Scientists just solved a 50-year-old geological puzzle: the mystery of ancient animal traces dating back to an era before animals had evolved on Earth.

The traces in question are embedded in quartzite rock, discovered in Mount Barren in southwestern Australia, and they look a lot like the burrows that crustaceans make in sand. The only problem is, the rock would have solidified from sand around 600 million years before animal life first appeared.

It seemed that either animals were burrowing way earlier than previously thought, or some species had developed teeth capable of chomping through solid rock. And neither explanation was particularly plausible.

"Quartzite is as hard as concrete and impossible for burrowing animals to penetrate," says paleontologist Bruce Runnegar from the University of California, Los Angeles (UCLA). "The traces would therefore have had to be made while the sand was still loose."

"But the sand was deposited 1.7 billion years ago – a billion years prior to the appearance of the first animals in the fossil record, and its transformation to quartzite occurred more than 1.2 billion years ago, much earlier than the oldest animal fossils, which are less than 0.6 billion years old."

A new study provides an explanation: the sand that formed the burrows is actually much younger than a lot of the quartzite surrounding it. The trace fossils are now thought to be around 40 million years old, made during the Eocene epoch.

The mysterious burrows were first described in a paper in 1977, but the team decided to revisit them using some of the latest technology.

Using a variety of radioactive materials and scanning techniques, including scanning electron microscopy, the researchers were able to more accurately identify the ages of the animal burrows. Further investigation revealed that the first breaks in the rock were made at surface level.

What seems to have happened is that weathering and flooding opened up a window of opportunity for burrowing creatures to explore the rock, which then hardened again. Similar geological processes have been observed elsewhere – in the stones of Stonehenge, for example, and quartzite caves in Venezuela.

"The age turned out to be more than a billion years younger than the enclosing quartzite," says geologist Birger Rasmussen from the University of Western Australia. "The burrows could therefore have been made by animals."

The team thinks it's likely that the animals in question were crustaceans, invading southwestern Australia as a result of the expansion of the Southern Ocean at the time. The contemporary climate would have been moist, and temperate to tropical.

Trace fossils like these – so named because they show the activity of animals rather than the animals themselves – are some of the oldest evidence of complex life on our planet that scientists have.

Everywhere they're discovered, the fossils can teach us more about when organisms evolved to be more sophisticated, and how certain species first got started. Part of that means getting the dates as accurate as possible.

"These trace fossils in the 'wrong' rocks have been a mystery for half-a-century," says paleontologist Stefan Bengtson from the Swedish Museum of Natural History. "We are glad to have been able to demonstrate geological processes that resolve this conundrum."

The research has been published in PNAS.