Ordovician-silurian extinction

AROUND 440 MILLION YEARS AGO

Species made extinct 85%

The first mass extinction occurred when life itself was still in its infancy. Organisms such as corals and shelled brachiopods filled the world’s shallow waters, but hadn’t yet ventured onto land. When a climatic shift caused sea temperatures to change, the majority of life in the ocean died. By the end of the Ordovician Period, the rapid onset of mass glaciation covered the southern supercontinent, Gondwana. Glaciation on this scale locked away high percentages of the world’s water and dramatically lowered global sea levels, which stripped away vital habitats from many species, destroying food chains and decreasing reproductive success. It’s thought that the cooling process may have been triggered by the formation of the North American Appalachian Mountains. Large-scale erosion of these mountainous silicate rocks is associated with the removal of carbon dioxide from the atmosphere, which keeps the planet warm.

Alternative theories suggest that toxic metal may have dissolved into ocean waters during a period of oxygen depletion, wiping out marine life. Others suggest that a gammaray burst from a supernova ripped an enormous hole in the ozone layer, allowing deadly ultraviolet radiation to kill life below.

By the end of the Ordovician Period the seafloor was teeming with shelled creatures such as trilobites

The supercontinent Gondwana reached the South Pole by the late Ordovician Period Source: Wiki/fama Clamosa