PALEOBIOMECHANICAL INVESTIGATION OF EDIACARAN FROND MORPHOLOGIES

The Ediacaran Biota is an enigmatic group of fossils from the latest Precambrian and consists of the oldest known architecturally complex multicellular fossils. Frond shaped-forms, such as Charniodiscus, exhibit a leaf-like shape with a stem and are attached to the substrate by a holdfast. This basic construction is present in all three multicellular kingdoms, making phylogenetic placement difficult. In addition, detailed reconstruction of frond morphology is hampered by inadequate preservation. Regardless of taxonomic association, organisms depend on flow for nutrients and reproduction, but run the risk of being damaged by high levels of flow. The negative effects of flow can be ameliorated by altering fine-scale flow by varying morphology. Flow tank experiments with model fronds were used to quantify forces and stresses 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 stiffness from rigid to compliant. The models were attached to a force transducer and then placed in a recirculating flow tank to record drag. Models were oriented at 0, 30, 60, and 90 degrees 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 (CD) for each model and orientation. The coefficients of drag decreased, as expected, as frond orientation approaches parallel to substrate. There is significant difference in CD between rigid models and compliant models, though no significant difference between the compliant materials silicon and agar, despite differences in density. The effect of changes in surface morphology was consistent with previous investigations on kelp and found to have no significant effect on drag. These results allow us to constrain interpretations of Ediacaran frond life habits.