Historical fossils: Thriller of early animal burrows in Australia solved

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Animal burrows (dark cylinders) in ancient quartzite rock

Stefan Bengtson

The mystery of how animal burrows in Western Australia could be 400 million years older than the earliest animals in the fossil record has finally been solved. A new analysis suggests they were made far more recently than previously thought.

These holes were thought to be 1.2 billion years old. “That’s twice as old as any known animal in the fossil record,” says Stefan Bengtson at the Swedish Museum of Natural History.

He and his colleagues have re-analysed the 1.7-billion-year-old rocks of West Mount Barren on the coast of Western Australia in which the holes were dug. The burrows are about 15 to 20 millimeters wide and 15 centimetres deep and were first analysed 20 years ago.

The researchers at the time determined that the rocks had hardened 1.2 billion years ago. And so the holes either had to have been made beforehand or by animals millions of years later that would have been able to penetrate the hard quartzite rock – which is essentially impossible to do.

But Bengtson and his team discovered that the quartzite particles showed evidence of so much weathering that for about five to 10 million years, the rocks would have been easy to burrow into. The weathering had caused the rocks to turn into crumbly sandstone. “It’s not very common, but it happens, especially in hot and humid climate,” says Bengtson.

By comparing the samples to other rocks and fossils in the area, and uranium-lead dating the minerals found in the burrows, the team estimated the holes were made 40 to 50 million years ago. The team found that the sandstone had subsequently hardened due to the arid conditions of the region, giving the impression that the burrows had been made much earlier than they actually were.

The team is unsure what specific animals dug the holes, as burrows look different depending on the sediment they are made in. But Bengtson says they were most likely crustaceans.

Anthony Shillito at the University of Oxford in the UK says this was a mystery that needed to be solved. “The fact that the authors have now shown that the burrows were most likely only 50 million years old fits much better with our current understanding of early animal evolution,” he says.

Journal reference: PNAS, DOI: 10.1073/pnas.2105707118

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