FLUID PALEOBIOMECHANICS OF THE EDIACARAN FROND CHARNIODISCUS

Frond-shaped forms, such as Charniodiscus, dominate the oldest Ediacaran assemblages. In addition to uncertainly over their phylogenetic affinities, their life habits remain a source of continuing debate, with filter feeding and osmotrophy being currently favored. It has also been suggested that other Ediacaran taxa, such as Apsidella, may represent holdfasts from which the upper portion, perhaps the stem and petalodium of a frond, has been torn away by high flows. In order to constrain interpretations of Ediacaran frond life habits, flow tank experiments with model Charniodiscus fronds were used to quantify forces and observe flow patterns associated with alternative morphologies, orientations, and compliances. Surface morphology reconstructions were taken from the literature and models were cast in epoxy, silicon, and agar to test a range of possible compliances. The models were attached to a force transducer and then placed in a recirculating flow tank to record drag forces and visualize flow. Models were tested at 0, 30, 60, and 90 degrees relative to substrate and perpendicular to flow, reflecting the range of orientations most common to modern organisms. Performance was evaluated by calculating the coefficient of drag (C_D) for each model and orientation. The coefficients of drag decreased, as expected, as frond orientation approaches parallel to substrate and for compliant materials as compared to rigid materials. A surprising outcome of the study was that the fronds may have been self exciting oscillators, with flow induced repetitive oscillations. These oscillations are similar to those seen in some kelp species that increase gas exchange in low velocity environs by altering blade morphology to increase turbulent flow. If it is assumed that the drag coefficients are relative constant at higher velocities, constraints can be placed on the forces required to separate the petalodium from the holdfast.