Biology:Beecher's Trilobite type preservation
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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
- ↑ 1.0 1.1 Template:PalAss2008
- ↑ 2.0 2.1 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.
- ↑ 3.0 3.1 3.2 Paul A. Selden; John R. Nudds (2005). Evolution of Fossil Ecosystems. University of Chicago Press. p. 192. ISBN 978-0-226-74641-8. http://www.mansonpublishing.com/sample/selden_evolution_sample.pdf. "see page 41"
- ↑ 4.0 4.1 4.2 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. doi:10.1130/0091-7613(1991)019<1221:POSBFB>2.3.CO;2. Bibcode: 1991Geo....19.1221B.
- ↑ 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. doi:10.2475/02.2008.01. Bibcode: 2008AmJS..308..105R.
- ↑ Petrovich, R. (2001). "Mechanisms of fossilization of the soft-bodied and lightly armored faunas of the Burgess Shale and of some other classical localities". American Journal of Science 301 (8): 683–726. doi:10.2475/ajs.301.8.683. Bibcode: 2001AmJS..301..683P. http://earth.geology.yale.edu/~ajs/2001/Oct/qn0801000683.PDF.
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