The End Ordovician Mass Extinction
This plot shows “extinction intensity”, defined as the fraction of genera that go extinct over a short time period. There are thought to be 5 major mass extinctions during the Phanerozoic, marked and labeled on this graph. The first of these on the list is the end-Ordovician.
Because it is so long ago, scientists are still working to understand the cause. A new paper measured geochemistry of rocks crossing this boundary from what is today South China and found a pair of high-mercury layers at that boundary. One possibility is that this high mercury content represents the output from extremely active volcanoes, as is thought to be the driver of the end-Permian Mass Extinction (https://tmblr.co/Zyv2Js2OUCRsV). However, as of now there is no known “Large Igneous Province” at the time, leaving plenty more work to do to understand this extinction.