Traces of the last dinners of some of the world’s oldest known animals have been discovered in their 558-million-year-old fossils.
Key Points:
- The Ediacaran biota is a strange group of soft-bodied organisms that lived between 600 and 540 million years ago.
- An international team has analyzed organic molecules in fossils from Russia to see if the organisms had guts
- Their analysis suggests that some ancient creatures known as Kimberella digested food in a way similar to ours
In the moments before the animals were buried forever, they grazed on green algae silt on the bottom of the shallow sea, 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 some 600 million years ago.
Most scientists had assumed these animals dined on mats of cyanobacteria, said study co-author Jochen Brocks of the Australian National University.
“But we can see now that they were actually already eating mats of algae,” Professor Brocks said.
“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 the same way we do.
Scientists analyzed well-preserved Dickinsonia fossils discovered in Russia.
Failed experiments, weirdos and modern animals
The shallow seas of the Ediacaran period were filled with lots of large, soft-bodied organisms.
“It’s a mixture of different creatures at a very early stage of evolution, with failed and strange experiments, but among them the modern animals that would later arrive in 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 like those in South Australia, where they were first discovered.
Dickinsonia fossils were first discovered in the Ediacara Hills of the Flinders Ranges in South Australia. (Provided by: Ilya Bobrovskiy)
“These fossils are usually poorly preserved. They are like a death mask in very rough, metamorphosed sandstones,” Professor Brocks said.
“All we have are surface molds, we don’t know what they look like 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 on the remote cliffs of the White Sea in Russia.
Ilya Bobrovskiy (left) with a Dickinsonia fossil excavated from the cliffs of the White Sea in Russia. (Provided by: Ilya Bobrovskiy)
The fossils included round-shaped creatures known as Dickinsonia that settled on slime, the worm-like Calyptrina, and Kimberella, a primordial organism that resembled a slug with a snout.
Analysis of the Dickinsonia fossils by Dr. Bobrovskiy in Dr. Brock’s lab revealed that they contained cholesterol found in animals.
The team then decided they wanted to find out if any of these fossilized animals had guts.
Dickinsonia and Calyptrina were largely thought to be passive feeders that absorbed nutrients through their skin, similar to the small, transparent placozoans and tubeworms that live in our oceans today.
Modern tuber worms do not have guts, but absorb nutrients through their skin from hydrothermal vents. (Flickr: Oregon State University)
But Kimberella was thought to be more advanced and had a gut, based on pairs of scratch marks and what could be poo balls seen around the fossils.
“The only explanation for this was scraping things off, putting them in your mouth and digesting them,” Professor Brocks said.
“But if you look at the fossils, even the organically preserved ones from the White Sea, you don’t see guts; 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 could figure out if Kimberella had a gut because of what she had been eating.
Algae contain a particular cocktail of plant sterols and some fatty molecules (cholesteroids such as cholesterol and ergosteroids) found in animals, fungi and mold.
On the other hand, the bacteria contain another fat molecule known as hopanol, instead of cholesterol.
A Kimberella fossil excavated from the cliffs of the White Sea in Russia. (Provided by: Ilya Bobrovoskiy)
The analysis suggested that Kimberella mainly gobbled up algae.
The structure of these molecules also indicated that they had been broken down in an anoxic environment such as the gut.
“We can see algal sterols in the Kimberella fossil, but they disintegrated in the typical anaerobic gut way, which is very different to the decomposition pattern of mats,” Professor Brocks said.
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To their surprise, they could also see that the animal’s gut was taking up cholesterol instead of plant sterols.
“Kimberella had absorbed the cholesterols for her own use and left only ergosteroids and plant steroids in the gut,” Professor Brocks said.
“That’s a bit of sophistication, it’s just like us.”
The team’s analysis also suggested that the worm-like Calyptrina had instinct.
“It was probably feeding on what it got on the surface, like maybe floating microalgae.”
But the iconic Dickinsonia was a bottom feeder that absorbed its food while living in its algal mat.
“We looked at 17 fossils and no trace of a gut … so it’s almost certain that Dickinsonia was a pretty ancient stranger,” Professor Brocks said.
Debate on the Ediacaran enigmatics is far from over
Palaeontologist Jim Gehling of the South Australian Museum is a world expert on the Ediacaran biota of the Precambrian Red Sandstone of the Flinders Ranges.
Dr Gehling said the findings Kimberella actively sought “fit what we find in the fossil record” in South Australia.
But the debate about how these bodies functioned is far from over.
“This has been one of the most controversial parts of the fossil record for a long time,” Dr Gehling said.
“All the textbooks say real animals started in the Cambrian and the Ediacarans were a failed experiment.”
While he has argued that organisms like Dickinosonia were animals, he said others were not convinced that the Russian fossils could have preserved material for millions of years.
The cliffs surrounding the White Sea in Russia contain a large amount of organic material that has not been compressed into layers of hard rock. (Provided by: Ilya Bobrovskiy)
“I’m not an organic geochemist and I don’t claim to have any wisdom on the matter, but what I do know is that they have rich organic material trapped in that sediment. [in Russia],” he said.
“It hasn’t been buried by 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 consult the science and see if the same signatures could be found in more fossils from Russia and from different parts of the world such as China and Canada.
“When you think everything you read in the textbooks is correct, then you have a problem because the textbooks will always be behind what’s going on in active research,” Dr. Gehling said.
“We’re looking for people who are willing to try, and if necessary, we have to abandon ideas and accept better explanations.”