The trees responsible for the Devonian extinction?

The Devonian extinction event occurred around 380 to 359 million years ago. It saw the disappearance of about 75% of species in one fell swoop, especially for marine organisms. Plants and animals seem to have resisted better on dry land, but the poverty of the terrestrial fossil record makes accurate assessment difficult. The dynamics of this extinction raise questions as scientists estimate it may have occurred in 3 episodes spanning 20 million years. Likewise, the causes of these massive disappearances are still debated, and a new, already differently worded, hypothesis is being proposed by scientists at the University of Indiana.

The appearance of trees

The Devonian is also called “the Age of Fishes” because of this period an important diversification of this group and the appearance of lineages that represent the origin of modern vertebrates as well as the first life forms that will experience the exit from the Devonian ‘water. But a great transformation also took place on the mainland: the rise of the first trees. with l’Archeopteris as a symbol considered by professionals to be the closest tree to current forms. The advent of trees profoundly changed the appearance of the Earth and perhaps its oceans, paleontologists claim in a study published in the journal Bulletin of the Geological Society of America. According to them, the rotting of the roots of the first trees led to a massive discharge of organic matter into the oceans. It caused widespread algal blooms, which themselves consumed almost all ocean oxygen, leading to periods of marine extinction.

This process, known as eutrophication, is similar to what is now happening on a smaller scale in some regions around the world. The cause is different: it is the agricultural waste (manure, fertilizers, etc.) that provides the organic matter that feeds the algae. In fact, nature has adapted over 350 million years and can compensate for the organic contribution associated with tree rot. A large part is now absorbed by the much deeper soils than in the Devonian and therefore does not end up in the water.

Collecting samples from the island of Ymer in East Greenland, one of the many sites whose analysis has led to a better understanding of the chemical composition of the Devonian seabeds. Photo credit: John Marshall, University of Southampton.

wet and dry cycles

To support their hypothesis, the researchers examined the rock deposits of ancient lake beds, remnants of which exist around the world, particularly in Greenland. The scientists were particularly interested in the level of phosphorus, a chemical element present in all life on Earth. Their analysis thus indicates the existence of a Devonian phosphorus cycle, with dry periods corresponding to high concentrations of phosphorus released by the dying roots of trees, and wet periods with lower concentrations of this element and more roots growing in soils. A result consistent with their theory, which implies root decay as the main cause of the Devonian extinction.

But other hypotheses such as volcanism or even the explosion of a star close to the sun could also explain the lack of oxygen in the oceans. This last explanation applies primarily to the last Devonian extinction event marking the end of this period and the beginning of the Carboniferous, dubbed the “Hangenberg Crisis”.

Leave a Reply

Your email address will not be published. Required fields are marked *