Geologists have been baffled by perforations in an Australian quartzite (rock), identical in shape to burrows made in sands by crustaceans; the original sandy sediment is a billion years older than the oldest known animals. An international team of scientists has now resolved the mystery.
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 2.5 billion years to 1.6 billion years ago), and no other fossils have ever been found.
When animals move, they leave traces, such as dinosaur footprints or the burrows of worms. These reveal how ancient animals moved, how they foraged and how they interacted with one another. Trace fossils are as old as the animal world.
Geologists were therefore stunned by the discovery in Western Australia of traces of burrowing animals in ancient quartzite, a rock type that was formed when sandy sediments were subjected to high pressures and temperatures.
“Quartzite is as hard as concrete and impossible for burrowing animals to penetrate,” said Bruce Runnegar, UCLA professor emeritus in the Department of Earth, Planetary, and Space Sciences and co-author of the new research, published today in the journal Proceedings of the National Academy of Sciences.
“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.”
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.
Half-a-billion year old worm fossils
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.
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 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 creatures 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.
“Most likely, the traces were made by crustaceans, which invaded southwestern Australia during a short-lived marine transgression associated with the opening of the Southern Ocean,” said senior author Stefan Bengtson, professor emeritus and paleontologist at the Swedish Museum of Natural History.
“These trace fossils in the ‘wrong’ rocks have been a mystery for half-a-century,” Bengtson said. “We are glad to have been able to demonstrate geological processes that resolve this conundrum.”