The Late Devonian crisis

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The least event-like of the Big Five and the one that most tests the definition. Rather than a single horizon it is a prolonged interval of elevated extinction punctuated by two severe pulses: the Kellwasser event near the Frasnian–Famennian boundary around 372 million years ago, and the Hangenberg event at the Devonian–Carboniferous boundary around 359 million years ago. Both are associated with black shales indicating widespread ocean anoxia. The most-discussed hypothesis links the crisis to the spread of the first forests and deep-rooted vascular plants across the continents, which would have accelerated chemical weathering, delivered nutrients to the sea, driven eutrophication and anoxia, and drawn down atmospheric carbon dioxide enough to cool the climate. The Devonian reef systems, built by stromatoporoids and tabulate corals, collapsed and were never rebuilt in the same form. Several analysts argue that the interval is better characterised as a collapse in the rate of new species originating than as a spike in the rate of species dying.

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