The high abundance of well preserved fossils is a result of anoxic conditions in the fine ooze at the bottom of the lake. At other Tertiary sites fossils are broken and disarticulated, but the conditions here mean that few benthic epifaunal and infaunal organisms can survive, so there is no bioturbation. Low current activity also leaves the bottom undisturbed and anoxic, but some palaeocurrent activity can be examined by looking at the orientation of fishes. The weight of the water (10s of metres deep) would have been sufficient to stop decomposing, gas-filled carcasses from floating to the surface. The primary soft bottom substrate is thought to be made up of masses of dead seaweed.
Many Messel fossils are preserved as 'Hautschatten' or 'skin-shadows' (eg. Messel birds), with layers of bacteria aligning themselves with soft tissue fibres (also called bacterial mapping or bacteriography). Products of the bacteria's own metabolic processes caused fine clay particles to adhere to their surface preserving the fossil's soft tissue morphology as bacterial ultrastrucure. This process has also been seen to occur in Brazil's Santana and Crato Formations, where the fine structures of fish muscle fibres are preserved.
Outlines are also preserved by impregnation of plant matter into the void left by decayed tissue. This only preserves the outline, though, not the detailed structure.
Flowchart showing the factors involved in the preservation of Messel fossils (Briggs & Crowther, 1992. Palaeobiology a Synthesis)
The Messel oil shale contains 40%+ water and exposure to air causes rapid drying and crumbling of the rock and fossils. Specimens should be stored in cool, damp conditions. Leaving the fossils in the rock is not recommended and today transfer to resin is common. Colourless, transparent resin is built up in thin layers.
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