444 million years  –  444 metres

The continents freeze while moving through the southern polar region. The forming glaciers bind water and the sea level drops. Eighty-five per cent of the organisms in shallow seas go extinct.

Image: Edubucher | Wikimedia

Continents moved towards the southern pole, which cooled the climate and caused glaciations.  Deposits of these Ordovician glaciers can be found in the Sahara and in Argentina. A large fraction of the oceans was bound in the glaciers, causing the sea levels to sink, and the coastal areas to dry.  This destroyed habitats for the marine life that had flourished in shallow sees and led to the first large mass-extinction at the Phanerozoic eon. About 85 % of all species of the shallow seas went extinct.  Also changes in the global carbon and oxygen cycles contributed to the mass exinction.