U. OREGON (US) — Ancient fossils, which scientists have considered ancestors of early marine life, are instead remnants of land-dwelling lichen or other microbial colonies, a geologist proposes.
Ediacaran fossils date to 542-635 million years ago and have been considered fossil jellyfish, worms, and sea pens. However, they are preserved in ways distinct from marine invertebrate fossils, says Gregory Retallack in a paper published online ahead of print in Nature.
The fossils were first discovered in 1946 in Australia’s Ediacara Hills in iron-colored impressions similar to plant fossils and microbes in fossil soils. Retallack, a native of Australia, examined soils gathered from the Flinders Ranges with an array of state-of-the-art chemical and microscopic techniques.
The soils with fossils, he writes, “are distinguished by a surface called ‘old elephant skin,’ which is best preserved under covering sandstone beds.” The healed cracks and lumpy appearance of sandy “old elephant skin” are most like the surface of microbial soil crusts in modern deserts.
“This discovery has implications for the tree of life, because it removes Ediacaran fossils from the ancestry of animals,” says Retallack, professor of geological sciences and co-director of paleontological collections at the University of Oregon Museum of Natural and Cultural History.
“These fossils have been a first-class scientific mystery. They are the oldest large multicellular fossils. They lived immediately before the Cambrian evolutionary explosion that gave rise to familiar modern groups of animals.”
Retallack studied numerous fossils and determined that their diversity reflects a preference by the ancient organisms for “unfrozen, low salinity soils, rich in nutrients, like most terrestrial organisms.”
Fossils in Australia’s iconic red-rock ranges, he concludes, were landlubbers. In his closing paragraph, Retallack outlines implications for a variety of other Edicaran fossils, that could have been lichens, other microbial consortia, fungal fruiting bodies, slime molds, flanged pedestals of biological soil crusts, and even casts of needle ice.
Ediacaran fossils, he says, represent “an independent evolutionary radiation of life on land that preceded by at least 20 million years the Cambrian evolutionary explosion of animals in the sea.” Increased chemical weathering by large organisms on land may have been needed to fuel the demand of nutrient elements by Cambrian animals.
Independent discoveries of Cambrian fossils comparable with Ediacaran ones is evidence, he says, that even in the Cambrian, more than 500 million years ago, life on land may have been larger and more complex than life in the sea.
Retallack leaves open the possibility that some Ediacaran fossils found elsewhere in the world may not be land-based in origin, writing in his conclusion that the many different kinds of these fossils need to be tested and re-evaluated.
“The key evidence for this new view is that the beds immediately below the cover sandstones in which they are preserved were fossil soils,” he says.
“In other words the fossils were covered by sand in life position at the top of the soils in which they grew. In addition, frost features and chemical composition of the fossil soils are evidence that they grew in cold dry soils, like lichens in tundra today, rather than in tropical marine lagoons.”
Source: University of Oregon