Ocean Oxygenation during Mid-Devonian Enabled Expansion of Animals into Deeper-Water Habitats

About 390 million years ago (Devonian period), marine animals began to colonize the depths previously uninhabited. New research by scientists from the Duke University, the University of Washington, the Virtual Planetary Laboratory of NASA and Caltech, indicates that this underwater migration occurred in response to a permanent increase in deeply oceanic oxygen, caused by the higher propagation of woody plants; This increase in oxygen also coincided with a remarkable diversification period in jaw fish.

Artistic rendering of Brindabellaspis Stensioi (foreground) with a range of other Devonian fossil fish; The white shark and the human diver in the upper right corner represent vertebrates in modern jaw. Image credit: Hongyu Yang / Qiuyang Zheng.

“We know that oxygen is a necessary condition for the evolution of animals, but the measure in which it is the sufficient condition which can explain the trends in the diversification of animals has been difficult to identify,” said Dr. Michael Kipp, researcher at Duke University.

“This study gives a strong vote according to which oxygen dictated the moment of the evolution of early animals, at least for the appearance of vertebrates in the jaw in the habitats of the deep ocean.”

For a certain time, the researchers thought that the deep oxygenation of the ocean occurred once at the start of the Paleozoic era, about 540 million years ago.

But more recent studies have suggested that oxygenation has occurred in the phases, the water in depth first becoming habitable to respiratory organizations, followed by deeper environments.

Dr. Kipp and his colleagues went at the time of these phases by studying the sedimentary rocks which were formed under deep sea water.

More specifically, they analyzed the selenium rocks, an element that can be used to determine whether oxygen existed at survival levels in old seas.

In the marine environment, selenium occurs in different forms called isotopes which vary depending on the weight.

When oxygen levels are high enough to support animal life, the relationship of heavy to light selenium isotopes vary considerably.

But at prohibitive oxygen levels to most animal lives, this report is relatively coherent.

By determining the report of selenium isotopes in marine sediments, researchers can deduce if oxygen levels were sufficient to support animals that breathe underwater.

In collaboration with research standards worldwide, the team has produced 97 rock samples dating from 252 to 541 million years.

The rocks had been excavated in areas of five continents which, hundreds of millions of years ago, were located along the most external continental shelves – the edges of the continents while they exceed underwater, just before giving way to steep drops.

After a series of steps that involved spraying the rocks, dissolving the resulting powder and purifying selenium, the team analyzed the selenium isotopes that occurred in each sample.

Their data indicated that two oxygenation events occurred in the deeper waters of the external continental shelves: a transitional episode approximately 540 million years ago, for a paleozoic period known as Cambrien, and an episode which started 393 to 382 million years ago, during a interval called the average devonian, which continued to date.

During intermediate millennia, oxygen fell to inhospitable levels for most animals.

“Selenium data tell us that the second oxygenation event was permanent,” said Kunmanee ‘Mac’ Bubphamanee, a doctorate. candidate at the University of Washington.

“It started at the average Devonian and persisted in our young rock samples.”

This event has coincided with many changes in ocean evolution and ecosystems – which some researchers call the medium -Paleozoic marine revolution.

While oxygen became a permanent characteristic in deeper environments, jaw fish, called gnathostomes and other animals began to invade and diversify in such habitats, according to the fossil file.

Animals have also become larger, perhaps because oxygen supported their growth.

The oxygenation event of the average Devonian also overlooked with the propagation of plants with hard wooden stems.

“Our thought is that, as these woody plants have increased in number, they released more oxygen in the air, which led to more oxygen in deeper ocean environments,” said Dr. Kipp.

The cause of the first temporary oxygenation event during the Cambrian is more enigmatic.

“What seems clear is that the drop in oxygen after this initial impulse has hampered the spread and diversification of marine animals in these deeper environments of external continental shelves,” said Dr. Kipp.

“Today, there is an abundant of oceanic oxygen in balance with the atmosphere.”

“But in some places, oxygen ocean can drop to undetectable levels.”

“Some of these areas occur through natural processes.”

“But in many cases, they are drawn by nutrients that escaped continents of fertilizers and industrial activity which feed the flowers of plankton which aspire oxygen when they decompose.”

“This work very clearly shows the link between oxygen and animal life in the ocean.”

“It was a balance about 400 million years ago, and it would be a shame to disturb it today in a few decades.”

The study appears this week in the Proceedings of the National Academy of Sciences.

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Kunmanee Bubphamanee and al. 2025. The oxygenation of the Mid-Devonian Ocean allowed the expansion of animals in deeper habitats. PNA 122 (35): E2501342122; DOI: 10.1073 / PNAS.2501342122