Beecher's Trilobite type preservation

{{Short description|Replacement of soft tissues of a fossil with pyrite}}

File:Triarthrus eatoni (pyritized fossil trilobite with appendages) (Whetstone Gulf Formation, Upper Ordovician; Lewis County, New York State, USA) 2.jpg with preserved appendages. From upper New York, United States|alt=A metallic shape emerging from black rock.]]

The preservational regime of Beecher's Trilobite Bed (Upper Ordovician) and other similar localities involves the replacement of soft tissues with pyrite, producing a three-dimensional fossil replicating the anatomy of the original organism.{{cite journal

| title = Exceptional Fossil Preservation and the Cambrian Explosion

| year = 2003

| journal = Integrative and Comparative Biology

| volume = 43

| issue = 1

| pages = 166–177

| doi = 10.1093/icb/43.1.166

| author = Butterfield, Nicholas J.

| pmid=21680421

| doi-access = free

}} 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).{{Cite book

| author1 = Paul A. Selden

| author2 = John R. Nudds

| title = Evolution of Fossil Ecosystems

| page = 192

| quote = see page 41

| publisher = University of Chicago Press

| year = 2005

| isbn = 978-0-226-74641-8

| url = http://www.mansonpublishing.com/sample/selden_evolution_sample.pdf

| url-status = dead

| archiveurl = https://web.archive.org/web/20110714030045/http://www.mansonpublishing.com/sample/selden_evolution_sample.pdf

| archivedate = 2011-07-14

}}

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.{{PalAss2008

| title = Pyritized olenid trilobite faunas of upstate NY: Palaeoecology and taphonomy

| author = Farrell, Úna C. |author2=Briggs, Derek E. G.}}{{cite journal

| title = Pyritization of soft-bodied fossils: Beecher's Trilobite Bed, Upper Ordovician, New York State

| year = 1991

| journal = Geology

| volume = 19

| issue = 12

| pages = 1221–1224

| doi = 10.1130/0091-7613(1991)019<1221:POSBFB>2.3.CO;2

| author1 = Derek E.G. Briggs

| author2 = Simon H. Bottrell

| author3 = Robert Raiswell

|bibcode = 1991Geo....19.1221B }}{{cite journal

| title = Turbidite depositional influences on the diagenesis of Beecher's Trilobite Bed and the Hunsrück Slate; sites of soft tissue pyritization

| journal = American Journal of Science

| volume = 308

| issue = 2

| year = 2008

| pages = 105–129

| doi =10.2475/02.2008.01

| author1 = Robert Raiswell

| author2 = Robert Newton

| author3 = Simon H. Bottrell

| author4 = Patricia M. Coburn

| author5 = Derek E. G. Briggs

| author6 = David P. G. Bond

| author7 = Simon W. Poulton

| bibcode = 2008AmJS..308..105R

| doi-access = free

}}

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.{{cite journal

| title = Mechanisms of fossilization of the soft-bodied and lightly armored faunas of the Burgess Shale and of some other classical localities

| journal = American Journal of Science

| volume = 301

| issue = 8

| year = 2001

| pages = 683–726

| url = http://earth.geology.yale.edu/~ajs/2001/Oct/qn0801000683.PDF

| doi = 10.2475/ajs.301.8.683

| author = Petrovich, R.| bibcode = 2001AmJS..301..683P

}}