The tree may have wiped out 70 percent of Earth’s life
Now a new theory is being launched about the cause of the mass death more than 300 million years ago. During the Devonian geological period, a tree developed its first roots – which had major consequences for the ecosystem.
What is the background?
Dominant species have always had to give way to new flora and fauna, but during the history of the earth there have been five major mass extinctions in which species richness has suddenly decreased on a broad front.
One of them occurred at the end of the Devonian geological period, about 364 million years ago. However, it is unclear how long the Devonian extinction lasted, and estimates vary from half a million years to 15 million years.
What explanations have there been?
The extinction during the Devonian occurred mainly in the sea and primarily among animals in warm waters. Oil-rich shale shows that the seabeds were deoxygenated, but the reason for the change is not determined.
An explanatory model is a meteorite, and there Siljan has been mentioned as a candidate. Opinions also differ as to whether these are two major events or a larger number of smaller events.
A fossilized tree trunk from the Late Devonian. Photo: Retallack / Wikipedia
What is the new theory?
A study conducted at Indiana University-Purdue University Indianapolis shows that one reason for the mass extinction may be that trees belonging to the fern genus Archaeopteris began to develop complex roots.
They picked up nutrients from the still thin soil – and according to the researchers, the main problem was dead roots.
Since today’s microorganisms were not yet in place in the soil, the nutrients could not be returned to the soil, but were washed into the oceans. That’s what Indiana University writes in one press release.
What effects did the emissions have?
The main culprit in the drama was the soil’s phosphorus. Just as with emissions from today’s agriculture, the nutrients led to eutrophication, which in turn produced a massive algal bloom – with a lack of oxygen in the oceans as a consequence. The Devonian extinction is thought to have wiped out close to 70 percent of life on Earth.
Archaeopteris was the first plant with complex roots, and first with leaves. Photo: Retallack / Wikipedia
What evidence do you rely on?
The researchers analyzed traces of phosphorus in former seabeds, including in Greenland and Scotland. In the rock formations you could see that the amount of phosphorus increased and decreased cyclically. Traces of the roots pointed to dry periods where many trees died, and this coincided temporally with high phosphorus levels in the seabed. In this way, the researchers believe they can connect the roots to the mass death.
What lessons can we learn?
Eutrophication and algal blooms are a global problem, and scientists believe that the Devonian natural disaster can serve as a warning of the consequences of an imbalance in the world’s oceans as a result of human activity. The study has been published in The Geological Society of America Bulletin.
