A 372-million-year-old extinction wiped out reefs and helped create modern oil fields
About 372 million years ago, the Kellwasser event struck the oceans, erasing perhaps 40 percent of marine species and nearly every reef-building animal. The oxygen-starved seas that came with it preserved so much organic matter that Devonian rocks now supply oil across Canada and the United States.
The Late Devonian extinction sits at the boundary between the Frasnian and Famennian ages and ranks among the Big Five mass extinctions, though how severe it was is debated. Its timing is fuzzy too. Estimates of its duration range from half a million to 25 million years, and some researchers split it into as many as seven separate pulses. A second crisis, the Hangenberg event, closed the Devonian about 13 million years later, and the two have often been blurred together.
The world looked unfamiliar. Gondwana covered much of the southern hemisphere while Laurussia drifted toward it, beginning the assembly of Pangaea. On land, plants had recently evolved roots, seeds and internal plumbing, letting forests spread onto dry highlands, and fish relatives such as Tiktaalik were developing leg-like fins. The extinction, however, seems to have struck only marine life, hitting warm shallow waters and low latitudes hardest.
The great Devonian reefs, built by stromatoporoid sponges and rugose and tabulate corals, collapsed so completely that reefs did not regain their earlier extent until the Mesozoic, built by stony corals that had not yet evolved. Three orders of trilobites vanished, leaving just two. Roughly 80 percent of ostracod species died out worldwide, reaching 91 percent in Eastern Europe. Survivors show telling changes: trilobites evolved smaller eyes before the event, hinting at darker or murkier water, and wider head rims that may have helped them breathe in low-oxygen seas.
The cause remains unclear. Leading ideas include sea-level change and ocean anoxia, perhaps driven by global cooling or undersea volcanism, and an impact such as the Siljan Ring event in Sweden has been proposed. Some statistics suggest diversity fell more because new species stopped appearing than because existing ones died faster.
Source: Late Devonian mass extinction
