TURNING BACK THE CLOCK: DECIPHERING TIME AVERAGING AND RECONSTRUCTING COLONIZATION HISTORIES OF EDIACARA COMMUNITIES

Many studies have demonstrated that “snapshots” of modern communities contain a considerable component of dead organisms. Identifying the extent of time averaging preserved in fossil assemblages has historically, however, proved extremely challenging.

Preservation of the Ediacara Biota as in situ assemblages has commonly led to the interpretation that fossil assemblages record biologically brief snapshots of temporally discrete communities. While we would expect a similar community “time-averaging”, the in situ nature of these assemblages allows for a much more detailed evaluation of the sequence and extent of time averaging. Excavation of a series of 28 beds in the Ediacara Member of the Rawnsley Quartzite (South Australia) allows for close consideration of this issue.

Here we examine one bed containing more than 400 preserved fossils, including more than 200 Dickinsonia. Nearly 70 specimens of Dickinsonia show strong evidence for current-mediated morphology and orientation, resulting from the lifting of a portion of the Dickinsonia from the seafloor. These specimens, though clearly not transported, have an appearance ranging from a “pacman” morphology to specimens where up to half of the Dickinsonia has been lifted off the organically-bound substrate by current-driven sands. These specimens exhibit a non-random orientation, distributed over an approximately 180 degree range. On this bed, the holdfast pullout structure “mop” as well as the current-mediated microbial mat structure “weave” are characterized by a very strong current alignment that is not consistent with that of the lifted Dickinsonia. Furthermore, Dickinsonia occurs on top of “weave” structures. We interpret these data to suggest that the ‘first-generation’ community consisted of frondose organisms, which were plucked out of the microbially-bound seafloor by a storm event, leaving mop structures, along with current-oriented mat deformation textures such as weave. After this event, the seafloor was colonized by a ‘second-generation’ community, composed of Dickinsonia and other taxa, that was eventually smothered by sands.