23% of marine animals were lost over 420 million years ago. Rapid and widespread depletion of oxygen was preceded by the Lau/Kozlowsii extinction. The Lau/Kozlowsii extinction event coupled with a carbon disruption event one hundred thousand years apart required science to link the events. The lesson from this discovery can assist in monitoring existing conditions by understanding deoxygenation causes incredible stresses on organisms who rely on oxygen for survival.
https://phys.org/news/2019-08-oxygen-depletion-ancient-oceans-major.html