50-year-old puzzle solved, animal tracks that date back before animals

A mystery that has spanned almost half a century has just been answered, according to a team of researchers.

The mystery involved the above image, which is a trace fossil. A trace fossil shows the activity of an animal rather than the animal themselves, and these types of fossils provide really valuable information to researchers looking to understand the evolution of complex life on Earth. The quartzite rock was discovered in Mount Barren in southwestern Australia, and the tracks embedded in it appear to have been made by crustaceans that burrow into the sand.

The only problem is, "quartzite is as hard as concrete and impossible for burrowing animals to penetrate. 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," said paleontologist Bruce Runnegar from the University of California, Los Angeles (UCLA).

In the new study, researchers propose that animals created the marks on the quartzite, but what had happened as that weathering and flooding caused the rock to soften, giving an opportunity for burrowing animals to make their mark before the rock hardened again.

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

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

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