A team of researchers has succeeded in solving the mystery of mysterious animal burrows that have puzzled geologists for nearly half a century.

In the mid-seventies of the last century in southwestern Australia, scientists found holes about one centimeter in diameter resembling burrows dug by crustaceans in the sand, inside quartzite rocks (a type of metamorphic rock of sandy sedimentary origin) that are one billion and 700 million years old.

This discovery was first reported in a study published in 1977.

The problem was that the oldest fossils of animals capable of digging were no more than 600 million years old.

Several explanations have been put forward for this mystery, including that these burrows were dug by primitive animals in the ancient Proterozoic period (which spanned from 2500 million years to 1600 million years ago), and no other fossils have ever been found.

When animals move, they leave footprints in the sand, like dinosaur footprints (Getty Images)

fossil trace

When animals move, they leave footprints on the sand, such as the footprints of dinosaurs, and these traces reveal the ways of life of ancient animals and ways of searching for food and their interaction with other animals. These burrows are one of the oldest fossils found of multicellular animals.

In 2017, an international team of scientists in western Brazil found holes and burrows dug by organisms in rocks between 555 and 542 million years ago.

The University of Manchester announced on Twitter that this research may invite us to reconsider the history of animal evolution.

But the burrows found in southwestern Australia baffled geologists because they were dug in hard quartzite rocks that form when sandy sediments are exposed to high temperatures and pressures.

Bruce Ronegar, Professor Emeritus in the Department of Earth, Planetary and Space Sciences at the University of California Los Angeles (UCLA), who participated in the preparation of this new research, says, according to the statement published on the university's website on September 27 last - Quartzite rocks are so hard that it is impossible for animals to penetrate."

He added, "These traces that were found must have been excavated when the sand was loose, but the sand was deposited one billion and 700 million years ago, that is, about one billion years before the appearance of the first animals in the fossil record."

Manchester research has shed new light on animal evolution thanks to Half-a-billion year old worm fossils https://t.co/IX1dIwkyGF pic.twitter.com/E6T3UCKNoY

— About Manchester (@AboutMcr) September 13, 2017

Solve the mystery of the mysterious animal burrows

The new study, published in the journal "Proceedings of the National Academy of Sciences" (PNAS), provides an explanation for the mysterious burrow problem, which is that the sand that formed the burrows is more recent than the other layers of the surrounding quartzite rock.

The team used radioactive materials and various techniques, including a scanning electron microscope, to measure the age of the sand in these burrows, and they discovered that the sand grains were initially separated from the surface of the rock by weathering and flooding factors, which provided the opportunity for living organisms to dig burrows and holes in the rock, and then the rock regained its hardness once Others passed through silica deposits, and again turned to the solid state of quartzite.

The researchers compared the ages of the rock layers, and discovered that the layer in which these creatures dug their burrows dates back to the Eocene era, about 40 million years ago.

The team believes that these organisms are crustaceans, which invaded southwestern Australia due to the expansion of the Southern Ocean at this time, and the area was at that time a tropical climate.

The importance of these fossil traces is that they reveal how living organisms evolved from primitive single-celled organisms to complex multicellular organisms, and accurately date each stage of their evolution.

Stefan Bengtsson of the Swedish Museum of Natural History says, "The traces of these organisms have puzzled scientists for nearly half a century, because they were misplaced, and we were finally able to reveal the geological processes that contribute to solving this dilemma."