The preservational regime of Beecher's Trilobite Bed (Upper Ordovician) and other similar localities[1] involves the replacement of soft tissues with pyrite, producing a three dimensional fossil replicating the anatomy of the original organism.[2] Only gross morphological information is preserved (unlike Orsten type phosphate replacement), although the fossils are compressed some relief is preserved (unlike Burgess Shale type preservation).[3]
The pyrite formed in voids left when soft tissue had decayed, and the tough exoskeleton formed a cavity which could be filled by euhedral pyrite.[2] Pyrite replacement of soft tissue can only occur in exceptional circumstances of sediment chemistry when there is a low organic content, but a high concentration of dissolved iron.[1][4][5]
When a carcass is buried in such sediment, sulfate-reducing anaerobic bacteria break down its organic matter producing sulfide. The high concentration of iron in the sediment converts this to iron mono-sulfide. Finally, aerobic bacteria convert this by oxidation to pyrite.[4] The requirement of early anaerobic and later aerobic bacteria means that the pyritisation must occur in the upper levels of the sediment, close to the aerobic-anaerobic interface.[3] If the organic content of the sediment is too high the dissolved iron precipitates in the sediment and not in the carcass.[3] Seawater sulfate ions diffusing toward animal carcasses enabled sulfate-reducing bacteria to oxidize the reactive organic matter of these remains, but the sulfide produced reacted promptly with the abundant Fe2+ ions of the pore water and pyrite precipitated right on the organic remains.[4][6]
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