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Devonian Mass Extinction: Causes, Facts, Evidence & Animals

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

In the late Devonian period, something happened and more than half of life on Earth died. In this lesson, we're going to explore the Devonian mass extinction and see what could have caused it.

The Devonian Mass Extinction

Imagine winning the lottery (yay!) Now imagine going bankrupt 100 days later (no!) How could such a dramatic change happen? Well, it could be that one big, traumatic event happened. Maybe the stock market crashed. Or, maybe you spent too much, made a bad investment, broke your arm and had medical bills, had to repay some loans, hail ruined your car, your television broke and then lost your job unexpectedly. By itself, a single event on this list is unlikely to eliminate your entire lottery paycheck. Together, however, they have that potential.

This is what happened in the late Devonian, a geologic period lasting 465-359 million years ago. The Devonian began with a flourishing of life and genetic diversity, and ended with mass extinction. Some mass extinctions were caused by a single major, traumatic incident, but that doesn't seem to be the case with the Devonian mass extinction. In fact, this mass extinction was really 8-10 different extinction events over 20 million years; compounded together, they created one massive loss of genetic diversity, as about 75% of the world's species went extinct.

Life in the Late Devonian

After surviving a mass extinction at the end of the Ordovician, life rebounded in the Silurian, and then Devonian periods. The oceans flourished as ammonites, bony fish, and sharks all first appeared. This was also the era when life really started taking hold on land, as the first true forests developed, along with amphibians and insects.

Life exploded in the Silurian and Devonian periods, especially in warm and shallow waters
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The real winners of the Devonian, however, were the coral reefs. A massive amount of life on Earth at this time was focused around coral and reef-building species. In fact, Devonian oceans had some of the largest and most diverse coral reefs in the history of the planet. These reefs provided a habitat for thousands of species, all of which were perfectly adapted to tropical, shallow, coastal waters. As long as nothing happened to this specific environment, life would be fine.

Unfortunately, things did start happening to that specific environment. Changes in the late Devonian hit shallow, warm waters extremely hard and fossil records indicate that this is where the most extinction occurred. In all, about 20% of all marine families went extinct. Groups particularly impacted included jawless fish, brachiopods, ammonites, and trilobites. In fact, of all the orders of the class Trilobita, only one survived beyond the Devonian. As for the corals, they were hurt so badly that it would be another 100 million years before they recovered.

Causes of the Devonian Extinctions

Interestingly, terrestrial plants and animals of the late Devonian were barely impacted at all, so we know that whatever happened during the Devonian period was focused around those shallow, tropical waters. So, what was it? What caused the 8-10 extinction events within 20 million years that compounded into a mass extinction? There are a few theories.

For one, some have looked for evidence of a meteor or asteroid impact (because thanks to the extinction of the dinosaurs, that's always the first assumption.) There is some evidence to support this. In particular, the Siljan structure is an impact crater in Sweden that was created in the late Devonian. It's been proposed that this contributed to one of the extinction events of the period (called the Kellswasser event,) but its connection to the greater Devonian extinction is questionable. It doesn't seem to have had a large enough impact to have killed 3/4 of all life on Earth (although, admittedly, it didn't help.)

A more intriguing clue can be found in thick layers of black shale that date to the late Devonian. Shale like this is created in very low-oxygen environments. This indicates that the seabed at this time was almost entirely de-oxygenated. Some scientists believe that there may have been so little oxygen near the seafloor that only bacteria could have survived.

Devonian shale like this may provide clues as to what cause the mass extinction
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