Traces of the last suppers of some of the world's earliest known animals have been discovered in their 558-million-year-old fossils.
In the moments before the animals were entombed forever, they grazed on green algal slime on the shallow sea floor, according to an analysis of different types of molecules known as sterols, which were preserved in the fossils.
The finding, 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 around 600 million years ago.
"This is the first direct evidence we have for the diet of Ediacaran animals," said palaeontologist Ilya Bobrovskiy, who led the study.
Most scientists had assumed these animals dined on cyanobacterial mats, said study co-author Jochen Brocks of the Australian National University.
"But we can see now these were actually already eating algal mats," Professor Brocks said.
"It's probably the difference between eating a raisin and eating a watermelon."
Their nutritious diet may have helped these early animals become very large, very quickly.
And while some animals absorbed nutrients through their skin, the analysis also suggested others were more advanced and digested food in much the same way as we do.
Failed experiments, weirdos and modern animals
The shallow seas of the Ediacaran Period were filled with a rag-tag bunch of large, soft-bodied organisms.
"It's a mixture of different creatures at the very early stage of evolution, with failed experiments and weirdos, but amongst them the modern animals that would then make it into the Cambrian Explosion," Professor Brocks said.
What we know about this relatively idyllic time before the rise of predatory animals with claws, shells and spikes in the Cambrian Period is imprinted in rocks such as those in South Australia, where they were first discovered.
"These fossils are usually poorly preserved. They're like a death mask in metamorphosed, very rough sandstones," Professor Brocks said.
"All we have is casts of the surface, we don't know what they look like from the underside or inside."
As a result, scientists have long debated whether these creatures were animals or strange types of plants or algae.
Then in 2013, Dr Bobrovskiy from GFZ-Potsdam in Germany discovered exquisitely preserved fossils in the remote cliffs of the White Sea in Russia.
The fossils included round-shaped creatures known as Dickinsonia that settled on slime, worm-like Calyptrina and Kimberella, a primordial organism that resembled a slug with a snout.
Analysis of the Dickinsonia fossils by Dr Bobrovskiy at Dr Brock's lab revealed they contained cholesterol found in animals.
The team then decided they wanted to find out if any of these fossilised animals had guts.
It was largely thought that Dickinsonia and Calyptrina were passive feeders that absorbed nutrients through their skin, similar to tiny, transparent placozoa and tube worms that live in our oceans today.
But Kimberella was thought to be more advanced and have a gut, based on pairs of scratch marks and what could potentially be balls of poo seen around fossils.
"The only explanation for that was it scraping stuff up, pulling it into a mouth and digesting it," Professor Brocks said.
"But if you look at the fossils, even at the 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 gut when they die, so the researchers thought they might be able to work out if Kimberella had a gut by what it had been eating.
Algae contains a particular cocktail of plant sterols and some fatty molecules (cholesteroids such as cholesterol and ergosteroids) found in animals, fungi and mould.
On the other hand, bacteria contain another fat molecule known as hopanol, instead of cholesterol.
The analysis suggested that Kimberella gobbled up mainly algae.
The structure of these molecules also indicated they'd broken down in an anoxic environment like the gut.
"We can see the algal sterols in the Kimberella fossil, but they decayed in the typical anaerobic gut way, very distinct from the decay pattern in the mats," Professor Brocks said.
To their surprise, they could also see that the animal's gut was taking up cholesterol rather than plant sterols.
"Kimberella had absorbed the cholesterols for its own use and only left ergosteroids and plant steroids in the gut," Professor Brocks said.
"That is quite some sophistication — that's like us."
The team's analysis also suggested the worm-like Calyptrina had a gut.
"It was probably feeding off what it was getting at the surface, like maybe microalgae floating by."
But the iconic Dickinsonia was a bottom feeder that absorbed its food as it lived on its algal mat.
"We analysed 17 fossils and not a trace of a gut ... so that almost certainly confirmed Dickinsonia was a pretty ancient weirdo," Professor Brocks said.
Debate about engimatic Ediacarans far from over
Palaeontologist Jim Gehling of the South Australian Museum is a world expert in Ediacaran biota from the red Precambrian sandstone of the Flinders Ranges.
Professor Gehling said the findings that Kimberella actively foraged "fits what we find in the trace fossils" in South Australia.
But the debate about how these organisms functioned is far from over.
"This has been one of the most controversial parts of the fossil record for a long time," Professor Gehling said.
"Every textbook tells you that real animals began at the Cambrian and Ediacarans were a failed experiment."
While he has advocated organisms such as Dickinosonia were animals, he said others were not convinced the Russian fossils could have preserved material for millions of years.
"I'm not an organic geochemist and don't claim to have any wisdom on that, but what I do know is they do have rich, organic material trapped in that sediment [in Russia]," he said.
"It's not been buried by kilometres of rock like the Flinders Ranges material has been."
That means that the geochemical signatures seen in the fossils could be valid.
But, he said, it was still important to query the science and see if the same signatures could be found in more fossils from Russia and different parts of the world such as China and Canada.
"When you think everything you read in the textbooks is right, then you've got a problem because textbooks always will be behind what's happening in active research," Professor Gehling said.
"We are looking for people who are willing to test, and if necessary, we have to abandon ideas and accept better explanations."
