PUCKERS AND PULL-THROUGHS: AN EDIACARAN ACTION SHOT

The earliest animal ecosystems, consisting of large, complex, soft-bodied organisms of bizarre morphology and mysterious phylogenetic affinity, are preserved in the ~555 Ma Ediacara Member of the Rawnsley Quartzite, west of the Flinders Ranges of South Australia. However, up to 50% of Ediacaran biogenic structures (including many of the most abundant enigmatic forms) have yet to be described. One particular structure, informally known as “mop,” for its similarity to the cleaning aid, is well-represented in the Ediacara Member, occurring on four and dominating two of twenty excavated beds. Hundreds of specimens, representing a wide range of mop sizes and morphologies, were examined on the bases of these previously-excavated beds.

Based on morphological examination, mop is not interpreted as a body fossil. Rather, data suggest that mop was formed by the uprooting of Aspidella-type holdfasts by unidirectional currents. Mop specimens range in size from centimeter- to decimeter-scale. Each mop specimen consists of two parts. The probable “upstream” component consists of parallel to subparallel lineations of a “streaming” morphology. The “downstream” component consists of a well-defined margin, ranging from arcuate to straight. Mop is the product of interactions between current activity, the microbial mat and sediment substrate and the biology and living environment of the holdfast-bearing organisms. The wide range of mop morphologies and taphonomy observed is due to the variable influence of these three factors.

In the Ediacara Member, especially, the exquisite preservation of soft-bodied forms as siliciclastic casts and molds on bed soles (Droser et al., 2006) indicates that unusual taphonomic processes, uncharacteristic of anything seen in modern marine sediments, were at work in terminal Neoproterozoic shallow marine settings. The presence of mop in the Ediacara Member demonstrates that not only the fossils of soft-bodied organisms, but also the interaction of such organisms with their environment and associated physical processes can be captured in the fossil record; that the moment of this interaction can be caught in a biostratinomic snapshot.