Experimental fluid mechanics of an Ediacaran frond

Ediacaran fronds are iconic members of the soft-bodied Ediacara biota, characterized by disparate morphologies and wide stratigraphic and environmental ranges. As is the case with nearly all Ediacaran forms, views of their phylogenetic position and ecology are equally diverse, with most frond species considered as sharing a similar ecological guild rather than life history. Experimental biomechanics can potentially constrain these interpretations and suggest new approaches to understanding frond life habits. We examined the behavior in flow of two well-know species of Charniodiscus from the Mistaken Point Formation of Newfoundland, Canada (Avalon assemblage): Charniodiscus spinosus and C. procerus. Models reflecting alternative interpretations of surface morphology and structural rigidity were subjected to qualitative and quantitative studies of flow behavior in a recirculating flow tank. At the same velocities and orientations, model C. procerus and C. spinosus experienced similar drag forces; the drag coefficient of C. procerus was smaller, but it is taller and thus experiences higher ambient flow velocities. Reorientation to become parallel to flow dramatically reduces drag in both forms. Models further demonstrated that C. procerus (and to a lesser extent C. spinosus) behaved as self exciting oscillators, which would have increased gas exchange rates at the surface of the fronds and is consistent with an osmotrophic life habit. Amy Singer. Geosciences Department, The University of Montana, 32 Campus Drive #1296, Missoula, MT 59812-1296; amy.singer@umontana.edu Roy Plotnick (correspondence author). Department of Earth and Environmental Sciences, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607; plotnick@uic.edu Marc Laflamme. Department of Paleobiology, Smithsonian Institution, PO Box 37012, MRC 121, Washington, DC 20013-7012; LaflammeM@si.edu