Short answers, pulled from the story.
The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction that occurred around 372.15 million years ago at the boundary between the Frasnian and Famennian ages. It is ranked among the Big Five most severe mass extinctions in Earth's history, with likely around 40 percent of marine species going extinct.
The Kellwasser event of the Late Devonian mass extinction occurred around 372.15 million years ago. A second extinction, the Hangenberg event, followed about 13 million years later around 358.86 million years ago, bringing an end to the Devonian period.
The causes remain unclear, but leading hypotheses include ocean anoxia marked by black shale, global cooling, and oceanic volcanism such as the Viluy Large igneous province. The spread of deep-rooted land plants may have driven weathering, nutrient runoff, algal blooms, and the resulting anoxia.
Likely around 40 percent of marine species went extinct in the Late Devonian mass extinction. A 1996 survey estimates 22 percent of marine animal families were eliminated, while 57 percent of genera and at least 75 percent of species did not survive into the Carboniferous.
Hard-hit groups included reef-building stromatoporoids and rugose and tabulate corals, along with brachiopods, trilobites, ammonites, conodonts, and graptolites. Three trilobite orders vanished, cystoids disappeared, and most jawless agnathan fish died out by the end of the Frasnian.
It is named for its type locality, the Kellwassertal in Lower Saxony, Germany. The Kellwasser was the first pulse to be detected from the marine invertebrate record and was the most severe of the Late Devonian extinction crises.