The history of the Late Ordovician mass extinction, the first one in history

 It was almost twice as deadly as the mass extinction that killed the dinosaurs.

85% of marine species on Earth were eliminated during this extinction.

It was the first one, and it's the second most severe to date. It occurred more than 440 million years ago (mya). The Late Ordovician mass extinction, also known as the Ordovician-Silurian or the end-Ordovician mass extinction, destroyed the 49-60% of marine genera and the 85% of marine species. Only the Permian-Triassic killed more species -it's not known as the Great Dying for no reason!-. But how did the first ever mass extinction happen? Let's discover it.

As its own name indicates, this event occurred between the late Ordovician and the early Silurian period. There were two different phases or pulses in the extinction. 

The first pulse began roughly as the same time as the Hirnantian stage, the last of the Ordovician Period, 445.2 mya. What happened was a major drop in temperatures -possibly caused by a descent in carbon dioxide levels due to the erosion of silicate stones-, which led to a glaciation (the Late Ordovician one) and an important decrease in sea level. All of that guided to the death of many marine species, especially those who inhabited the open, shallow parts of the sea. Lots of brachiopods, bryozoans, conodonts, trilobites, echinoderms, corals, bivalves and graptolites passed away. A fact that displays the severity of the conditions is that some brachiopods -who were cold-water animals- travelled to tropical zones. The global cooling was huge.

After that, when the temperature started rising -during the last years of the Hirnantian stage-, the second pulse of the extinction began. There was intense euxinia in the entire world. Euxinia is a state in which the water has no oxygen and a high amount of hydrogen sulfide. These euxinic conditions expanded to the Rhuddanian stage, the first of the Silurian period.

Despite the intensity of the Late Ordovician mass extinction, diversity came back to the levels where it was before this event in 5 million years, and the ecosystems of the Silurian period weren't significantly different to the ones of the Ordovician.