A pair of geologists with the University of Cambridge has found a possible link between the evolution of ancient rootless plants and a marked increase in the production of ancient mud rock (fine-grained rock formed from silty clay deposits). In their paper published in the journal Science, William McMahon and Neil Davies describe their study and analysis of published papers reporting work with mud rock, and how they connected its increase to the arrival of rootless plants. Woodward Fischer with the California Institute of Technology offers a Perspectives piece on the work done by the team in Britain in the same journal issue.
As the researchers point out, geologists have on occasion noted that long-ago sediment deposits tend to grow muddier in some places, which might have something to do with plants. In this new effort, the researchers have tested this idea by studying 1200 published papers describing work surrounding mud rock—they also collected and analyzed samples from 125 river outcrops.
Mud rock has been around for a long time, the pair note, but it was sparse, making up just 1 percent of ancient river deposit material prior to 458 million years ago, the data showed. For some reason, the amount of mud rock began to rise, reaching 26 percent around 359 to 299 million years ago. They note that the increase was steady, suggesting it was not cyclical or due to events such as the movement of glaciers. That left just one option—plants.
Bryophytes, the researchers note, became common around 450 million years ago, right about the time mud rock started to become more common. The rootless plants, similar to liverworts and mosses, would have thrived in the wet environment of ancient river systems. But the researchers wondered why the plants might have caused more mud rock to develop. They suggest such plants could have tempered the impact of wind and rain against rocks. They also think it was possible that they secreted organic acids that caused changes in soil chemistry. There is even a chance, they note, that they caused changes in landscape—altering the paths of rivers and streams, for example, by adding a stabilizing factor to riverbanks.
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William J. McMahon et al. Evolution of alluvial mudrock forced by early land plants, Science (2018). DOI: 10.1126/science.aan4660