Why Did 75% of Marine Life Vanish During the Late Devonian Extinction 375–360 Million Years Ago? — Here’s What Researchers Found

The Devonian Period, more than 375 million years ago, was often referred to as the age of fishes just because of how diverse and rich the ecosystems are underwater and in the world’s oceans. There were large coral reefs that were home to different species, like armored fish and sponge-like sea creatures. But near the end of this period in Earth’s history, something happened that caused a widespread catastrophe for the present lifeforms.

About 375 to 360 million years ago, a series of different crises hit the oceans, and along with it came the downfall of countless marine animals and other beings. However, what exactly could have caused this period that is brimming with so much life to come to an end? And how did it change our seas forever? Let’s explore these questions in this article.

What Was the Late Devonian Extinction?

The Late Devonian Extinction refers to the mass annihilation of marine life in the Earth’s ocean around 375 to 360 million years ago. An estimated 75% of all species were eradicated during these phenomena. But it was not caused by one single disaster. Instead, it is believed that it happened because of a series of events spread over millions of years.

Two of the biggest extinction pulses or events during the Late Devonian Extinction were the Kellwasser event and the Hangenberg event. The Kellwasser event was thought to have happened around 372 million years ago in the Frasnian-Famennian boundary, impacting warm-water marine organisms such as reef-building corals. These sea creatures were nearly wiped out, but some survived.

However, the Hangenberg event that occurred later on, around 359 million years ago, finished off many of the organism groups that survived the Kellwasser event, leading to near total disappearances of some reef ecosystems. By the time the end of the Devonian period came and the beginning of the transition to the Carboniferous period, about three-quarters of all animal species had disappeared forever.

Read more: Wikipedia

Which Sea Creatures Suffered the Most?

What Killed 75% of Life? The Devonian Mass Extinction Explained

During the Devonian period, life on land of the Earth was only getting started, so the main victims of the extinction event were primarily marine animals and other sea creatures. Before the different crises began, the Devonian seas were filled with coral-building reefs and even sponge-like reef builders.

Just like the modern coral reefs we have today, these creatures were home to a lot of different species. Moreover, just like today, once these organisms were destroyed, the animals living in them also suffered the same fate. So, during the late Devonian extinction event, many types of corals and fish that considered them home were nearly wiped out. Some types of coral reefs did not even recover until millions of years later down the line on Earth’s history.

Famous fossil animals like the trilobites were already thought to be in decline during this era, but this specific extinction event made them lose even more diversity, leading to their eventual disappearance later on. Another type of creature that was heavily impacted was the shell-bearing marine animals, including brachiopods, an animal that has two shells.

Additionally, one of the most prominent groups of organisms living in the oceans during the Devonian period was the armored fishes. They were highly successful ecologically and have dominated the marine and freshwater ecosystems. However, by the end of the period, they were also gone forever.

The impact of this extinction event was immense, but we still have not yet discussed what could have been the reasons for the disappearance of these lifeforms. So, let’s get over some of the theories that evidence and research point to.

What Do Scientists Think Caused the Die-Off?

Late Devonian Mass Extinction Event ~360 Million Years Ago | GEO GIRL

Scientists do not believe that the cause was just one single reason or factor, but instead, it is most likely a combination of challenges that happened over a vast period of time. One of the most important theories is that it was caused by ocean anoxia. It simply refers to the lack of oxygen in the water.

Researchers think that this is one of the likely reasons because most sea creatures would not have survived without adequate oxygen, in both shallow and deep waters. What they found is that there are many rock layers that form during this time period that contain black shales, which often form when deep waters lack oxygen.

Some of the theories explaining this phenomenon were linked to the growth of early forests on land. The plant invasion may have caused some minerals and nutrients to be washed away into the oceans, causing algae to bloom massively. This scenario could have caused the oxygen levels to go down because the dense mass of algae would have needed a massive amount of dissolved oxygen during their bacterial decomposition when they perish, which would have created “dead zones” or areas in the water that have little to no oxygen.

Another major idea that could have caused the Late Devonian Extinction was the rise and fall of sea water levels. As the glaciers grew and melted, sea levels may have shrunken, causing the reefs and shelf animals living in warm, shallow waters to lose their homes and face an extinction-level event. Then, if the sea level rises again, it could give them additional stress and another challenge to adapt to.

These are some of the most prominent theories, but it is likely that the combination of both, together with a few other possible factors such as volcanism, asteroid impacts, and depletion of ozone layers, is the reason why the Late Devonian Extinction occurred.

Read more: Berkeley Museum of Paleontology

How Did the Late Devonian Extinction Change Life’s Future?

Even though the Late Devonian Extinction was terrible for many species, as they are now gone forever, it also paved the way for new types of animals and creatures to rise and dominate the new ecosystem. When the reef builders vanished, it took tens of millions of years for reef systems in the ocean to fully return. During this time, space opened up for other types of communities to evolve and develop, such as sea buds and sea lillies like blastoids and crinoids.

Additionally, the loss of large fish that dominated the oceans, like armored fish, created a period of recovery and evolution that led the surviving fish and the first land-walking vertebrates to a new form of life. Eventually, over millions of years, the modern fish groups appeared, and also the first four-limbed animals on land. So, another way of looking at it is that the Late Devonian Extinction event helped in setting the stage for the rise of the vertebrates on land and the development of future ecosystems in the waters.

Author's Final Thoughts

The Late Devonian Extinction was a series of crises that mainly struck the Earth’s marine ecosystems and organisms. Over millions of years, different challenges slowly and gradually erased various species and animal groups, to the point that about 75% of all marine life was estimated to have gone extinct.

Yet, from this loss of life, a new chapter begins. One that may have possibly only happened after the end of the last period. With old ecosystems gone, new groups of fish and other creatures were given room to develop. In time, these survivors were able to give rise to the more modern marine life and land creatures, from whom some of our present animal species may have descended.

What we could learn about the Late Devonian Extinction is that Earth’s life and environment were deeply linked, and the development of one part could have caused the downfall of another. But, just as likely, the collapse of one could have led to the development of new life on the planet.

Read more: Scientists Say Gut Feelings Could Be Memories From the Future — And Here’s the Strange Theory Behind It

References & Further Reading

McGhee, G. R., Jr. (1988). The Late Devonian extinction event: Evidence for abrupt ecosystem collapse. Paleobiology. https://doi.org/10.1017/S0094837300011982

Joachimski, M. M., & Buggisch, W. (1993). Anoxic events in the late Frasnian—Causes of the Frasnian–Famennian faunal crisis. Geology. https://www.researchgate.net/publication/236862035_Anoxic_events_in_the_Late_Frasnian-Causes_of_the_Frasnian-Famennian_faunal_crisis

Carmichael, S. K., Waters, J. A., Königshof, P., Suttner, T. J., & Kido, E. (2019). Paleogeography and paleoenvironments of the Late Devonian Kellwasser event: A review of its sedimentological and geochemical expression. Global and Planetary Change. https://doi.org/10.1016/j.gloplacha.2019.102984

De Vleeschouwer, D., Da Silva, A.-C., Sinnesael, M., Chen, D., Day, J., Whalen, M. T., Guo, Z., & Claeys, P. (2017). Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity. Nature Communications. https://doi.org/10.1038/s41467-017-02407-1

Stigall, A. L. (2012). Speciation collapse and invasive species dynamics during the Late Devonian “Mass Extinction”. GSA Today. https://doi.org/10.1130/G128A.1