The Table Manners Of Tribrachidium

The Ediacara biota were a highly unusual group of organisms. Not a strict biological group in the first place, and soft-bodied, they are difficult to study as fossil evidence is limited. Some species, such as Kimberella, possess reasonable anatomical links to modern day groups of organisms, making it easier to infer their lifestyles. Others have weird and wonderful body plans which obfuscate both their place in the tree of life and their day to day existence. How the Ediacara biota fed is particularly enigmatic. Kimberella displays feeding traces while Charnia had a large surface body area ideal for filter feeding.

Fossils and 3D computer models of Tribrachidium

Tribrachidium on the other hand has a body structure whose benefits are not immediately apparent. Fossils of this species preserve little in the way of how it may have lived. It is likely, however, that it was sessile, lying motionless on the sea bed. It could not forage for food so it must have relied on some other method. Its body plan offers few clues.

Tribrachidium was small and possessed a highly unusual triradial symmetry characteristic of a group of Ediacarans known as trilobozoans. This form of symmetry is seen nowhere else in the animal kingdom, both living and extinct. It is a failed experiment in body building.

A recent study by an international team of researchers, led by Dr Imran Rahman from the University of Bristol has shown that Tribrachidium's unusual design was well suited to a mode of nutrition known as suspension feeding - previously undocumented in the Ediacara biota. Suspension feeding is the capture of suspended particles from a water column which are too light to settle. This was achieved using computational fluid dynamics. More commonly encountered in the world of engineering, this mode of analysis is used to model the flows of water or air around and within pieces of machinery. This study, represents one of the first applications of the technique in palaeontology, following previous research also carried out by researchers from Bristol University.

The computational flow dynamic models of Tribrachidium. The grey
arrows represent flow direction. The double height set show the
 recirculation of currents in the eddy behind the organism

CT scans were used to build 3D computer models of Tribrachidium. They were subjected to different flow conditions to match the environments in which the creature lived. To account for the varying degrees of compression which the fossils would have been subjected to, the models were altered to include a range of heights. The results showed that an elongated, low velocity flow region formed downstream of the organism. This is similar to the way a rock in the middle of a river creates an eddy on its downstream side.

In just the same fashion, the low velocity region behind Tribrachidium developed currents which recirculated back towards the body, and chanelled by the structure of the body, towards pits at the apex of the organism. Here nutritious particles in the water could be absorbed.

'For many years, scientists have assumed that Earth's oldest complex organisms, which lived over half a billion years ago, fed in only one or two different ways,' said Dr Simon Darroch, from Vanderbilt University, Tennessee. 'Our study has shown this to be untrue, Tribrachidium and perhaps other species were capable of suspension feeding. This demonstrates that, contrary to our expectations, some of the first ecosystems were actually quite complex.'

The Ediacara biota were responsible for creating the first dynamic ecosystems on the planet. Yet this is a relative term. The Ediacaran ecosystems have been viewed as simple compared to those of the Cambrian and later periods. However, this study shows the gulf between them is not as vast as previously thought.