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Rock doc: Soils versus sea beds

By
March 20, 2013 |

There’s a new debate in paleontology, one that took me by surprise but that shows nicely how some science works.

There’s a particular type of ancient fossil called the “Ediacara fauna” found in rocks about 550 million years old. The term Ediacara is reference to a place in Australia where the fossils were located and well-described. In a complex tale that unfolded over decades both before and a bit after the Australian discovery, similar fossils were found around the world at several locations. In time people connected the separate discoveries and a unified set of fossils was understood as being from around the same time in Earth history.

The Ediacara fauna is made up of several types of small impressions left in what’s now solid rock. The impressions show simple life-forms that were flat like little pancakes or long like simple worms. They had no eyes and no legs but they were the first multicellular organisms to grace the Earth, so they were advanced forms of life in their day.

I was taught the simple little guys were animals that were flat or long because they needed to exchange gases through their skin and thus they needed considerable surface area to stay alive in the shallow seas in which they lived. I was also taught they disappeared from planet Earth during the “Cambrian explosion,” that part of Earth history in which advanced sea creatures with hard shells, eyes and legs first appear in the fossil record. One hypothesis about what happened is simply that the Cambrian animals were able to move around and eat up the Ediacara fauna, which had no defenses or ability to skittle away from predatory Cambrian animals. Under this hypothesis, the predators had quite a feast day, gobbling up the Ediacara life-forms until they were all extinct.

There has always been more than one way to interpret the Ediacara fauna. They may not have been animals, but perhaps lichens – an interesting life form that’s a combination of fungi and algae that help one another survive. Some paleontologists reject that view and have considered putting the Ediacara into their own “kingdom” in terms of the classification of life forms sketched by science – meaning the Ediacara were organisms that were quite unlike plants, animals, or fungi.

The limited information available from the trace impressions the Edicara left behind is what makes many different hypotheses possible. Some issues in science can be resolved by relatively clear-cut experiments in a laboratory. Paleontology isn’t like that, and unfortunately we don’t have time machines that would let us travel back to ancient times and study live and wiggling little Ediacara organisms. Instead we must do what we can with the samples of rocks and fossils we have.

Recently I was surprised to hear of a new and quite different hypothesis about our simple, little friends from prehistory. Gregory Retallack of the University of Oregon argues that the rocks of at least some Ediacara are paleosols – that’s geospeak for ancient soils. The rocks have variations in trace chemicals and different types (or isotopes) of carbon and oxygen similar to what we’d expect in soils, he says. Another point of evidence is that some of the fossils are laced with gypsum crystals. Gypsum is the mineral in sheet-rock, and it’s soluble in water so the argument is that the little fossils could not have lived in water or the gypsum would have dissolved away. Lastly, the texture of some of the rocks has a wavy surface like “elephant skin,” a phenomenon seen in some soils.

Everything I was taught about the Ediacara emphasized they were creatures living in shallow water, not on land. And Ediacara fossils are found at some 30 locations around the world, many of which I believe don’t fit easily with Retallack’s point of view. Still, researchers can and should voice different ideas based on what they can come up with as they study the fossil record. It’s a sign that science is healthy when scientists disagree and have sometimes vigourous arguments about the same fossils.

But I really do wish for one simple time-machine to clear up many debates about the history of life on Earth.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University. 

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