SUSPENSION FEEDING IN THE GREGARIOUS EDIACARAN ORGANISM ERNIETTA PLATEAUENSIS

The latest Neoproterozoic Ediacara biota (571–541 Ma) is an enigmatic group of fossils that represents the first radiation of complex life. However, the paleobiology and -ecology of many of these organisms is poorly understood, with little idea of how they moved, fed, and reproduced. Here, we use a virtual modeling technique – computational fluid dynamics (CFD) – to test between osmotrophy and suspension feeding as hypothesized feeding modes for the semi-infaunal and sack-like organism Ernietta plateauensis. CFD is used to visualize flow vectors around virtual models, and so can help to identify areas of recirculation and other hydrodynamic attributes that organisms frequently take advantage of in feeding. Single model simulations were carried out at at three different inlet velocities (0.1, 0.2, and 0.5 m/s) and at three different burial depths. Multi-model simulations were performed in four separate model arrangements to assess whether flow patterns are significantly altered in arrangements that approximate actual communities. All simulations show the development of small-scale recirculation within the central cavity of the organisms, suggesting either a gravity-settling mode of suspension feeding, or alternatively an adaptation for maximizing the efficiency of nutrient particle capture. Simulations also reveal recirculation vortices forming directly down-current from the model where they direct flow towards other individuals, potentially providing an advantage to living in groups. Osmotrophy is rejected on the basis that calculated SA:V ratios fall far below those estimated for extant organisms that feed in this way. We combine the results of these analyses with observations made in the field, which illustrate that Ernietta lived gregariously, forming communities that achieved small-scale relief above the sediment-water interface. Accumulations of Ernietta may therefore represent Ediacaran soft-bodied bioherms, and possibly the earliest bioherms formed by macroscopic eukaryotes yet known from the fossil record.