Fossils show signs of earliest burrowing

Ancient diggers may have stirred up evolutionary forces

Worms may have first burrowed into mud more than 550 million years ago. The tunnels they apparently created, preserved in fossilized sediments and reported in a new study, could be the oldest example of animals significantly churning up the ground.

ANCIENT NEW DIGS Crescent-shaped trace fossils in mudstone, seen here in a horizontal slice, could be tunnels dug by primitive worms that spurred a diversification of life more than 550 million years ago. D. Grazhdankin

That newly plowed seafloor in turn might have helped to spur the rise of new kinds of macroscopic life late in the Ediacaran period — just before the Cambrian explosion produced most of the major animal groups around today.

“We think that Ediacaran organisms diversified as a reaction to habitat remodeling by … burrowing,” says Dima Grazhdankin, a paleontologist at the A.A. Trofimuk Institute of Petroleum Geology and Geophysics in Novosibirsk, Russia, and coauthor of a new paper published online March 19 in Geology.

Grazhdankin and colleagues found the fossils in central Siberia, in uplifted rock that had once been mud deep underwater. Tiny, crescent-shaped traces cutting through 5 centimeters of former sediment looked like small tunnels made by creatures scooping and flinging dirt from front to back.

Only a creature with bilateral symmetry — not just a front and back, but a top and bottom as well — could have made such a pattern, says Grazhdankin. Previous evidence of bilaterians this early in the fossil record has proved controversial. Still, he credits a primitive worm.

Digging could have helped the worm find food, as well as open up new living spaces for other creatures. Mud that’s been churned up, or bioturbated, would be softer, freed of the stiff microbial mat that covered much of the seafloor at the time. The softer mud would be easier to colonize, possibly helping to explain why macroscopic life spread and became more diverse late in the Ediacaran.

Shuhai Xiao, a paleontologist at Virginia Tech in Blacksburg, says the appearance of bioturbation so early could even have had a planetwide impact. Churning up seafloor sediments could have helped to recycle carbon and sulfur in the sediments back into the ocean, for instance, changing seawater chemistry and even oxygen levels in the atmosphere.

“If these fossils do represent evidence for bioturbation, they would have important implications for early animal evolution and its impact on the Earth system,” he says.

But Xiao still needs some convincing. Since none of the densely packed traces in the rock crisscross, he wonders whether the patterns are burrows at all — or simply the imprint of a creature that died an inglorious death in the mud.

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