The preservational regime of

Beecher's Trilobite Bed Beecher's Trilobite Bed is a Konservat-Lagerstätte of Late Ordovician (Caradoc) age located within the Frankfort Shale in Cleveland's Glen, Oneida County, New York, USA.Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

) and other similar localities involves the replacement of soft tissues with

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

, producing a three-dimensional

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

replicating the anatomy of the original organism. Only gross morphological information is preserved (unlike

Orsten The Orsten fauna are fossilized organisms preserved in the Orsten lagerstätte of Cambrian (Late Miaolingian to Furongian) rocks, notably at Kinnekulle and on the island of Öland, all in Sweden. The initial site, discovered in 1975 by Klaus M� ...

type

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

replacement), although the fossils are compressed some relief is preserved (unlike

Burgess Shale type preservation The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 69 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion. Additi ...

). 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. 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. 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. 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. If the organic content of the sediment is too high the dissolved iron precipitates in the sediment and not in the carcass. 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.

References

{{Cambrian preservational modes Fossilization Lagerstätten Ordovician fossil record