Bizarre Ancient Sea Creature Was Well-Armed for Feeding
Tribrachidium, a bizarre sea creature that lived some 550 million years ago, is unlike any modern organism. New research suggests it fed on particles suspended in the water.
Credit: M. Laflamme

A bizarre creature that looks like nothing alive on Earth today probably used its unique shape to collect drifting particles from the ocean to feed on, new research finds.

Tribrachidium was a denizen of the shallow seas about 550 million years ago, during the late Ediacaran period. It looked like a disc with three tentaclelike arms protruding from its flat top. Oddly, Tribrachidium had three-fold symmetry, meaning three segments  were mirror images of each other. For comparison, humans have two-fold, or bilateral, symmetry, and starfish have five-fold symmetry. Nothing alive today has three-fold symmetry.

"Because we have no obvious modern comparison, that's made it really hard to work out what this organism was like when it was alive — how it moved, if it moved, how it fed, how it reproduced," said Imran Rahman, a research fellow at the University of Bristol, in the United Kingdom, who led the study.

Now, Rahman and his colleagues have used fluid dynamics to show that Tribrachidium probably was a suspension feeder, meaning it ate floating organic particles out of the water. Modern suspension feeders include brittle stars, many crustaceans and bivalves.  

Tribrachidium lived about 40 million years before the Cambrian explosion, when life on Earth expanded and diversified relatively rapidly. Scientists once thought Ediacaran organisms were very simple, Rahman told Live Science, but the new findings paint a more complex picture of this time period. It's possible that Tribrachidium even altered its environment. [See Images of the Wacky Creatures of the Cambrian Period]

Suspension feeding "mobilizes organic material that was being carried around in the water column," Rahman said. "It can increase passage of sunlight through water and potentially increase oxygenation, as well."

A computer simulation of <em>Tribrachidium</em> shows the flow of water over the top of a 3D virtual reconstruction of the organism.
A computer simulation of Tribrachidium shows the flow of water over the top of a 3D virtual reconstruction of the organism.
Credit: Imran Rahman

There's no evidence that Tribrachidium could move around, so researchers thought perhaps it fed by osmotrophy, or absorbing dissolved nutrients from the water. Alternatively, it could have captured and digested larger particles by suspension feeding.

To demystify Tribrachidium's feeding habits, Rahman and his colleagues created a 3D digital model of the organism based on a cast of a fossil from south Australia. (Tribrachidium fossils have also been found in Russia and Ukraine.) They then subjected this digital model to virtual currents mimicking what would have existed in its shallow seafloor environment.

The currents slowed as they hit Tribrachidium, and then eddied in the organism's wake. These eddies served to recirculate the water back toward Tribrachidium, directing it into the nooks between its three symmetrical arms. It's very likely that gravity then settled out any waterborne particles into these crevices, allowing Tribrachidium to snag the particles and chow down.

"This is really exciting, because we didn't really have any good evidence of suspension feeding in organisms of this time period previously," Rahman said.

Other Ediacaran creatures remain mysterious, with equally weird body designs. Some, like Tribrachidium, are disc-shaped, Rahman said. Others look like fronds. He said he'd like to use similar models of fluid dynamics to figure out how those creatures might have fed.

"This approach has been really valuable for us to try to understand these highly mysterious and enigmatic organisms," Rahman said.

The research is detailed in the Nov. 27 issue of the journal Science Advances.

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