Ecology and Biology of Dickinsonia, an Iconic Member of the Ediacara Biota From Nilpena, South Australia
Dickinsonia, an iconic member of the Ediacara biota, is abundant in the Ediacaran deposits found at the Nilpena field site, South Australia. Despite exquisite preservation at this site, many specimens of Dickinsonia appear to be incomplete. Orientation measurements from specimens on fossil beds suggest that “missing pieces” are aligned irrespective of the axial orientation of Dickinsonia. The nonrandom orientation of incomplete specimens matches that of aligned structures found on two beds. The directionality of this feature suggests the molding of incomplete specimens under the influence of current activity prior to or during burial. This feature originates where part of a Dickinsonia was lifted off of the substrate during a storm event and sand was deposited beneath this lifted portion. This suggests that Dickinsonia was easily separated from the sea floor and was not attached to the substrate. This is consistent with the data suggesting that Dickinsonia was mobile. Examination of Dickinsonia costata demonstrates ranges in length from 3.6 mm to 167.4 mm and occurs in varying abundance within and across facies. Density data show that this organism preferred a relatively shallow water, fair-weather wave base habitat. The substrate dominated by fossils of Aspidella and Funisia limited the presence of D. costata. Ubiquitous Funisia in the absence of Aspidella preserves larger populations of D. costata, while juvenile forms occur preferentially in an inferred algal-dominated seafloor. The timing of depositional events controlled the maximum size ranges of D. costata, with small populations representing communities buried before they fully developed. The biology of D. costata is reflected in the overall right-skewed, log-normal size frequency curve, demonstrating a pattern of high infant mortality. A single bed contains more than half of the D. costata at Nilpena and the absence of cohorts within this large population is suggestive of continuous recruitment.