Tribrachidium – three arms and a lot of mystery

If you were to have been wading out into the Ediacaran seas, waves lapping against your legs, the slime of the microbial mats squidging under your feet, you might have noticed groups of a unique little organism called Tribrachidium. About the size of a coin, Tribrachidium had three ‘arms’ spiralling gently away from its centre, resembling a Celtic pattern. It rested on the sea floor in a variety of settings, mostly shallow with wave action, not moving, just letting the waves wash over it. It’s one of those organisms which is difficult to classify, but it has been possible to make inferences about its ecology as a recent study has done.

The Ediacaran period has been perceived as being ecologically quite simple, followed by the Cambrian explosion which saw a sharp rise in the number of ecological habits of animals. Bush, Bambach, and Daley (2007) developed a classification of life modes based on the area of marine ecospace inhabited relative to the seafloor, the level of movement of the organism, and feeding mechanisms. Theoretically, there are 216 distinct combinations, 92 of which are found in the Phanerozoic, 98 are possibly not viable, and 26 are possible but have not evolved. When applied to the Ediacaran  (Bush, Bambach, and Erwin 2011), there are just two modes of life in the older Avalon assemblage which is dominated by frond-like organisms, there are ten in the younger White Sea assemblage, and five in the younger Nama assemblage which sees the first skeletal forms. Contrasted with this, the first half of the Cambrian saw a rise to around 30 different modes of life (still only a third of modern life-modes).

A new study by Rahman et al. (2015) looked at the possible feeding habits of Tribrachidium and suggested that it had a mode of life previously unknown from the Ediacaran, indicating that Ediacaran ecosystems were more complex than thought (add this to the optimistic approach to the Ediacaran period). They narrowed down the possible feeding habits of Tribrachidium to just two: osmotrophy and suspension feeding. Osmotrophy involves the passive absorption of organic matter dissolved in the water, a common approach in the Ediacaran due to increased amounts of organic matter in the water, found in organisms such as Charnia and Charniodiscus which had large surface areas to absorb nutrients. Suspension feeding involves the trapping of organic matter in specific parts of the organism and requires a method of directing the water towards those traps.

With the two possible modes of feeding in mind, the researchers used computational fluid dynamics (CFD) to observe how water would have flowed over the organism, a method commonly used in engineering. Osmotrophy requires that the water flow over as much of the organism as possible, as has been observed in some of the frondose Ediacarans. By contrast, suspension feeding requires that the flow would be directed and focussed. What their tests found was that the water was directed passively by the arms, funneling it towards three depressions called ‘apical pits’ where it slowed down so that food particles would fall out of suspension. This directed movement fits neatly with a rare ‘gravity settling’ mode of suspension feeding, rather than with osmotrophy. They explained it as follows:

In summary, our CFD analyses demonstrate that the external surface morphology of Tribrachidium altered ambient water flow to produce low-velocity circulation above extremely localized areas around the organism, which is consistent with the interpretation of Tribrachidium as a suspension feeder rather than as an osmotroph. Specifically, we find that the three primary branches act to slow water flow and direct it up toward the apex of the organism, where small-scale recirculation develops directly above apical pits. This recirculation occurs at a range of simulated current velocities regardless of the organism’s orientation to the principal direction of flow. We suggest that this low-velocity zone of recirculation allowed larger particles to fall out of suspension, whereupon they were collected in the apical pits and subsequently metabolized (suspension feeding via “gravitational settling”). This hypothesis suggests that Ediacaran organisms used a larger diversity of feeding strategies than is currently appreciated and that they may have played a role as rudimentary ecosystem engineers, albeit in a fashion that became rare in the Phanerozoic with the disappearance of microbial matgrounds.

Computer simulation of water flow around Tribrachidium.

If it is indeed accurate, this study sheds light on the nature of the taxonomically tricky Tribrachidium, whilst expanding our understanding of Ediacaran ecosystems as more complex than previously thought. However, it is worth noting that fossils of Tribrachidium found so far show no signs of a mouth or other feature for ingesting filtered particles, it may simply be part of a larger organism – the holdfast of a frond-like organism, for example.

References

Click here for an interview with one of the authors.

Bush, AM., Bambach, RK., and Daley, GM. 2007. Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Palaeozoic and Late Cenozoic. Paleobiology 33:76-97.

Bush, AM., Bambach, RK., and Erwin, DH. 2011. Ecospace utilization during the Ediacaran radiation and the Cambrian eco-explosion. In Quantifying the Evolution of Early Life, edited by Laflamme, M., Schiffbauer, JD., and Dornbos, SQ, 111-33. Dordecht, Netherlands: Springer.

Rahman, IA., Darroch, SAF., Racicot, RA., and Laflamme, M. 2015. Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems. Science Advances 1, 10.

Singer, A., Plotnik, R., and Laflamme, M. 2012. Experimental fluid dynamics of an Ediacaran frond. Palaeontologica Electronica 15:1-14.

 

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Filed under Cambrian, Cambrian Explosion, Ediacarans, Palaeontology

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