If you were to travel back 550 million years ago, you would be forgiven for thinking that you’d accidentally arrived on an alien planet. The Earth’s ecosystem, entirely confined to the ocean, would have been populated by primitive and bizarre organisms. One such sea monster is the Tribrachidium, a three-armed sea creature that is unlike any living thing found today.
Tribrachidium lived during the late Ediacaran period, over 100 million years before the first plants would appear on land. The late Ediacaran immediately preceded the Cambrian Explosion, which ushered in unprecedented ecological diversity and complexity. An early evolutionary experiment, Tribrachidium‘s body consisted of a flat disc from which three arms extend upward. This form is unique in that it demonstrates three-fold symmetry. Nearly all vertebrates, including humans, and many invertebrates demonstrate two-fold symmetry. While there are some creatures like the starfish that maintain five-fold symmetry, there are no living creatures today that are three-fold.
Analyzing the unique form of Tribrachidium was a challenge for researchers. “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,” says Imran Rahman, research fellow at University of Bristol and lead researcher on a recent study of Tribrachidium. The research team created a digital model of Tribrachidium using a fossil mold. This model was then inserted into simulations which recreated various current flows that Tribrachidium may have encountered in the shallow seas of the Ediacaran.
From these results, Rahman and his peers concluded Tribrachidium was likely a suspension feeder, sustaining itself on dissolved organic matter as it floated through the water. Although there is no evidence that the creature could move, it may have been able to affect its environment. Suspension feeding “mobilizes organic material that was being carried around in the water column,” says Rahman. “It can increase passage of sunlight through water and potentially increase oxygenation, as well.”
Images via M. Laflamme and