Late Ordovician mass extinction

The Late Ordovician Mass Extinction (LOME) is also known as Ordovician-Silurian extinction. They are the second largest of the five major extinction events of the earth. The first largest was the Permian-Triassic Mass Extinction.

Five Major Extinction Events of the Earth

• Ordovician-Silurian Extinction: 440 million years ago
• Devonian Extinction: 365 million years ago
• Permian-Triassic extinction: 250 million years ago
• Triassic-Jurassic Extinction: 210 million years ago
• Cretaceous-tertiary Extinction: 65 million years ago

About LOME

• The LOME eliminated around 85% of the marine species from the earth. It killed more than one-third of bryozoans and brachiopods. It also destroyed several corals, conodonts, echinoderms, trilobites, bivalves and graptolites.
• It did not affect the ecosystem of the structures as that of the other mass extinctions.
• After the LOME, the lost biodiversity recovered during the first five million years of the Silurian period.
• The LOME occurred in two distinct pulses. They are Katian and Hirnantian.
• During the first pulse of LOME, called Katian, the earth shifted to ice cold climate from a green house climate. It led to continental glaciation. The continental glaciation was centred around Gondwana land which was then located in south pole. The falling sea levels and cooling destroyed the habitats. The fall in temperature killed several living organisms.
• During the second pulse of LOME, called Hirnantian, the glaciers receded and warm conditions returned. Hirnantian was associated with Anoxia (meaning depletion of oxygen) and euxinia (meaning production of sulfides).
• The LOME was followed by the Great Ordovician Biodiversification Event. This is the largest evolutionary surge in the biological history of the earth.

Causes of LOME

The major causes of LOME were believed to be anoxia, glaciation, euxinia. However, the causes are debatable.

Findings about LOME in 2021

The most contentious issue of Late Ordovician Mass Extinction is if oxygen in the sea triggered the extinction. In November 2021, a group of scientists solved this issue. They found that oxygen had no role in LOME. In fact, there were no oxygen at all in deep sea during this period. The LOME was caused by climate cooling. This discovery that oxygen had no role in species extinction is important as it removes several hypotheses around the relation between temperature change, oxygen levels and species extinction.