In Lagerstätten the normal processses of decay after death were less active or halted as sediment was deposited. This allowed the preservation of soft parts and exceptional detail.
There are several possible causes which allow this to happen:
- Anoxic or near anoxic conditions (e.g. oxygen-free mud) which suppresses common bacterial decomposition long enough for the initial casts of soft body parts to register.
- Rapid burial in volcanic ash from a nearby eruption. This prevents scavengers and burrowing animals from reaching the dead body. It also produces a low-oxygen environment.
- Rapid burial by mud-slides.
- High salt content in the water, which few organisms can live in. This condition occurred in the lagoons which produced the Solnhofen limestone.
In these conditions, which are exceptional, scavengers,
burrowers and bacteria are eliminated or reduced. Then the normal processes of deposition and fossilisation produces strata containing exceptional fossils.