Some of the fossils celebrated as sea life’s big breakout beyond mere soups and slimes might actually have dwelled on land, argues a controversial new study.
Named the Ediacaran fauna after Australia’s Ediacara Hills, these creatures dating from roughly 575 million to 542 million years ago mark life finally growing beyond the microscopic. Found in some 30 locations around the world, Ediacarans grew in discs, fronds and other fairly simple shapes with a quilted look, and paleontologists usually consider them some sort of marine creatures.
A new detailed analysis of the rocks where Australian Ediacarans are found suggests the rocks are fossilized soils, or paleosols, instead of a sea bottom, says Gregory Retallack of the University of Oregon in Eugene. The placement of fossils and tiny tubes in the rocks suggests to him that at least some of the Ediacarans actually lived in those soils instead of just washing up on them.
Retallack received “some pushback,” as he calls it, for earlier proposals that Ediacarans were not sea animals but land-living lichens. With the new study, published online December 12 in Nature, he says he knew “people were going to be irate.”
If Ediacarans did turn out to be terrestrial, the implications would go far beyond rethinking where Earth’s earliest complex multicellular organisms lived, says longtime Ediacaran researcher Guy Narbonne of Queen’s University in Kingston, Canada. He doesn’t agree with the new results, but if they were correct, it could mean that decades of studies of ancient environments were based on flawed assumptions.
Also, putting some Ediacarans on soil would add a chapter to the story of life adapting to land. The specimens Retallack studied were about 550 million years old and would be the first big organisms (larger than 2 centimeters) on land. There’s plausible evidence for smaller life that early, he says. But the great rise of seed-making land plants was still some hundred million years in the future.
Retallack has specialized in analyzing ancient soils, but only in recent years has he analyzed in detail at the Australian geological formations that hold the Ediacarans. The rocks show soillike gradations in titanium and some other elements, he says. Patterns of uncommon forms, or isotopes, of carbon and oxygen track each other as they would in soils.
Gypsum crystals and some other nodules are junked up with internal grains of sand that Retallack would not expect if the structures had formed with plenty of water around. He saw cracked and nubbly texture like old elephant skin that he would expect to see on a soil surface. The red color comes from ancient exposure to air instead of more recent weathering, he says.
What he’s found, Retallack suggests, could have been somewhat like modern tundra. Some Ediacarans appear to have grown in these soils, he says. Fossils he’s examined, such as disclike Dickinsonia, look embedded in the fossilized soil without overlapping and show a range of growth stages. In microscopic views of the fossilized soils, he saw tiny tubes branching like modern fungal filaments or cyanobacterial ropes.
“Unconvincing,” says Shuhai Xiao of Virginia Tech in Blacksburg. He says he still thinks ocean processes could explain the Ediacaran rock details. The weight of overlying rock underwater might have deformed a surface into the bumpy elephant-skin texture, for example.
And then there are the other places around the world where Ediacaran fossils show up, protests paleobiologist Mary Droser of the University of California, Riverside. “Where most of the fossils occur are not beds that anyone would consider anything but shallow marine,” she says. Many fossils, for example, show up in rocks with wave-generated ripples as well as underwater-style hummocks and swales.
“Biology matters,” Droser says: Organisms have physiological limits and “cannot live both in the deep sea and in soils, they cannot live on land and in shallow marine settings.”
Identifying ancient soils is not easy. “It is almost always possible to interpret sedimentary textures and structures in alternative ways,” says geologist Paul Knauth of Arizona State University in Tempe. Retallack “has new observations and a reasonable interpretation that should be published and seriously considered.”