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An ancient mollusc-like animal has been caught in the act of having its last meal.
Algae and bacteria were on the menu for the early animal Kimberella shortly before it was buried over 500 million years ago.
One of the oldest meals ever eaten may have been discovered in a fossil over half a billion years old.
A mollusc-like animal known as Kimberella appears to have enjoyed a meal of green algae and bacteria shortly before its death 558 million years ago. Chemical traces preserved in its fossil and surrounding rock from Russia's White Sea region suggest that it had an internal gut capable of digesting prey.
Meanwhile, other fossilised animals such as the enigmatic Dickinsonia appear to have gone without a gut, and instead obtained food by absorbing nutrients directly through its body.
Dr Ilya Bobrovskiy, the lead author of a study revealing these findings, says, 'Our findings suggest that the animals of the Ediacaran biota, which lived on Earth prior to the Cambrian Explosion of modern animal life, were a mixed bag.'
'While weird forms such as Dickinsonia were very unusual, more advanced animals like Kimberella already had some physiological properties similar to present-day animals.'
While the research provides tantalising evidence of some of the earliest animal behaviour, not all of the chemical traces could be accurately distinguished. This means that alternative suggestions, such as communities of bacteria leaving the traces, are possible if unlikely.
The findings of the study were published in the journal Current Biology.
The Ediacaran Period is one of the most important in the development of life, as organisms made the jump from simple structures such as microbial mats to animals with bodies, heads, tails and guts.
This allowed new ways for animals to live to evolve, including the development of strategies such as predation. The earliest known animal predator, Auroralumina attenboroughii, was named earlier this year, and is believed to have used tentacles to catch passing food in a similar way to modern corals.
While it is uncertain if this species was able to move, Kimberella was. Dating to around two million years after A. attenboroughii, fossils of Kimberella have been associated with preserved tracks left behind by the animal as it grazed on microbial mats.
It's not quite sure how this feeding took place, with some researchers suggesting that it ate while moving backwards, while others think it stayed still and used an extending proboscis to eat in a fan-shaped arc.
This method of feeding is similar to modern molluscs, many of which have a structure known as a radula that they use to scrape food off from rocks. Some researchers have suggested that Kimberella could be a very early relative of molluscs, though others think it could be a relative of jellyfish or be within its own group of animals.
It would have lived alongside a range of other weird animals, including a worm-like species known as Calyptrina and the more unusual Dickinsonia, which looked like a large, rippled pancake up to 1.4 metres long.
Professor Jochen Brocks, a co-author on the paper, says that their diet might help explain why these animals are a lot bigger than their predecessors.
'The energy-rich food they consumed, such as algae, may explain why the organisms of the Ediacaran biota were so large,' Jochen says. 'Nearly all fossils that came before the Ediacaran biota were single-celled and microscopic in size.'
Their large size may also have been helped by cholesterols, which help animals to maintain their cell membranes. Previous research detecting this molecule, which is only found in animals and their closest relatives, has added to the evidence that Dickinsonia was an animal, although this is still debated.
The team have now taken this one step further by investigating the products of cholesterol breakdown within the bodies of these Ediacaran animals, which could provide evidence that digestion was taking place and identify what they may have been eating.
To investigate what Ediacaran animals could have been eating, the team looked for evidence of a group of molecules known as steranes. The steranes are generally formed by processes such as digestion from molecules like cholesterol, but can also be formed naturally in the environment.
The ratio between different forms of these steranes can indicate how they formed. Dickinsonia specimens, for instance, had a ratio significantly different from the background of the White Sea area, which is believed to have been caused by microbes degrading its body after death.
Researchers couldn't find any evidence of gut contents preserved within these animals, which suggests Dickinsonia didn't have a gut and instead absorbed food across its surface.
Meanwhile, the steranes around the Kimberella specimen are preserved in three distinct groups. The layers surrounding the fossil are indistinguishable from the background, including a microbial mat preserved alongside Kimberella.
The next layer immediately next to the animals contained elevated levels of steranes derived from cholesterol, which are thought to have been formed by the degradation of the body after death.
The specimen itself contains the third group, which show a depletion in one form of the molecules known as C27. This means that either the rest of the steranes have been enhanced somehow to shift the ratio of molecules, or that the C27 molecules have been removed.
Removing C27 only occurs naturally through digestion. This is considered the more likely cause of the observed change, as the ratio pattern is similar to those seen in modern invertebrates. This suggests that the third group of steranes represents a gut.
'Kimberella knew exactly which sterols were good for it and had an advanced fine-tuned gut to filter out all the rest,' Jochen says. 'This was a Eureka moment for us; by using preserved chemicals in the fossils, we can now make gut contents of animals visible even if the gut has long since decayed.'
The researchers hope that their technique can be applied to other extinct organisms, revealing the diets of the distant past.