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Beecher's Trilobite type preservation

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A metallic shape emerging from black rock.
A Triarthrus eatoni with preserved appendages. From upper New York, United States

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]

References

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  1. ^ a b Farrell, Úna C. (2008). "Pyritized olenid trilobite faunas of upstate NY: Palaeoecology and taphonomy" (PDF). In Cusack, M.; Owen, A.; Clark, N. (eds.). Programme with Abstracts. Palaeontological Association Annual Meeting. Vol. 52. Glasgow, UK.
  2. ^ a b Butterfield, Nicholas J. (2003). "Exceptional Fossil Preservation and the Cambrian Explosion". Integrative and Comparative Biology. 43 (1): 166–177. doi:10.1093/icb/43.1.166. PMID 21680421.
  3. ^ a b c Paul A. Selden; John R. Nudds (2005). Evolution of Fossil Ecosystems (PDF). University of Chicago Press. p. 192. ISBN 978-0-226-74641-8. Archived from the original (PDF) on 2011-07-14. see page 41
  4. ^ a b c Derek E.G. Briggs; Simon H. Bottrell; Robert Raiswell (1991). "Pyritization of soft-bodied fossils: Beecher's Trilobite Bed, Upper Ordovician, New York State". Geology. 19 (12): 1221–1224. Bibcode:1991Geo....19.1221B. doi:10.1130/0091-7613(1991)019<1221:POSBFB>2.3.CO;2.
  5. ^ Robert Raiswell; Robert Newton; Simon H. Bottrell; Patricia M. Coburn; Derek E. G. Briggs; David P. G. Bond; Simon W. Poulton (2008). "Turbidite depositional influences on the diagenesis of Beecher's Trilobite Bed and the Hunsrück Slate; sites of soft tissue pyritization". American Journal of Science. 308 (2): 105–129. Bibcode:2008AmJS..308..105R. doi:10.2475/02.2008.01.
  6. ^ Petrovich, R. (2001). "Mechanisms of fossilization of the soft-bodied and lightly armored faunas of the Burgess Shale and of some other classical localities" (PDF). American Journal of Science. 301 (8): 683–726. Bibcode:2001AmJS..301..683P. doi:10.2475/ajs.301.8.683.
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