First land plants trigger mass extinction / Ordovician

If present day life is getting you down, try a getaway to the Ordovician.

Why it’s great to have a geologist in the house

By Nancy Shute, March 22, 2018

Science has a way of surprising us when we least expect it. Like with mud rocks.We science journalists can be a cranky lot, eternally skeptical as to whether a touted advance is really significant enough to warrant coverage. So when Science News’ managing editor Erin Wayman waxed enthusiastic about a study explaining how ancient plants may have played a key role in making Earth muddier, I perked up.

Geologists have long known that mud started to take hold at some point, but as earth and climate writer Carolyn Gramling reports in this issue, “no one had ever pinpointed when that muddening happened.”

Clearly erosion must have been a factor, but that’s as far as my mud expertise goes. A geologist I am not. Fortunately, Gramling is a geologist, with bachelor’s degrees in both geology and European history and a Ph.D. in marine geochemistry. I asked her what it was about this study that convinced her it was worth a look. “It struck me because I like to know what makes things tick,” she said during a conversation in my office. “It was surprising.”

This wasn’t a big sexy science story: no neutron star collisions, no gene-editing breakthroughs, no advances in immunotherapy. Instead, we have grayish rocks. But they have something to tell us. The researchers, at the University of Cambridge, looked at ancient riverbed deposits and found that the amount of mud rock, which is primarily made of clay, silt and other fine particles, increased about 458 million years ago. (Ordovician) That’s also when a group of primitive land plants known as bryophytes, which include modern mosses and liverworts, became common on Earth. The fact that bryophytes could have had that much impact is another surprise, Gramling said. “These are not rooted plants,” she added. “They’re these little mats of mosses on the surface, but they still have this profound effect.” Indeed, the author of a commentary accompanying the study in Science called the plants “tiny, little scrappy things.”

I like the notion of scrappy little underdog plants helping to transform the face of our planet. And I very much like having a writer on staff who’s a scientist with deep expertise who can say, yes, this is as neat as it sounds.

Gramling was quick to note that we don’t know exactly how ancient plants made Earth muddier. But even simple plants can help keep wind and water from eroding sediments. Plants may also help break down rock chemically, too.

Researchers study ground-covering plants, resembling the earliest land-dwelling plants, on a lava field in Iceland (Credit: Paul Kenrick)


Origin and early evolution of land plants

Problems and considerations


The origin of the sporophyte in land plants represents a fundamental phase in plant evolution. Today this subject is controversial, and scarcely considered in textbooks and journals of botany, in spite of its importance. There are two conflicting theories concerning the origin of the alternating generations in land-plants: the “antithetic” theory and the “homologous” theory. These have never been fully resolved, although, on the ground of the evidences on the probable ancestors of land plants, the antithetic theory is considered more plausible than the homologous theory. However, additional phylogenetic dilemmas are the evolution of bryophytes from algae and the transition from these first land plants to the pteridophytes. All these very large evolutionary jumps are discussed on the basis of the phyletic gradualist neo-Darwinian theory and other genetic evolutionary mechanisms.


First land plants plunged Earth into ice age

Never underestimate moss. When the simple plants first arrived on land, almost half a billion years ago, they triggered both an ice age and a mass extinction of ocean life.

The first land plants appeared around 470 million years ago, during the Ordovician period, when life was diversifying rapidly. They were non-vascular plants, like mosses and liverworts, that didn’t have deep roots.

About 35 million years later, ice sheets briefly covered much of the planet and a mass extinction ensued. Carbon dioxide levels probably fell sharply just before the ice arrived – but nobody knew why.

Tim Lenton of the University of Exeter, UK, and colleagues think the mosses and liverworts are to blame.

Moss versus rock

It’s not the first time that plants have been fingered as a cause of glaciation. Researchers already suspect that the rise of vascular plants in the Devonian period, some 100 million years later, triggered another ice age. The plants’ roots extracted nutrients from bedrock, leaving behind vast quantities of chemically altered rock that could react with CO2 and so suck it out of the atmosphere.

Non-vascular plants like mosses don’t have deep roots, so it was thought that they didn’t behave in the same way. Lenton suspected they might have played a role nevertheless. To find out, he set up an experiment to see what damage a common moss (Physcomitrella patens) could inflict on granite. After 130 days, rocks with moss living on them had weathered significantly more than bare ones – and about as much as they would have if vascular plants were living on them. “The secret seems to be that the moss secrete a wide range of organic acids that can dissolve rock,” Lenton says.

When Lenton added this effect of non-vascular plants to a climate model of the Ordovician, the CO2 dropped from about 22 times modern levels to just eight times modern levels. That was enough to trigger an ice age in the model of Ordovician Earth.

In his experiments, the non-vascular plants also released lots of phosphorus from rocks. Much of this would have wound up in the ocean, where we know it can trigger vast algal blooms. As other bugs feasted on the algae, they would have used up the oxygen in the water – suffocating oxygen-breathing animals and accounting for the mass extinction of marine life known to have occurred at the end of the Ordovician.

Although the first land plants were responsible for these mass deaths in their ocean-dwelling neighbours, Lenton says they themselves probably came out of the Ordovician ice age largely unscathed. That’s because the ice was concentrated around the South Pole, while the plants lived in the tropics.

Life may also have caused an even harsher cold snap much earlier in Earth’s history. The first complex animals appeared some time around 800 million years ago, and may have sucked so much CO2 from the atmosphere that the entire planet froze over in a “snowball Earth“.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo139

 Crinoids and Rugose Corals. 

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