The Messel Pit is a place where fossils are preserved in Hesse, central Germany. It is from the Eocene epoch, about 47 million years ago. The rock layer containing fossils there is called the Messel Formation. This layer is found only at the Messel Pit. It lies above older rocks, including crystalline Variscan basement, Permian cover rocks (called Rotliegend), and volcanic rocks from the Eocene era that formed from basement rocks. The Messel Formation is mainly made of lake-deposited, layered shale rich in bitumen, also known as oil shale. This shale is famous for containing fossils that are very well preserved, including plants, insects and spiders, and animals with backbones, such as mammals like Darwinius masillae.

Messel pit

The Messel Pit (German: Grube Messel) is an old quarry near the village of Messel in Hesse, Germany, about 35 kilometers (22 miles) southeast of Frankfurt am Main. Bituminous shale was mined there. Because of its many well-preserved fossils from the middle Eocene period, the site is important for geology and science. Over 1,400 species of plants, fungi, and animals have been documented there. After plans to turn the area into a landfill were stopped by local people, the Messel Pit became a UNESCO World Heritage Site in 1995. Scientists continue to find important information about the early evolution of mammals and birds there, and the site is now also a place for tourists.

Brown coal and oil shale were mined from 1859. The area became known for its fossils around 1900, but serious scientific digging started in the 1970s when mining became unprofitable due to falling oil prices. Oil shale mining stopped in 1971, and a cement factory built in the quarry failed the next year. The land was planned for a landfill, but this was stopped, and the Hessian state bought the site in 1991 to protect it for scientific study. Between the end of mining and 1974, when the state prepared the site for garbage disposal, amateur collectors were allowed to gather fossils. These collectors developed a method called the "transfer technique" to preserve small fossils, a method still used today.

Many fossils found at the site came from amateur collectors. In 1996, a program was started to return privately owned fossils to public collections for scientific use.

The current surface of the Messel Pit is about 60 meters (200 feet) below the surrounding land and covers an area of 0.7 kilometers (0.27 square miles). The oil shale bed originally extended to a depth of 190 meters (620 feet). About 47 million years ago, during the Eocene, the area was 10 degrees further south than it is now. The climate was much warmer, with an average temperature of 22°C, and the site had many lakes surrounded by lush subtropical forests that supported a wide variety of life. The Messel lake bed was likely a central point for water from nearby rivers and streams.

The deposits at the site formed during the Eocene Epoch of the Paleogene Period, as determined by dating basalt fragments beneath the fossil layers. Oil shale, created by the slow buildup of mud and dead plants in the lake, is the main rock type. The sediments are 13 meters (43 feet) thick and sit on top of older sandstone. The fossils in the shale are very clear and well-preserved because of the unique conditions of the lake. This makes the Messel Pit a Konservat-Lagerstätte, a place where fossils are exceptionally preserved. The upper layers of the lake supported many organisms, but the bottom had little water movement, creating a low-oxygen environment that limited the presence of certain species. Periodic changes in the lake’s layers reduced oxygen levels, causing some aquatic animals to die. Combined with a slow rate of sediment buildup, this environment helped preserve many animals and plants.

The area around the Messel Pit was likely geologically active during the Eocene. Some scientists, like Jens Franzen, suggest that events similar to the 1986 volcanic gas release at Lake Nyos in Cameroon could explain how non-aquatic animals died and were preserved. These events may have released large amounts of toxic gases, like carbon dioxide and hydrogen sulfide, into the lake and nearby areas, killing nearby animals. Birds and bats might have fallen into the lake if they were near the surface, and land animals could have been affected if they were near the shore.

Other scientists, like Wighart von Koenigswald, think that cyanobacterial blooms (a type of algae) could explain the many animals found at the site. Animals may have died after drinking water contaminated with toxins from these blooms. This idea is supported by the presence of animals that were pregnant or mating, suggesting a specific time of year when many animals died.

Some researchers, like Krister T. Smith, studied the number of bat fossils found at Messel and compared it to the number of bats that drown in modern swimming pools. They found that bat deaths at Messel are not unusually high, which challenges the idea that many animals died suddenly at the site.

No matter how the animals died, the excellent preservation at Messel—such as nearly complete skeletons—shows that the lake was very deep. Animals that fell into the lake sank to the bottom, where there were few scavengers. Layers of mud then covered them, eventually turning into rock and preserving the fossils. This process created a large collection of fossils with high quality, quantity, and variety.

Fossils from the Messel Pit can be seen in the Messel town, the Fossil Museum in Messel, the Hessisches Landesmuseum in Darmstadt (5 kilometers from Messel), and the Senckenberg Museum in Frankfurt am Main (about 30 kilometers from Messel). Visitors can park near the pit and walk 300 meters to a viewing platform. Entry to the pit is only allowed as part of a special tour.

Fossils

The Messel Pit is the best-preserved site for studying the plants and animals from the Eocene era, with over 1,400 types of species identified. Other sites usually only have parts of bones, but the Messel Pit has very well-preserved remains, including fur, feathers, and "skin shadows" of some animals. This unusual preservation has led to careful scientific study. For example, bite marks shaped like dumbbells on a fossilized leaf were found to be from a carpenter ant infected by a fungus called Ophiocordyceps unilateralis. The fungus changed the ant’s behavior to help it spread its spores. This is the oldest known example of a fungus controlling an animal’s actions.

The variety of life at the site is partly due to gas releases from the lake bed. Here are some examples of fossils found:

• Nine pairs of turtles (Allaeochelys crassesculpta) were found mating. The males were tucked under the females, as turtles mate. They likely died quickly after starting to mate in shallow, oxygen-rich water. As they sank deeper, they were poisoned by volcanic gas and buried in the lakebed. These turtles are still alive today, but they lack scales and absorb oxygen through their skin. This helps them stay underwater, but it also harms them in areas with little oxygen.
• Over 10,000 fish fossils from many species.
• Thousands of insects, some with colors still visible.
• Many small mammals, including pygmy horses, large mice, primates, hedgehogs, marsupials, pangolins, aardvark relatives, and bats. Bat fossils have helped scientists learn about how echolocation evolved.
• Many birds, especially predators.
• Crocodiles, frogs, turtles, salamanders, and other reptiles or amphibians.
• Remains of over 30 plant types, including palm leaves, fruits, pollen, wood, walnuts, and grapevines.

Some specific species found include:

• Wedelphus dichopteroides
• Ants from the family Formicidae, such as Casaleia eocenica, Cephalopone grandis, Cyrtopone curiosa, Gesomyrmex pulcher, Messelepone leptogenoides, Pachycondyla eocenica, Titanomyrma gigantea, and others.
• Bees like Protobombus messelensis and Friccomelissa schopowi.

The International Union of Geological Sciences (IUGS) called the Messel Pit the "richest geosite in the world" for studying the Eocene environment. It was added to its list of 100 "geological heritage sites" in 2022. A geological heritage site is a place with important geological features or processes that help scientists understand Earth’s history.