The end-Ordovician mass extinction, killing roughly 86% of all marine species, is now linked to nutrient-driven anoxia in the global ocean.
This marine catastrophe has previously been attributed to a cooling event of a warm Earth, as a glacier can be seen to grow on the South Pole, and to increased oxygen in the ocean.
The cooling appears to be real, but from a hot to less hot greenhouse planet which not necessarily led to stress for the animals.
Also, instead of increasing oxygen the resent results point towards severe lack of oxygen at depth in the ocean. The conclusion of ocean anoxia is inferred primarily from sulfur isotope data from sea floor sediments in three ocean basins worldwide.
The new data overthrows century old knowledge of why marine animals met their first of three major challenges, and highlights how evolution of life is tightly coupled to the dynamics of oxygen in the ocean.