SECULAR DISTRIBUTION OF BURGESS-SHALE-TYPE PRESERVATION

Burgess-Shale-type preservation is defined as a taphonomic pathway inv olving the exceptional organic preservation of non-mineralizing organi sms in fully marine siliciclastic sediments. In the Phanerozoic it occ urs widely in Lower and Middle Cambrian sequences but subsequently dis appears as a significant taphonomic mode. The hypothesis that this dis tribution derives solely from a secular increase in the depth of biotu rbation is falsified: low bioturbation indices do not prevent the rapi d 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 organ ic-walled fossil preservation suggests a long-term recurrence in Burge ss-Shale-type taphonomy that is independent of metazoan activity. A mo del based on the potentially powerful anti-enzymatic and/or stabilizin g 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 interactio n with organic molecules are likely to have induced the pronounced sec ular distribution of these fossil biotas, while regional variations in tectonism, weathering, etc., explain their non-uniform geographic dis tribution; the close correlation between exceptional, organic-walled f ossil preservation and volcanogenic sedimentation in Tertiary lake dep osits provides a compelling analogue. Recognition of a temporal contro l on Burgess-Shale-type preservation constrains the evolutionary scena rios that can be drawn from such biotas; significantly, neither the in itial rate of appearance, nor the ultimate fate of Burgess-Shale-type taxa can be directly assessed.