Organisms grouped together to access food

A lifeform that lived half a billion years ago was one of the first to benefit from communal living, modelling reveals.

The organism, known as Ernietta plateauensis, inhabited shallow marine waters towards the end of the Ediacaran period, around 540 million years ago.

The period, named after fossils found in the Ediacara Hills of South Australia, marks the first big explosion of complex, multicellular lifeforms on Earth.

Many of the soft-bodied Ediacaran biota that have survived in the fossil record have no known modern counterparts. This has presented a challenge for scientists trying to understand how the organisms lived and behaved.

Ernietta is one such organism. Individuals resemble up-turned bells semi-submerged in the seafloor sediment. But how they lived and fed has been a mystery.

Now, Brandt Gibson from Vanderbilt University in Nashville, Tennessee, US, and colleagues, think they’ve figured out some of the details of Ernietta’s ancient ecology using computational models 

First, they built a model Ernietta – based on three complete fossils from the Witputs sub-basin in Namibia – and submerged it in a virtual tank.  

Next, they used a computational fluid dynamics model to work out how water – and nutrients – circulate in and around the upturned bell-shaped cavity under different conditions.

“No matter what velocity [the water flow] was, what orientation we put the organism, we were always seeing this consistent re-circulation within the cavity,” says Gibson. 

This fits with Ernietta being a suspension feeder, plucking nutrients and organic particles from water whooshing past some sort of specialised feeding structures that haven’t been preserved in the fossil record. 

Ernietta fossils are frequently found in groups, says Gibson, so the team was curious to know how this affected feeding, according to the model.

“If we start stacking multiple of these organisms together, does our feeding interpretation break down?” he asks. “Or does it still work?”

It turns out that clumping together improves each organism’s chances of getting a good feed. Downstream organisms enjoyed greater flushing of water into their cups, as well as greater turbulence in the water above them. This would have diluted waste from upstream neighbours, as well as delivering more fresh nutrients.

“This is one of the oldest examples of commensalism, or that kind of helping-thy-neighbour interaction that we see in the fossil record,” says Gibson. “We didn’t expect to see that kind of behaviour.”

The team also ruled out an alternative feeding strategy. Ernietta’s volume was too large compared to its surface area to absorb enough nutrients from the water simply by osmosis.

Mussels, oysters and other marine creatures stuck in their spot also benefit from being gregarious suspension feeders. Now it seems that those benefits have their origins all the way back in the Ediacaran.

“While these organisms look really weird, they’re actually following similar rules and behavioural patterns that we see with organisms that we do understand better,” Gibson notes.