Have we misunderstood the first known animal? Dickinsonia, a weird organism from half a billion years ago, may have inflated its body to feed. The strategy is not seen in animals today, which raises questions about whether Dickinsonia really belongs in the animal kingdom after all.
Dickinsonia is one of a bunch of enigmatic organisms聽called the Ediacarans that lived a few tens of millions of years before familiar animals like sponges began forming fossils.
The Ediacarans have sometimes been interpreted as a , unlike anything alive today. But recently palaeontologists have become more confident that they were animals, with Dickinsonia singled out in particular as a candidate for the earliest known animal聽we have a fossil of.
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But if Dickinsonia was an animal, it may have been a very odd one, according to Nicole Law and Scott McKenzie at Mercyhurst University, Pennsylvania.
Dickinsonia fossils look a bit like round flat ribbed blobs, some are about a metre in length but all . Law and McKenzie have聽studied unusual radial scratch lines in the rock fringing one of the fossils and say they suggest that聽the organism was larger in life and then聽shrank after it died, leaving scratch lines in the sand beneath its body as it shrivelled up.
鈥淎bout 21 per cent of the total fossil area is taken up by this fringe,鈥� says Law. She thinks that聽whilst alive聽Dickinsonia聽was longer, wider and taller than our fossils suggest. Its body was inflated like a balloon, and most fossils show it in a smaller and thinner deflated state.
Inflatable feeder
Law says this is an idea that Rex Powell at the University of California in Berkeley, has been working on for about a decade. The three researchers presented the聽 idea at an annual meeting of the Geological Society of America in Indianapolis聽last week.
Inflating could have been a key feeding strategy. Dickinsonia has been interpreted as some sort of seabed grazer that fed by digesting the organic matter beneath its body and absorbing it directly through its body wall. 鈥淚nflating could have allowed it to cover more of the seabed,鈥� says Law 鈥� meaning it聽could consume more food before it had to move to a new grazing site.
Evidence that Dickinsonia might have inflated its body in life adds to the sense that the organism was very odd. 鈥淚 don鈥檛 know of any animals that inflate in the way we think Dickinsonia may have been able to do,鈥� says Law.
Pufferfish can inflate their stomachs to deter predators, but聽Dickinsonia聽seems to have inflated its entire body for feeding. As such, Law says it might not fall on the animal branch of the tree of life, but instead on an unidentified nearby branch. 鈥淪o perhaps it wasn鈥檛 an animal. But I don鈥檛 believe we have enough evidence to know for sure right now.鈥�
