Around 360 million years ago, the Devonian Period came to an end.
Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian Period, it is not clear over how long a period these extinctions took place, with estimates varying from 500 thousand to 15 million years. Neither is it clear whether it was a single mass extinction or a series of several smaller ones one after the other. Nevertheless, the balance of evidence suggests that the extinctions took place over a period of some three million years beginning about 374 million years ago.
During the late Devonian extinction, as many as seventy percent of all species vanished. Marine species were more severely affected than those in freshwater. Brachiopods, ammonites, and many other invertebrates suffered heavily, as did agnathan and placoderm fish. On land, where plants were diversifying and amphibians were beginning their evolution, there seem to have been far fewer losses.
The causes of the Devonian extinction are unclear. The disproportionate losses amongst warm water species suggest that the Earth's climate changed -- most likely for the cooler. The global cooling was an important factor and it has been suggested that this was associated with (or may have even caused) a drop in the oxygen levels of the shallower waters.
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