It was originally speculated that it represented body contents forced out by the pressure of overlying rocks,[1] although decay contents would form more quickly and thus be a more likely interpretation.[3] This is supported by their presence around organisms whose external wall apparently ruptured; dark stains are also found in Carboniferous fish, and oily liquids have been seen to ooze out of the mouths and anuses of decaying seals.[3] A high abundance of elemental copper has been taken to suggest that it represents "blood" (i.e. the hemolymph of arthropods and molluscs).[7] Although copper was not analyzed in the canonical work on Elemental Mapping of Burgess Shale organisms, the dark stain was higher in silicon and lower in aluminum and potassium than elsewhere.[8]