Beecher's Trilobite type preservation

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] 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.

[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] 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.