Spatial distributions of Tribrachidium, Rugoconites, and Obamus from the Ediacara Member (Rawnsley Quartzite), South Australia

The spatial distribution of in situ sessile organisms, including those
from the fossil record, provides information about life histories, such as
possible dispersal and/or settlement mechanisms, and how taxa are
interacting with each other and their local environments. At Nilpena
Ediacara National Park (NENP), South Australia, the exquisite preservation
and excavation of 33 fossiliferous bedding planes from the Ediacara Member
of the Rawnsley Quartzite reveals in situ communities of the Ediacara
Biota. Here, the spatial distributions of three relatively common taxa,
Tribrachidium, Rugoconites, and Obamus, occurring on excavated surfaces
were analyzed using spatial point pattern analysis. Tribrachidium have a
variable spatial distribution, implying that settlement or post-settlement
conditions/preferences had an effect on populations. Rugoconites display
aggregation, possibly related to their reproductive methods in combination
with settlement location availability at the time of dispersal and/or
settlement. Additionally, post-settlement environmental controls could be
affecting Rugoconites on other surfaces, resulting in lower populations
and densities. Both Tribrachidium and Rugoconites also commonly occur as
individuals on a number of beds constraining possible reproductive
strategies and environmental/substrate preferences. The distribution of
Obamus is consistent with selective settlement, aggregating near
conspecifics and on substrates of mature microbial mat. This dispersal
process is the first example of substrate selective dispersal amongst the
Ediacara Biota, thus making Obamus similar to numerous modern sessile
invertebrates with similar dispersal and settlement strategies.