'Marine' Fossils May Instead Represent Early Land Dwellers

12 December 2012 1:20 pm

G. Retallack

Land ho! Many aspects of fossil-bearing Australian rocks from the Ediacaran period suggest that they formed on land, which in turn suggests that organisms such as Dickinsonia (fossils shown) were terrestrial and not marine.

The fossils of various frondlike and sacklike organisms that supposedly lived at the bottom of ancient oceans may actually represent some of the earliest organisms to dwell on land. That's the controversial interpretation of a new study, which suggests that rocks long thought to have been formed from sediments deposited on ancient seafloors may actually be the remnants of early soils. If true, the finding would push back life's transition from sea to land by tens of millions of years—and possibly by 100 million years or more.

Fossils reveal that life on Earth diversified rapidly during the Cambrian period, which began about 542 million years ago and lasted until about 485 million years ago. The so-called Cambrian explosion yielded most of the major groups of animals known today, but fossils of a host of organisms bearing little resemblance to modern life forms are embedded in Precambrian rocks—including those of the Ediacaran period, which began about 635 million years ago and lasted until the onset of the Cambrian. Most researchers have considered these unusual organisms—some resemble segmented worms or fronds, and others look like nothing more than bags of tissue—to have lived in the sea, because the types of rocks that entombed them typically accumulate as sediments in marine environments.

Not so fast, says Gregory Retallack, a paleobotanist at the University of Oregon in Eugene. His new analysis of fossil-bearing Ediacaran rocks from southern Australia suggests that those rocks formed from paleosols, or ancient soils. That makes the fossils found within the rocks terrestrial and not marine, he contends.

For example, the texture of the rocks, as well as the arrangement of the angular, interlocking grains, indicates that the accumulated sediments had been windblown, he argues. "These features aren't found in marine sediments."

Moreover, he notes, the distinctive color patterns of the Australian rocks, as well as the hues of the strata above and below, suggest that these Ediacaran rocks were weathered by exposure to the elements at the time they were formed, not in the millions of years since.

Finally, the rock immediately beneath many of the fossils contained branching tubular structures a centimeter or more in length. The lack of such structures above the fossils suggests that the tubular structures are similar to those anchoring modern-day mosses and lichens to underlying soils, Retallack says.

Together, the evidence hints that fossils long thought to represent seafloor organisms are actually the remnants of terrestrial organisms similar to today's lichens or other microbial colonies, Retallack reports online today in Nature.

But Shuhai Xiao, a paleontologist at the Virginia Polytechnic Institute and State University in Blacksburg, doesn't find Retallack's evidence compelling. For example, he says, the varying colors of the Ediacaran rocks that Retallack analyzed don't necessarily point to erosion at the time soon after the rocks were formed. That's because over time—and especially over the hundreds of millions of years since the strata were formed—rocks with different chemical compositions and different permeabilities to water and oxygen can weather at dramatically different rates.

The purported rootlike structures associated with the fossils are much too irregular to have been formed by microbes, as are those in today's lichens, Xiao contends in a commentary, also published online today in Nature. Finally, he notes, the fossils of the same species found in the Ediacaran rocks have turned up in rocks that, based on their stratigraphy and composition, definitely formed in a marine environment. "It's unlikely that the same species, with the same anatomy and physiology, were adapted to both land and salty water," he says. "I think the traditional interpretation of the Ediacaran rocks [as being derived from marine sediments] still stands."

Other scientists, including Paul Knauth, an isotope geochemist at Arizona State University, Tempe, are more open to Retallack's arguments. "We're trying to interpret what happened long ago, and none of the evidence so far is unequivocal," he says. "The truth is, we just don't know."

The Ediacaran period was a time of frequent and dramatic sea level change, Knauth says. In many parts of the world, rocks from that era that formed under shallow seas were later exposed to the elements, undergoing the same types of erosion and weathering seen in the Australian rocks that Retallack analyzed. Some of those Australian strata "are dead ringers for paleosols," he notes.

Few scientists agree on what soil that formed during the Ediacaran period would look like, Knauth says. And altogether, he notes, Retallack "makes a whole slew of new arguments, many of them reasonable and worth looking at. … It would be a mistake to beat this [interpretation] down."

If Retallack's ideas are correct, Knauth says, researchers would have further evidence that land areas during the Ediacaran period weren't biologically barren, as is commonly assumed. That, in turn, could inspire new notions about how and when Earth's first soils formed—and how scientists can recognize remnants of them in today's rocks.

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