Fossil formation. Sequence of ilustrations of the formation of a trilobite fossil. The process begins when the trilobite dies (top) and the body sinks to oxygen-free sediments on the seabed (upper centre). The soft tissues decay, while minerals in the trilobite's body are gradually replaced by minerals from the mud as sediment layers accumulate (lower centre). These minerals solidify into a form of stone or rock, preserving the shape and structure of the trilobite. The fossil may remain underground, or may eventually be exposed by erosion or removal of the surrounding material (bottom). For this sequence without labels, see image C048/2768.