Age: Lower Cambrian
The fossils of the Sirius Passet fauna have been preserved similarly to those of the Burgess Shale. There is no consensus on the exact taphonomic processes involved during preservation (see, for example, Butterfield 1990, Towe 1996 and Orr et al. 1998), but it is possible to make some general statements.
Most of the organic carbon has been lost by exposure to relatively high metamorphic temperatures (Butterfield 1995). The presence of quartz, chlorite, muscovite and chloritoid in the muds indicates that temperatures were elevated to greenschist grade.
The fossils have undergone no distortion other than flattening, and therefore it is assumed that any mineralisation that has occurred would have to have post-dated sedimentation and compaction (Butterfield, 1995). The original taphonomic process must have been preservation of the organic constituents of the organisms.
Anoxia and a lack of bioturbation are key elements of this mode of preservation (Butterfield, 1995). However, Budd and Peel (1998) state that the fossils "are associated, perhaps directly, with trace fossils", which suggest that the environment of deposition wasn't constantly lethal. It is conceivable that anoxic conditions could have been maintained for a sufficiently long period to preserve the organism from decay, and then oxic conditions could return to permit bioturbation to occur in a higher sediment layer. The fact that few taxa with "shelly skeletons" are found in the Sirius Passet fauna could be an indication of anoxic conditions; "the number of shelly species drops dramatically in areas of the sea floor with very low levels of oxygen" (Conway Morris, 1998 pp.121).