Paleontologists have examined how Tribrachidium, Rugoconites, and Obamus — three relatively common members of the Ediacaran biota (550 million years ago) — were geographically distributed.
Life restoration of Obamus coronatus, a tiny, toroidally shaped, soft-bodied animal from the Ediacaran biota of South Australia. Image credit: Nobu Tamura.
“The Ediacaran seas were a largely foreign place compared to today’s marine environments,” said University of California, Riverside graduate student Phillip Boan and his colleagues.
“They were dominated by a mat on the sea floor composed of bacteria and layers of other organic materials. In addition, predatory creatures were uncommon.”
For their study, the authors selected three animals — Tribrachidium, Rugoconites, and Obamus — found in relatively large numbers at Nilpena Ediacara National Park, South Australia, and examined how they were geographically distributed.
Given the alien nature of Ediacaran Earth, they were surprised to find an animal, Obamus, that lived much the way barnacles do today.
First discovered in 2018 and named for the former U.S. president, this creature lived on specific parts of the sea floor in the company of other Obamus.
“The animal averaged about a half-inch in diameter and was shaped like a French cruller donut with ribbons on top,” Boan said.
“It did not move of its own accord, and likely spent its entire life embedded in its preferred spot on the sea floor.”
“We think about the very oldest animals and maybe you wouldn’t expect them to be so picky,” said University of California, Riverside’s Professor Mary Droser.
“But Obamus only occurs where there is a thick mat, and it’s a pretty sophisticated way of making a living for something so very old.”
Obamus coronatus. Image credit: University of California, Riverside.
The other two animals, Tribrachidium and Rugoconites, are also immobile creatures with no modern descendants.
“They are tri-radially symmetrical, like the Mercedes Benz logo. And they would have lived their entire lives embedded in the sea floor, as Obamus did,” Boan said.
“Distribution for these other two animals was varied. Sometimes they could be found living in the company of other organisms like themselves, but not in every instance. However, Obamus displayed a clear preference.”
“This is really the first example of a habitat-selective Ediacaran creature, the first example of a macroscopic animal doing this. But how did they get where they wanted to go? This is a question we don’t yet know the answer to.”
The researchers theorize that Obamus were likely motivated by the need to reproduce.
“There are a limited number of reproductive strategies, especially for animals like these,” Professor Droser said.
“There are more strategies today, and they’re more elaborate now. But the same ones used today were still being used 550 million years ago.”
Obamus likely spread itself via selective larva that preferred locations with thick microbial mat and near other Obamus.
“We don’t entirely understand how Obamus offspring spread out, but we know that when they picked a place to live, it was very specific,” Boan said.
The findings were published in the journal Paleobiology.
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Phillip C. Boan et al. Spatial distributions of Tribrachidium, Rugoconites, and Obamus from the Ediacara Member (Rawnsley Quartzite), South Australia. Paleobiology, published online March 13, 2023; doi: 10.1017/pab.2023.9
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