Virginia, United States

A team of researchers at Virginia Tech has discovered a 550 million-year-old sea sponge fossil, which can help fill a critical 160 million-year gap in the fossil record. 

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Million of years old sea sponge fossil

This finding provides a roadmap for palaeontologists in finding prehistoric sponges.

The sea sponge is unassuming: no brain, no gut; it can quickly be dated to seven hundred million years ago. Yet convincing sponge fossils only go back 540 million years. That means there is a hundred and sixty million-year gap in the fossil record.

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In a paper published June 5 in the journal Nature, geobiologist Shuhai Xiao and his colleagues report finding the 550 million-year-old sea sponge fossil from the "lost years".

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As per SciTechDaily, the mystery of the missing sea sponges rested on a paradox: molecular clock estimates, which project the history of genetic mutations back in time. These estimates suggest that sponges evolved almost 700 million years ago—much older than any known sponge fossil. This discrepancy has long puzzled zoologists and palaeontologists alike.

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Xiao's find completes part of the evolutionary story for one of our earliest animals and answers Darwin's questions about the time of sponge evolution.

He first saw the fossil some five years ago when a colleague sent him an image of a specimen recovered from the Yangtze River region of China. "I had never seen anything like it before," said Xiao. 

"Almost immediately, I realised that it was something new."

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Xiao and his colleagues from the University of Cambridge and the Nanjing Institute of Geology and Palaeontology carefully sifted through the other likely candidates, finally proposing that it could be an ancient sea sponge. 

The group has offered their guess on why early sponges never left fossils; these animals had not evolved spicules – the complex elements constituting the modern sea sponge skeleton by this time.

The team has been able to observe in the fossil record how much more organic and less mineralised are the spicules of older sponges. 

"If you extrapolate back, then perhaps the first ones were soft-bodied creatures with entirely organic skeletons and no minerals at all," Xiao said. 

Only under scarce circumstances, in which very rapid fossilisation outpaces degradation, could the organisms with such soft bodies be fossilised.

(With inputs from agencies)