Counted among the five great extinctions, the Late Devonian crises were not a single bad day but a sequence: two main pulses spread over time with smaller events between. The first is the Kellwasser Event (around 372 million years ago, at the Frasnian–Famennian boundary); the second is the Hangenberg Event (around 359 million years ago, at the Devonian–Carboniferous boundary). Marine life took the worst of it: shallow-reef builders (stromatoporoids, tabulate and rugose corals), brachiopods, and trilobites declined sharply, and the armored placoderms — the icon of the age — were wiped out entirely.

The strongest explanations point to environmental system change. The rapid spread of vascular land plants drove deep roots into rock, accelerating weathering and washing nutrients into the oceans; that triggered algal blooms in shallow seas and widespread anoxic conditions. The climate cooled, sea level fluctuated, and there is rock-record evidence of large volcanic episodes. The combined effect left its mark less as outright disappearance than as a long silence of speciation — a slowing of the generation of new forms.

The Devonian's legacy is also the first steps of land vertebrates (the earliest tetrapods are known from about this same window). The extinctions did not close that scene but reshaped it: the door to the Carboniferous — giant insects, vast coal forests, and the first reptiles — opened through the gap.