Prehistoric worms populated the seagrass for 500 million years ago, evidence that life was active in an environment uninhabited so far, according to research from the University of Saskatchewan (USask).
The seabed in the deep ocean during the Cambrian period was thought to be inhospitable to animal life because it lacked enough oxygen to support it.
But the research was published in the scientific journal Geology It reveals the existence of fossil worm tunnels that go back to the Cambrian period 270 million years before the evolution of the dinosaurs.
The discovery, by Professor of USask Brian Pratt, suggests that animal life in sediment at that time was wider than previously thought.
The tunnels-towers where the worms lived and crossed the sediments are invisible to the naked eye. But Pratt "had a drink" and cut off the rocks and analyzed them to see if they revealed signs of old life.
The rocks came from a zone in the remote Mackenzie Mountains of the Northwest Territories in Canada that Pratt found 35 years ago.
Then, digitally enhanced images of rock surfaces so that you can examine them more closely. Only then happened the hidden "freeway" of burrows made by various sizes and types of prehistoric worms on the rock.
Some were just one millimeter in size and others as big as a finger. The smallest ones were probably manufactured by simple gauze or polyglyphs, but one of the great forms was a predator that attacked unsuspecting arthropods and surface worms.
Pratt said he was "surprised" by the unexpected discovery.
"For the first time, we have seen evidence of large populations of worms living in the sediment – which was thought to be barren," he said. "There were critical worm-burrows tunnels-in mud on the continental shelf some 500 million years ago, and more animals replanted, or bioturbated, the sea bed of which no one thought."
Pratt, a geologist and paleontologist and member of the Geological Society of America, found the tunnels on sedimentary rocks that are similar to Burgess Shale, a famous fossil deposit on the Canadian rocks.
The discovery can provoke a repensation of the oxygenation level in the ancient oceans and continental shelves.
The Cambrian period saw an explosion of life on Earth in the oceans and the development of multicellular organisms, including prehistoric worms, prunes, snails and ancestors of crabs and lobsters. Previously the seas had been inhabited by simple and unicellular microbes and algae.
It was always assumed that the creatures of Burgess Shale, known for the richness of their fossils, remained so immaculately because the lack of oxygen at the bottom of the sea stopped decaying and because they did not live animals in the mud to eat the carcasses.
The discovery of Pratt, with co-author Julien Kimmig, now from the University of Kansas, shows that there was enough oxygen to support various types of worms in the seagull.
"Serendipity is a common aspect for my type of investigation," said Pratt. "I found these unusual rocks accidentally all these years. With a raffle I prepared a lot of samples and when I improved the images I was really surprised at what I found," he said.
"This has many implications that will now have to be investigated, not only in Cambrian shales, but also in the youngest rocks. People should try the same technique to see if they reveal signs of life in their samples."
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