More than half a billion years ago, during the Ediacaran Period, animal life looked nothing like today. Strange cup- and disk-shaped creatures sat and crawled along an ocean floor covered in thick microbial mats made of bacteria and algae. The only clues these organisms left to their lifestyles exist in the fossil record.

Many enigmatic fossils from the Ediacaran belong to the genus Dickinsonia, which were oval-shaped, worm-like organisms that varied in size but could grow up to one meter in length. Scientists hypothesize that Dickinsonia were animals that fed by absorbing microbial food sources through the bottom tissue layer of their bodies as they slithered across the shallows.  

“Imagine if you and your stomach were just one layer, opened up like a pancake,” said Christopher Mulligan, a Ph.D. candidate in the Department of Earth and Planetary Sciences at the UC Davis College of Letters and Science. “They may have just exuded enzymes into the environment and then pulled the digested food up.” 

In a study appearing in Geobiology, Mulligan and Associate Professor of Earth and Planetary Sciences David Gold bolster this hypothesis by reexamining biomarkers found in Dickinsonia fossils. Specifically, they analyzed the biomarker coprostanol, a microbial degradation product of cholesterol. 

Dickinsonia fossils and early animal diets

Today, coprostanol is mainly found in the guts and feces of vertebrates, including mammals and birds, and some reptiles and fish. But Dickinsonia were invertebrates, which in their modern form aren’t typically associated with producing coprostanol. 

Researchers previously hypothesized that the presence of coprostanol in Dickinsonia fossils was the result of the organisms’ decay after death, produced by bacteria eating its body. 

“It’s possible that the bacteria could be causing decay, but that does seem to conflict with the exceptional quality of the fossils,” said Gold.  

Mulligan assembled 115 metagenomes from present-day animals to look for evidence of coprostanol-producing enzymes in invertebrate stomachs. Metagenomes contain all the genetic material of the organisms in a single sample, giving scientists a window into the microbial environment. 

Mulligan searched for a specific gene associated with the production of coprostanol called ismA. He then used computational modeling to test the biosynthesis of the biomarker in modern invertebrate guts. 

“At the end, we were left with this tiny branch from our metagenome tree that was entirely vertebrates,” Mulligan said. “It really reinforced this idea that invertebrates don’t have the capacity to produce coprostanol.”

The depths of diet

Mulligan and Gold said their findings support the idea that Dickinsonia didn’t have an anaerobic, internal gut. That idea introduced a question. If coprostanol was not coming from the decay of Dickinsonia bodies or digestion in their guts, where was the signal coming from? 

This led the researchers to a new hypothesis about how Dickinsonia fed.

Gold said that coprostanol is found in microbial mats, with its presence increasing deeper in the mat where there is less oxygen. It can also be found in Ediacaran mat fossils, and interestingly, its levels increase when associated with Dickinsonia remains. 

“This compound is found at really high levels in organisms that we think are digesting the mat,” Gold said.

Lower levels of coprostanol have been found in Dickinsonia relatives like Andiva and Yorgia, the latter two feeding higher in the mat than Dickinsonia.

"Dickinsonia might eat lower in the mat where you have less oxygen and where this compound would be more abundant,” Gold said. 

These differences led Gold and Mulligan to hypothesize that coprostanol levels indicate the depth at which these organisms fed.    

“Instead of being some signature of the organisms decaying, this is actually telling us differences in how they fed,” Gold said. 

The hypothesis isn’t a stretch. Accumulation of coprostanol is found in modern mussels and fish exposed to wastewater, meaning it’s a biological possibility.  

“This might be the best direct evidence that Dickinsonia was eating the microbial mats,” Mulligan said.  

The research was supported by the National Science Foundation.