These mysterious animal burrows seem to pre-date animals. We finally know how
Scientists have just solved a 50-year-old geological conundrum: the mystery of ancient animal tracks dating back to an era prior to the evolution of animals on Earth.
The traces in question are embedded in quartzite rock, discovered at Mount Barren in southwest Australia, and they closely resemble the burrows that crustaceans dig in the sand. The only problem is that the rock would have solidified from the sand about 600 million years before the appearance of animal life.
It seemed that either the animals were digging much earlier than previously thought, or some species had developed teeth capable of gnawing at solid rock. And no explanation was particularly plausible.
“Quartzite is as hard as concrete and impossible for burrowing animals to penetrate,” says paleontologist Bruce Runnegar of the University of California, Los Angeles (UCLA). “The tracks should therefore have been made while the sand was still loose.”
“But the sand was deposited 1.7 billion years ago – a billion years before the first animals appeared in the fossil record, and its transformation into quartzite occurred over 1 , 2 billion years old, 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 much of the quartzite that surrounds it. The trace fossils are now believed to be around 40 million years old, made in the Eocene epoch.
The mysterious terriers were first described in an article 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 age of the animals’ burrows. Further investigation revealed that the first breaks in the rock were made at surface level.
What appears to have happened is that inclement weather and flooding opened 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 over a billion years younger than the encompassing quartzite,” says geologist Birger Rasmussen of the University of Western Australia. “So the burrows could have been dug by animals.”
The team believe it is likely that the animals in question were crustaceans, invading southwest Australia following the expansion of the Southern Ocean at the time. The contemporary climate would have been humid and temperate to tropical.
Fossil traces like these – so named because they show the activity of animals rather than the animals themselves – are among the oldest evidence of complex life on our planet that scientists have.
Wherever they are found, fossils can tell us more about when organisms evolved to become more sophisticated and how certain species began. Part of this means that the dates are as precise as possible.
“These traces of fossils in ‘bad’ rocks have been a mystery for half a century,” says paleontologist Stefan Bengtson of the Swedish Museum of Natural History. “We are happy to have been able to demonstrate geological processes that solve this puzzle.”
The research was published in PNAS.