Secular distribution of Burgess-Shale-type preservation



    1. Nicholas I. Butterfield, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England, UK; present address: Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 578, Canada; 1st June 1994
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Burgess-Shale-type preservation is defined as a taphonomic pathway involving the exceptional organic preservation of non-mineralizing organisms in fully marine siliciclastic sediments. In the Phanerozoic it occurs widely in Lower and Middle Cambrian sequences but subsequently disappears as a significant taphonomic mode. The hypothesis that this distribution derives solely from a secular increase in the depth of bioturbation is falsified: low bioturbation indices do not prevent the rapid enzymatic degradation of organic structure, nor do they account for the conspicuous absence of comparable preservation during the Vendian. An earlier, Late Riphean (ca. 750–850 Ma), interval of enhanced organic-walled fossil preservation suggests a long-term recurrence in Burgess-Shale-type taphonomy that is independent of metazoan activity. A model based on the potentially powerful anti-enzymatic and/or stabilizing effects of clay minerals on organic molecules is proposed to account for Burgess-Shale-type preservation. Long-term changes in average clay mineralogies and the ocean chemistry that determines their interaction with organic molecules are likely to have induced the pronounced secular distribution of these fossil biotas, while regional variations in tectonism, weathering, etc., explain their non-uniform geographic distribution; the close correlation between exceptional, organic-walled fossil preservation and volcano-genic sedimentation in Tertiary lake deposits provides a compelling analogue. Recognition of a temporal control on Burgess-Shale-type preservation constrains the evolutionary scenarios that can be drawn from such biotas; significantly, neither the initial rate of appearance, nor the ultimate fate of Burgess-Shale-type taxa can be directly assessed. □Taphonomy, exceptional preservation, organic preservation, fossil Lagerstätten, Burgess Shale, clay mineralogy, clay-organic interactions, secular change, Cambrian, Proterozoic.