Geologists solve half-century-old mystery of animal traces in ancient rocks

Quartzite with tracetraces of burrowing mammals. The scale is in centimeters. Credit: Stefan Bengtson/Swedish Museum of Natural History
Perforations in Australian quartzite (rock) have puzzled geologists. They are identical to burrows made by crustaceans in sands. The original sandy sediment is billions of years older than any known animal. The mystery has been solved by an international team of scientists.

Animals leave footprints when they move. These traces reveal the movements of ancient animals, their foraging habits and how they interacted with each other. Trace fossils can be as old as the animal kingdom.

The discovery of evidence of burrowing animals in quartzite in Western Australia stunned geologists. This rock type was formed from sandy sediments that were exposed to high temperatures and pressures.

Bruce Runnegar, UCLA professor and emeritus in Earth, Planetary, and Space Sciences, said that quartzite is hard like concrete and difficult for burrowing animals. He was also co-author of the research, which was published in Proceedings of the National Academy of Sciences. The traces had to be taken while the sand was still wet. The sand was formed 1.7 billion years before the appearance of the first fossil animals. Its transformation into quartzite took place more than 1.2 trillion years ago. This is much earlier than the oldest fossils of animal species, which are less that 0.6 billion years.

A Swedish-Australian-Chinese-American team has now offered a solution to this riddle. Scientists present a solution that doesn't require any unreasonably old animals or concrete-chewing, diamond-toothed worms.

The team used unusual radioactive minerals to determine the age of the sand in the burrows.

Birger Rasmussen adjunct professor at University of Western Australia, said that the age was more than a billion times younger than the surrounding quartzite. "The burrows could have been made by animals," he said.

How can animals burrow through quartzite hard? Microscopic studies revealed that the grains had separated at contact surfaces and formed a friable matrix. The rock was then fused through later depositions of quartz, which returned it to its original state of hard quartzite.

Runnegar stated that a similar process was responsible for the Stonehenge standing stones.

Researchers report that a window of time was opened for burrowing. The scientists were able to date the window by comparing it with the surrounding sedimentary strata. This was during the Eocene period of Earth's history.

Senior author Stefan Bengtson, professor of natural history at the Swedish Museum of Natural History, said that the most likely explanation is that the traces were left behind by crustaceans.

Bengtson stated that trace fossils found in the "wrong" rocks have been a mystery for over half a century. "We are happy to have been able show geological processes that solve this conundrum."

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