Hangenberg extinction

The Hangenberg event, at the Devonian-Carboniferous boundary about 359 million years ago, is a short but severe extinction. It takes its name from a black shale and sandstone sequence in the Rhenish Massif of Germany. Marine losses were heavy among the placoderms, most of the early ammonoids, many trilobites and ostracods, and reef animals, while some land vertebrate lines also suffered. The Late Devonian extinction itself came in pulses, and Hangenberg is the final big one.

The sequence of events is clearer than the cause. Black shale shows the oceans lost oxygen, and cooling brought glaciers to Gondwana, which dropped sea level and then raised it again as the ice melted. Some studies find mercury spikes pointing to volcanism, a link not agreed on. After the crisis, the surviving fish, including ray-finned fish and sharks, filled the empty roles, and tetrapods almost vanish from the record for about 15 million years in a stretch called Romer's gap, named for Alfred Romer, before reappearing in the Carboniferous. Whether the gap is a real bottleneck or a gap in fossil finding is debated.