The Messel Pit, formerly a lake, 1.000 meters long and 700 meters wide, contains fossils of the middle Eocene period, around 50 million years ago. At that time the climate on this location was (sub) tropical. The deposits were preserved in oily shale formed from seaweed which prevented them from erosion. Many different species of animals and plants are found like crocodiles, bats, birds, reptiles, fish, turtles, insects, and the primitive horse Propaleotherium. The fossils found are not only complete skeletons, but also soft tissues like hair and feathers and even stomach contents.