What drove the evolution of the earliest animal life? In modern animals, it's easy to infer a lot about an organism's lifestyle based on its anatomy. Even back in the Cambrian, with its large collection of bizarre looking creatures, these inferences are possible. Anomalocaris may have had a freakish, disk-shaped mouth, but it clearly was a mouth.

Go back to Earth's earliest animals in the Ediacaran, however, and things get much, much harder. There's only one species known so far that appears to have the right body plan to act as a predator of sorts. Beyond that, it's all a collection of soft-looking fronds and segments that are difficult to ascribe any obvious function to. Faced with a lot of questions without obvious answers, biologists turned to an unlikely source of help: physicists and engineers who understand fluid mechanics.

All of these creatures lived in an aquatic environment, so tracing how fluid flows across them can provide some hints as to how food might have arrived. Now, the same sort of research indicates that a strange cup-shaped species grew in communities because it improved the feeding of some of the community members.

Meet Ernietta

The Ediacaran period lasted almost a 100 million years, a period that included at least two snowball Earth periods. Despite the sometimes harsh conditions, it also contains the first indications of complex animal life, which appeared right after one of the global glaciations ended. Many of the creatures took forms that look almost plant-like; others are simple disks, sacs, or tubes. Few show any clear relationship to the animal life that appeared after it in the Cambrian, so figuring out their place on the tree of life has been a challenge.

Yet many of the fossil finds show clear evidence of entire ecosystems and indicate that different organisms flourished at different ocean depths.

Ernietta plateauensis, the species that's the subject of the new study, fits in well with its odd neighbors. The Wikipedia entry on the organism describes it as "bag-shaped," which is fairly accurate. Its body consists of a series of parallel tubes that form the wall of the bag. They're joined at a specialized structure at the bottom of the creature. Aside from an obvious inside and outside of the bag, there's not much in the way of an obvious internal structure.

Dave Mazierski

Dave Mazierski

Charlotte Kenchington

Charlotte Kenchington

One of the few things we can tell about Ernietta is that it was sedentary. Many of the fossils we've found of it contain multiple deposits of sediment layered on the bottom of the bag. This suggests it sat in the same place long enough to accumulate sediment from a number of separate events. The other thing we know is that they're gregarious, as large groups of them are found growing together.

Like many other Ediacaran organisms, there's no obvious mouth or anything apparent going on inside the organism that would indicate a mouth had somewhere to deliver food to. This raises an obvious question: how does it get enough food to survive?

We can rule out photosynthesis (either its own or that of a symbiotic algae), since the organism has been found among deposits formed in waters that are too deep for much light to reach it. Predatory behavior is extremely unlikely. Only one other Ediacaran species shows evidence of it, few organisms even had the ability to move about, and Ernietta was sedentary, so it wasn't going to move to where its prey was. The lack of internal structure also seems to rule out options like filter feeding.

That, in the view of the researchers behind the new paper, left two options. Ernietta could simply absorb the nutrients that it needed directly from materials dissolved in the water. Or, the second option is that it could somehow gather larger particles of food as they floated by and somehow move these to a location where they could be ingested, an approach called suspension feeding. But how can we tell these two options apart?

A model opportunity

This is where the fluid mechanics were brought in. The two different lifestyles only work with distinctive body forms. To pull enough nutrients directly out of the water, organisms that feed on dissolved material have to maximize their surface area relative to their volume. By contrast, suspension feeders have to have some way of directing particles to areas where they can be processed. With no apparent moving parts, Ernietta would have to rely on its body shape to do that.

A computer analysis of the reconstructed bodies of Ernietta suggested feeding on dissolved materials was out. Its bag-like shape and the extensive contacts between the neighboring tubes limit the amount of surface area available to extract material from the ocean water. In addition, the interior of the bag could potentially lead to an area of water that's somewhat nutrient depleted.

That latter issue, however, turned out not to be much of a problem, as the fluid mechanics simulations would show. As water flowed over the bag-like shape, it set off a rotating current within the bag, downward on the downstream side, and upward again on the side closer to the current source. This had the effect of circulating water through the interior of the structure and providing a constant supply of nutrients as they drifted by on the current.

While this would help with both forms of feeding, the lack of surface area remained an issue. Thus, the researchers conclude that Ernietta was a suspension feeder, and its internal surface was specialized to extract particles that local currents washed inside.

But the fluid mechanics weren't done yet. As noted above, fossil finds indicated that Ernietta grew in large communities. So the researchers also performed a number of simulations with groups of Erniettas spread across the floor. These showed that the flow of current across one didn't only create a downward flow within that organism; the same flow extended well downstream of that. Thus, in a large group of these organisms, any that are located downstream of the leading edge of the colony would receive additional nutrients pushed their way by the downward flow.

While the organisms at the leading edge might not benefit from this, they would if the current regularly changed directions or if a thriving colony expanded so that they'd end up in the interior eventually. Thus, the researchers claim that the Ernietta colonies represent the earliest known case of commensal feeding behavior.

Overall, Ernietta appeared fairly late in the Ediacaran, meaning there were tens of millions of years for animals to evolve more sophisticated feeding systems. Still, these innovations seemingly came to an end as the Ediacaran terminated in yet another global glaciation. By the time the planet warmed again, organisms like Ernietta had been replaced by the organisms of the Cambrian.

Science Advances, 2019. DOI: 10.1126/sciadv.aaw0260 (About DOIs).