Triarthrus eatoni preserved in pyrite with preservation of soft bodied legs. The classic Beecher's Trilobite Bed trilobite.

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, sulphate-reducing anaerobic bacteria break down its organic matter producing sulphide.^[4] The high concentration of iron in the sediment converts this to iron mono-sulphide.^[4] 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 SO₄²⁺ 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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