The Devonian is known as the Age of Fish. This period, from 419 to 359 million years ago, is the period when marine life “exploded”. Paleontologists have found a large number of fish fossils belonging to this period, showing geographical spread and diversity.
Now, scientists have found evidence that this period coincides with a rise in oxygen levels in deeper ocean waters. This could explain why the Devonian saw so much fish diversification.
The news expands our understanding of ancient oceans
For decades, scientists have debated when Earth’s deep oceans first became permanently rich in oxygen. Some studies placed the moment as far back as the Ediacaran Period (about 635–541 million years ago). Newer work pointed much later, into the mid-Paleozoic Era. Pinning down this timeline is important, because oxygen availability shaped how animal life spread and evolved.
The study concluded that there was no evidence for lasting oxygen in the deep ocean until the Middle Devonian (393–382 million years ago). This timing overlaps with the “mid-Paleozoic marine revolution”, a burst of animal innovation when marine ecosystems became more complex and active.
“In the Middle Devonian (393 to 382 Ma), our data document the onset of permanent deep-ocean oxygenation, coincident with the spread of woody biomass across terrestrial landscapes”, the researchers wrote.
“This episode is concurrent with the ecological occupation and evolutionary radiation of large active invertebrate and vertebrate organisms in deeper oceanic infaunal and epifaunal habitats, suggesting that the burial of recalcitrant wood from the first forests sequestered organic carbon, increased deep marine oxygen levels, and was ultimately responsible for the “mid-Paleozoic marine revolution.””
The answer could be trees
The rise in oxygen seems tied to life on land. As the first woody forests spread across Earth’s continents, they buried large amounts of carbon in the ground. This process would have freed up oxygen, and raised oxygen levels in the oceans.
With this new, stable oxygen supply, larger and more energetic invertebrates and vertebrates could colonize deeper habitats. “The oxygenation history of Earth’s surface environments has had a profound influence on the ecology and evolution of metazoan life.” said the researchers.