Ediacaran Fossils A 'First-class Scientific Mystery'
December 13, 2012

Ancient Sea Creatures Were Actually One Of World’s Earliest Land-dwelling Organisms

redOrbit Staff & Wire Reports - Your Universe Online

Mysterious multicellular fossils believed to be ancient sea creatures may actually be some of the earliest land-dwelling organisms, according to a paper published online on Wednesday in the journal Nature.

The controversial hypothesis has been fiercely criticized, with some paleontologists flatly rejecting the idea, but if true, the finding would push back life's transition from sea to land by as much as 100 million years or more.

The Ediacaran fossils were first discovered in 1946 in Australia's Ediacara Hills, and date to 542-635 million years ago. They were long considered fossil jellyfish, worms and sea pens.

Gregory Retallack, an Australian native who has been studying the fossil soils of South Australia, examined the ancient Ediacaran soils with a variety of chemical and microscopic techniques, including an electron microprobe and scanning electron microscope.

The soils with fossils "are distinguished by a surface called 'old elephant skin,' which is best preserved under covering sandstone beds,” wrote Retallack, professor of geological sciences and co-director of paleontological collections at the University of Oregon´s Museum of Natural and Cultural History, in his report.

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.”

"These fossils have been a first-class scientific mystery," he said.

"They are the oldest large multicellular fossils. They lived immediately before the Cambrian evolutionary explosion that gave rise to familiar modern groups of animals."

After studying numerous Ediacaran fossils, Retallack determined that the diversity reflects a preference by the ancient organisms for "unfrozen, low salinity soils, rich in nutrients, like most terrestrial organisms."

Thus, the fossils in Australia's iconic red-rock ranges were land-dwellers, he concluded.

In the closing paragraph of his paper, Retallack describes the implications for a variety of other Edicaran fossils, which could have been lichens and other microbial consortia, fungal fruiting bodies, slime molds, flanged pedestals of biological soil crusts and even casts of needle ice.

Ediacaran fossils 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,” he said.

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 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 said.

He leaves open the possibility that some Ediacaran fossils found elsewhere in the world may not be land-based organisms, and that 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 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."

Fossil soils are typically recognized from root traces, soil horizons and soil structures. However, in rocks of Ediacaran age, before the advent of rooted plants, only the second two criteria can be used to recognize fossil soils.

Retallack said Ediacaran fossil soils represent ecosystems less effective at weathering than the modern array of ecosystems, so 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.

"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."