Computer models reveal a richer complexity to Earth's first ecosystems

The complexity of life in the Ediacaran period (635 million to 541 million years ago) has been redefined by a new study.

SImonIn the Science Advances study researchers used computer simulations to reveal that the feeding habits of a 555-million-year-old organism called Tribrachidium were not as simple as previously thought. We spoke with one of the authors, Simon A. F. Darroch, Vanderbilt University about the new discovery.

ResearchGate: What was Earth like 555 million years ago?

Simon Darroch: In short, very different to today. The atmosphere had much lower concentrations of oxygen, and there was no complex life on land, the only large eukaryotic organisms in existence were living in the oceans. In addition, much of the ocean floor (especially the shallow ocean) would have been colonized by thick microbial mats, which would have made them very unpleasant to wade about in. In the modern day these mats are grazed by animals such as mollusks, arthropods, and other invertebrates, but back then these groups hadn't diversified yet, so bacteria were able to grow in thick mats just about everywhere. As a result, there wouldn't have been much oxygen below the sediment-water interface. If you were to grab a handful of sand underneath the microbial mat, it would be dark in color, be full of organic matter, and would smell powerfully of sulfur (also betraying a lack of oxygen). Although nothing much was able to live below the surface of the sediment, Ediacaran organisms were very happy living on the top of the mats, and you might come across vast communities of Ediacaran organisms all living at the sediment-water interface.

RG: Can you explain a day in the life of Tribrachidium, a 555 million-year-old organism which lived in the oceans?

SD: I can try! However, Tribrachidium is a very mysterious organism, so it would be foolish to claim that we know very much about it, even after our recent research. Tribrachidium is always found preserved on the sediment surface, without any evidence for movement or burrowing, so in all likelihood, it's day-to-day existence would not seem very exciting to us. Like a sponge or a coral, it would have just sat on the seafloor. Research by Christine Hall at UC Riverside suggests that Tribrachidium liked a wide variety of shallow marine environments influenced by moving currents. It doesn't seem to have particularly specialized habitat preferences, but it's almost always preserved with sedimentary evidence for moving water. So a day in the life of Tribrachidium would have been very relaxing from our point of view, chilling out in warm, shallow coastal waters while ocean currents sloshed around and above it.

Fossil of the Extinct Organism Tribrachidium. Image courtesy of M. Laflamme
Fossil of the Extinct Organism Tribrachidium. Image courtesy of M. Laflamme

RG: How did Tribrachidium feed?

SD: Central to this question is recognizing that Tribrachidium had a bizarre body plan, unlike anything seen today (or indeed, in the last 550 million years). Tribrachidium was broadly hemispherical in shape, like half a squash ball, with three raised ridges ('arms') that spiral out from the center to the edge of the organism. Our research simulating water flow around this organism shows that this bizarre body architecture slows water flow, recirculates flow around the back (i.e., downstream) of the organism, and then funnels it up along the arms towards the apex. Here, there are three depressions ('apical pits') where water flow enters, and then forms small, low-velocity vortices, before escaping vertically upwards. Our simulations suggest that this most likely represents a very unusual version of suspension feeding, whereby water velocity is slowed sufficiently in the 'apical pits' for organic particles to fall out of the suspension, and onto a collecting apparatus. This method of suspension feeding ('gravity settling') is not common in modern animals, but still exists in odd places. If our model is correct, then this would make Tribrachidium the oldest macroscopic ("large") suspension feeder we've yet identified in the fossil record.

RG: What does this mean for our understanding of Earth’s ancient ecosystems?

SD: Prior to this work, we'd assumed the Ediacaran ecosystems were relatively simple, characterized by only a few methods for feeding and nutrient acquisition. In a sense, what makes ecosystems 'complex' in the Phanerozoic (i.e., the last 540 million years) is the diversity of feeding modes that appeared after the end of the Ediacaran - predation, filter feeding, suspension feeding, substrate mining etc. A larger diversity of feeding modes builds a larger trophic pyramid, alters the immediate environment ('ecosystem engineering'), and creates new ecological niches that evolution and adaptation can exploit. Many paleontologists (including us) have assumed that the majority of Ediacaran organisms were engaged in wholly passive methods of feeding ('osmotrophy'), and thus that the organisms themselves built simple ecosystems, and had very little effect on their environment. However, the discovery that Tribrachidium was likely a suspension feeder shows us that these ecosystems may actually have been surprisingly complex, and that there may be more complex feeding behaviors represented among Ediacaran organisms that we just don't recognize, because their body plans are so bizarre.

RG: How did you know what the life and feeding patterns of Tribrachidium looked like and how did you come up with a model to simulate this?

SD: Many marine species in the present day have evolved unusual body plans in order to manipulate water flow, in order to aid in feeding. So…to gain some insights into how Tribrachidium probably fed, we used a method called Computational Fluid Dynamics, which allows us to simulate and visualize water flow around a 3D digital model of the organism.

Computer Simulation of Water Flow around a 3-D Model of Tribrachidium. Image courtesy of I.A. Rahman
Computer Simulation of Water Flow around a 3-D Model of Tribrachidium. Image courtesy of I.A. Rahman

RG: What is the future of computational modelling in your field of study?

SD: I think there is huge potential for these sorts of methods to help us understand the paleobiology of long-extinct organisms. Because these fossils look bizarre, and have no modern relatives that are alive today, it can be virtually impossible to work out how these organisms lived because we have nothing to compare them to. However, methods like Computational Fluid Dynamics allow us to objectively test hypotheses surrounding the ecology of these organisms, without having to make assumptions about their evolutionary relationships. This is one of the first studies to perform CFD on fossil organisms, but I wouldn't be surprised if many other groups were using these techniques in the next few years...

RG: What’s next in the study of Ediacaran organisms?

SD: The biology and behavior of Ediacaran organisms is only just beginning to be understood. Although we think we've made a big step forward with this work (and we're very excited about the results!), this will not be the last word on Tribrachidium, and future work will continue to test the conclusions we've reached here. It'll be fascinating to look at other Ediacaran fossils, and see how many other feeding modes and unusual life strategies are hidden there. It should be an exciting time for anyone interested in evolutionary biology!

Featured image courtesy of SQFP info.

Study:

'Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems' by Imran A. Rahman, Simon A. F. Darroch, Rachel A. Racicot and Marc Laflamme in Science Advances