The Kellwasser event

At the Frasnian–Famennian boundary, about 372 million years ago, widespread ocean anoxia destroys the Devonian reef ecosystems and devastates shallow-water faunas. Stromatoporoids never recover, and placoderms are badly hit. Two distinct black shale horizons record the crisis, and it is the first and larger of the two pulses that together make up the Late Devonian mass extinction.