Controversial claim shakes some limbs on the tree of life

December 12, 2012

Controversial claim shakes some limbs on the tree of life

University of Oregon researchers say that ancient multicellular fossils may not be ancestors of early marine life.

A controversial claim has been put forth by a researcher at the University of Oregon that could shake some limbs on the tree of life. Researcher Gregory Retallack argues that ancient multicellular fossils previously thought to be ancestors of early marine life are vestiges of land-dwelling lichen or other microbial colonies.

Mr. Retallack points to Ediacaran fossils as evidence that land creatures may not have come from the sea. Ediacaran fossils date to 542-635 million years ago. They have been thought of as fossil jellyfish, worms and sea pens, but this researcher says that Ediacaran fossils are preserved in ways that are completely different from marine invertebrate fossils. Ediacaran fossils are located in iron-colored impressions like planet fossils and microbes in fossil soils.

Mr. Retallack took a closer look at ancient Ediacaran soils with an assortment of high-tech chemical and microscopic techniques, including an electron microprobe and scanning electron microscope.

Mr. Retallack notes that the soils with fossils “are distinguished by a surface called ‘old elephant skin,’ which is best preserved under covering sandstone beds.”

“This discovery has implications for the tree of life, because it removes Ediacaran fossils from the ancestry of animals,” said Mr. Retallack, professor of geological sciences and co-director of paleontological collections at the University of Oregon’s Museum of Natural and Cultural History, in a statement.

“These fossils have been a first-class scientific mystery,” he posited. “They are the oldest large multicellular fossils. They lived immediately before the Cambrian evolutionary explosion that gave rise to familiar modern groups of animals.”

Mr. Retallack arrived at the conclusion that the Ediacaran fossils in Australia’s red-rock ranges were unfamiliar with the sea because he determined that the ancient organisms preferred “unfrozen, low salinity soils, rich in nutrients, like most terrestrial organisms.”

Ediacaran fossils symbolize “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.”

Mr. Retallack says that elevated chemical weathering by organisms on land may have been necessary to propel the demand of nutrient elements by Cambrian animals.

Findings of Cambrian fossils equivalent with Ediacaran ones are evidence, Mr. Retallack contends, that even in the Cambrian, life on land may have been more complex than life in the sea.

The researcher notes, however, that some Ediacaran fossils may not be land-based in origin. Determining their origin, he says, will require additional research.

“The key evidence for this new view is that the beds immediately below the cover sandstones in which they are preserved were fossil soils,” Mr. Retallack said. “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.”

Only soil horizons and soil structures can be used to identify fossil soils in rocks of Ediacaran age. Mr. Retallack notes that Ediacaran fossil soils represent ecosystems less effective at weathering than the modern array of ecosystems. This means that soil horizons and soil structures are not as well developed as they are in modern soils.

“The research conducted by Dr. Retallack helps to unravel the mystery of very ancient life on Earth,” said Kimberly Andrews Espy, University of Oregon vice president for research and innovation and dean of the graduate school, in a statement. “It also serves as an example of how technology, some of it developed at the University of Oregon, can be used to analyze materials from anywhere in the world.”

Mr. Retallack’s findings, however, were rejected by some in the scientific community. In an email to LiveScience, Martin Brasier, a paleobiologist at the University of Oxford, slammed the University of Oregon researcher’s claim.

“I find Retallack’s observations dubious, and his arguments poor. That this was published by Nature is beyond my understanding,” wrote Mr. Brasier.

Others, however, were mostly supportive of Mr. Retallack’s claim.

“I don’t have any problem with early evolution being primarily on land,” Paul Knauth, a professor in the School of Earth and Space Exploration at Arizona State University, told Cap Radio. “I think you can make a pretty good argument for that, and that it came into the sea later. It’s kind of a radical idea, but the fact is we don’t know.”

The study’s findings are described in detail in the journal Nature.


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