scientific importance. Over 1400 taxa of plants, fungi and animals have been documented at the site. After almost becoming a landfill, strong local resistance eventually stopped these plans and the Messel pit was declared a UNESCO World Heritage Site on 9 December 1995. Significant scientific discoveries about the early evolution of mammals and birds are still being made at the Messel Pit, and the site has increasingly become a tourist site as well.
History
Brown coal and later oil shale was actively mined from 1859. The pit first became known for its wealth of fossils around 1900, but serious scientific excavation only started around the 1970s, when falling oil prices made mining the quarry uneconomical. Commercial oil shale mining ceased in 1971 and a cement factory built in the quarry failed the following year. The land was slated for use as a landfill, but the plans came to nought and the Hessian state bought the site in 1991 to secure scientific access. In the few years between the end of mining and 1974, when the state began preparing the site for garbage disposal, amateur collectors were allowed to collect fossils. The amateurs developed the "transfer technique" that enabled them to preserve the fine details of small fossils, the method still employed in preserving the fossils today. Many of the known specimens from the site have come from amateur collectors and in 1996, an amnesty on previously collected fossils was put in effect, in the hope of getting privately owned collections back into public ownership and available to science.
Depositional characteristics
The current surface of the Messel pit is roughly 60 m (200 ft) below the local land and is about 0.7 km2 (0.27 sq mi) in area. The oil-shale bed originally extended to a depth of 190 m (620 ft). 47 million years ago in the Eocene when the Messel deposits formed, the area was 10° further south than it is now. The period was very close to the Early Eocene Climatic Optimum and the climate and ecology of the site were very different, characterised by a mean annual temperature of 22 °C and a large series of maar lakes surrounded by lush sub-tropical forests that supported an incredible diversity of life. The Messel lake bed was probably a center point for drainage from nearby rivers and creeks.
The pit deposits were formed during the Eocene Epoch of the Paleogene Period about 47 million years ago, based on dating of basalt fragments underlying fossilbearing strata. Oil shale, formed by the slow anoxic deposition of mud and dead vegetation on the lake bed, is the primary rock at the site. Its sediments extend 13 m (43 ft) downward and lie atop an older sandstone foundation. The fossils within the shale show a remarkable clarity and preservation due to the unique depositional characteristics of the lake and so the Messel pit represents a Konservat-Lagerstätte. The upper stratifications of the lake most certainly supported a variety of organisms, but the bottom was subject to little disturbance by current, spawning a very anoxic environment. This prevented many epifaunal and infaunal species from inhabiting this niche and thus bioturbation was kept at a minimum. Overturn of the lake layers (caused by seasonal variations) lowered oxygen content near the surface and led to a periodic "die-off" of aquatic species. Combined with a relatively low rate of deposition, 0.1 mm (0.0039 in) per year, this provided a prime environment for the preservation of fauna and flora.
Volcanic gas releases
The area around the Messel Pit is believed to have been geologically and tectonically active during the Eocene. Some scientists, especially Jens Franzen, have hypothesized that events much like the 1986 volcanic gas releases at Lake Nyos, Cameroon, could account for the deaths and preservation in the lake of non-aquatic species. Periodic subsurface shifts possibly released large concentrations of toxic gases (such as carbon dioxide and hydrogen sulfide) into the lake and adjoining ecosystems, killing susceptible organisms. During these releases, birds and bats might have fallen in if near the lake surface and terrestrials could be overwhelmed when near the lake shore. Other scientists, especially Wighart von Koenigswald, have hypothesized that cyanobacterial blooms could account for the rich faunal assemblage of Messel, with animals succumbing after drinking water contaminated with cyanobacterial toxins. This hypothesis drew support from the occurrence of gravid and or copulating animals at Messel, which supposedly reflect a dominant season of death. Others explored the null hypothesis of accidental death. In particular, Krister T. Smith and colleagues found that the annual number of fossilized bats in Messel is comparable to the number of bats that drown in modern swimming pools. That is, bat mortality is not actually elevated at Messel, which fails to support the "mass mortality" hypotheses above. Regardless of the manner of death, the exceptional preservation at Messel (e.g., mostly complete skeletons) indicates that the lake was very deep. Animals that fell in it drifted downwards into oxygen-poor water without scavengers, where they were overlaid by successive layers of mud that petrified later, thus producing an aggregation of fossils of exceptional quality, quantity, integrity, and variety.
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