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Mud can preserve an extraordinary amount if it accumulates in a particular way. It must settle fast, before decay dissolves the flesh and sinews that hold a skeleton together and before the bones are separated, washed away and broken into fragments. As more mud settles in thicker and thicker layers on the bottom, so the body beneath is squashed flat. The mud may be so glutinous that it shuts off oxygen - and then some relic of the flesh may survive. But even if that entirely disappears, the scales and bones may remain, and when the mudstone is cleared away the fossilised body is revealed in great detail, sometimes down to the delicate tracery of its fins.