WHAT WAS TRIBRACHIDIUM GOOD AT?

The fossil Tribrachidium heraldicum is a distinctive member of the latest Proterozoic "Ediacara biota", noted for its threefold spiral asymmetry. Although Tribrachidium is known from many specimens from Australia and northern Russia, its evolutionary affinities are still not well understood. It has been suggested that Tribrachidium's spiral asymmetry may have created currents that channelled food particles towards the mouth. Although spiral morphology is common among marine suspension feeders, the hypothesis has yet to be tested in this case.

A promising approach to studying such problematic fossils is testing how well they were adapted for various hypothesized functions. While this approach alone cannot determine the evolutionary relationships of a problematic fossil, it does allow us to constrain them. It also allows us to test known morphologies against morphologies that never existed; we can test whether the observed morphology of an organism is close to a functional optimum, and thus begin to disentangle the effects of functional adaptation and formal constraints on morphology. Using a simple wind tunnel, we have tested the efficiency of models of Tribrachidium at filtering out fine particulate matter under paleoecologically reasonable flow regimes. The results suggest that Tribrachidium may have been well adapted for a passive suspension feeding lifestyle.