There are three important extinctions in latter half of the Devonian Period, each separated by about 10 million years. Only one of these, at the end of a time interval called the Frasnian, is normally considered large enough to be one of the “Big Five.”
When did they happen?
The end-Frasnian extinction happened about 375 million years ago. The oldest of the three extinctions, towards the end of a time interval called the Givetian, occurred about 10 million years before the Frasnian event. The youngest extinction happened near the end of the Devonian period, about 365 million years ago, during a time interval called the Famennian.
Who became extinct?
The end-Frasnian extinction was most pronounced in tropical environments, particularly in the reefs of the shallow seas. Reef building sponges called stromatoporoids and corals suffered losses and stromatoporoids finally disappeared in the third extinction near the end of the Devonian. Brachiopods associated with reefs also became extinct. Groups of trilobites disappeared at each of the three extinctions and very few survived into the following Carboniferous Period. Examples of groups of brachiopods and trilobites that became extinct are shown below.
How did they happen?
As extinctions were mostly of tropical groups climate change may have been involved, and there is geological evidence for cooling of the global climate at the end-Frasnian event and near the end of the Devonian Period.
Cooling may have been caused by a drop in the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas that helps warm the planet, so if levels fall, cooling will follow. In the Late Devonian, large trees evolved and formed the first forests. As plant life expanded, they used up more carbon dioxide in photosynthesis. When dead plant material decays, carbon dioxide is returned to the atmosphere, but some plant material (e.g., leaves) will be buried in swamps, lakes and rivers. This buried plant material removes carbon permanently from the atmosphere and often forms coal. When we mine coal and burn it we return carbon dioxide to the atmosphere and warm the planet.