Scientists have solved a geological puzzle that has been brewing for 50 years: The mystery of ancient animal tracks dating back to a time before humans evolved on Earth.
These traces were found in Mount Barren, southwestern Australia. They look very similar to the burrows made by crustaceans in sand. Problem is that the rock would have formed from sand approximately 600 million years ago, long before animals first became aware of it.
Either animals burrowing earlier than thought or certain species have teeth that can eat solid rock. Neither explanation seemed particularly plausible.
Bruce Runnegar, a paleontologist at the University of California Los Angeles (UCLA), says that quartzite is hard like concrete and difficult for burrowing animals. "The traces had to be created while the sand was still wet."
"But the sand was formed 1.7 billion years before the appearance of the first animal fossils. Its transformation into quartzite took place more than 1.2billion years before the oldest fossils of animals, which are less that 0.6 billion years.
New research has revealed that the sand which formed the burrows was actually younger than most of the surrounding quartzite. These trace fossils, which were discovered during the Eocene Epoch, are thought to be approximately 40 million years old.
Although the mysterious burrows were first described by a paper in 1977. The team decided to revisit them using the most recent technology.
Researchers were able to identify the age of animal burrows using a variety radioactive materials and scanning techniques including scanning electron microscopy. Further research revealed that the rock had been broken at the surface.
It seems that the weathering and flooding created a window for burrowing animals to explore the rock. The rock then hardened again. Similar geological processes can be observed in Stonehenge's stones and in quartzite caves of Venezuela, for instance.
Birger Rasmussen, a geologist at the University of Western Australia, says that the age was more than a billion times younger than the surrounding quartzite. "The burrows could have been made by animals."
According to the team, it is likely that the animals were crustaceans who invaded southwestern Australia due to the expansion of South Ocean. The climate of the present day would have been humid, tropical to temperate.
Scientists have found trace fossils that show activity from animals, which are the oldest evidence of complex living on the planet.
Fossils can be found everywhere. They provide information about how and when certain species evolved, as well as the history of their origins. This means that we need to be as exact as possible with the dates.
Stefan Bengtson, a paleontologist at the Swedish Museum of Natural History, says that "these trace fossils found in the 'wrong rocks' have been a mystery for half a century." "We are happy to have been able show geological processes that solve this conundrum."
The research was published in PNAS.