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.