Traces of the last dinners of some of the world’s earliest known animals may have been discovered in their fossils from 558 million years ago.
- Ediacaran biota are a strange group of soft-bodied organisms that lived between 600 million and 540 million years ago
- An international team has analyzed organic molecules in fossils from Russia to see if the organisms have guts
- Their analysis suggests that some ancient creature known as Kimberella digested food in a similar way to us
In the moments before the animals were buried forever, they grazed on green algal slime on the shallow seabed, according to an analysis of several types of molecules known as sterols, which were preserved in fossils.
The discovery, published today in the journal Current Biology, sheds new light on an enigmatic group of organisms that first appeared on Earth in the Ediacaran period about 600 million years ago.
Most scientists had assumed that these animals ate on mats of cyanobacteria, said study co-author Jochen Brocks of the Australian National University.
‘But now we can see that they were actually already eating seaweed mats,’ said Professor Brocks.
“It’s probably the difference between eating a raisin and eating a watermelon.”
And while some animals absorbed nutrients through their skin, the analysis also suggested that others were more advanced and digested food in much the same way as us.
Failed experiments, weirdos and modern animals
The shallow seas of the Ediacaran period were filled with a ragtag group of large, soft-bodied organisms.
‘It’s a mixture of different creatures in the very early stage of evolution, with failed experiments and weirdos, but among them modern animals that would later enter the Cambrian explosion,’ said Professor Brocks.
What we know about this relatively idyllic time before the rise of predatory animals with claws, shells and spikes in the Cambrian Period is etched into rocks like those in South Australia, where they were first discovered.
‘These fossils are usually poorly preserved. They are like a death mask in metamorphosed and very rough sandstones,’ said Professor Brocks.
“All we have is casts of the surface, we don’t know what they look like from underneath or inside.”
As a result, scientists have long debated whether these creatures were animals or strange types of plants or algae.
Then, in 2013, paleontologist Ilya Bobrovskiy of GFZ-Potsdam in Germany discovered exquisitely preserved fossils in the remote reefs of Russia’s White Sea.
The fossils included round-shaped creatures known as Dickinsonia that settled on slime, worm-like Calyptrina, and Kimberella, a primeval organism that resembled a slug with a snout.
Dr Bobrovskiy’s analysis of Dickinsonia fossils at Dr Brock’s laboratory revealed that they contained cholesterol found in animals.
The team then decided they wanted to find out if any of these fossilized animals had the guts.
Dickinsonia and Calyptrina were largely thought to be passive feeders that absorb nutrients through their skin, similar to the tiny transparent placozoans and tube worms that live in our oceans today.
But Kimberella was thought to be more advanced and have intestines, based on pairs of scratch marks and what could potentially be poop balls seen around the fossils.
“The only explanation was to scrape stuff off, put it in your mouth and digest it,” said Professor Brocks.
“But if you look at fossils, even organically preserved ones from the White Sea, you don’t see a gut; you don’t see anything.”
An ancient animal with a modern gut
Animals usually have food in their intestines when they die, so the researchers thought they might be able to tell if Kimberella had intestines from what she ate.
Algae contain a particular cocktail of plant sterols and some fatty molecules (cholesteroids such as cholesterol and ergosteroids) found in animals, fungi and moulds.
On the other hand, bacteria contain another fat molecule known as opanol, instead of cholesterol.
The analysis suggested that Kimberella mainly engulfed algae.
The structure of these molecules also indicated that they were broken down in an anoxic environment such as the intestine.
‘We can see algal sterols in the Kimberella fossil, but they decayed in the typical anaerobic intestinal fashion, very different from the decay pattern in mats,’ said Professor Brocks.
To their surprise, they could also see that the animal’s intestines were absorbing cholesterol instead of plant sterols.
“Kimberella had absorbed the cholesterol for her own use and left only ergosteroids and plant steroids in her gut,” said Professor Brocks.
“This is pretty sophisticated – it’s like us.”
The team’s analysis also suggested that the worm-like Calyptrina had a gut.
“It was probably feeding on what was coming to the surface, like maybe microalgae that were floating around.”
But the iconic Dickinsonia was a bottom feeder that absorbed food while living on its algal mat.
‘We looked at 17 fossils and not a trace of an intestine… so we almost certainly confirmed that Dickinsonia was a pretty ancient weirdo,’ said Professor Brocks.
The debate on engimatic Ediacarans is far from over
Paleontologist Jim Gehling of the South Australian Museum is a world expert on Ediacaran biota from the Precambrian red sandstone of the Flinders Ranges.
Dr Gehling said the discoveries that Kimberella has actively funded ‘fit with what we find in trace fossils’ in South Australia.
But the debate on the functioning of these bodies is far from over.
“This has been one of the most controversial parts of the fossil record for a long time,” said Dr. Gehling.
“Every textbook tells you that real animals began in the Cambrian and the Ediacarans were a failed experiment.”
Although he argued that organisms like Dickinosonia were animals, he said others weren’t convinced that the Russian fossils could have preserved the material for millions of years.
“I’m not an organic geochemist and I don’t pretend to have any wisdom on this, but what I do know is that they have rich organic material trapped in that sediment [in Russia],” he said.
“It wasn’t buried under miles of rock like the Flinders Ranges material has been.”
This means that the geochemical signatures seen in the fossils could be valid.
But, he said, it was still important to interrogate the science and see if the same signatures could be found in more fossils from Russia and different parts of the world like China and Canada.
“When you think everything you read in textbooks is right, then you have a problem because textbooks will always underlie what’s happening in active research,” said Dr. Gehling.
“We look for people who are willing to test, and if necessary, we have to abandon ideas and accept better explanations.”