A new reason has been given for the first mass extinction on Earth, also known as the Late Ordovician mass extinction.
The Earth has seen four other great mass extinctions apart from the Cretaceous mass extinction known for wiping out non-avian dinosaurs.
The main reason was said to be cooling climate that changed the ocean circulation pattern.
The disruption in the flow of oxygen-rich water from the shallow seas to deeper oceans caused hypoxia.
Many marine creatures were destroyed as a result of this sudden change. This extinction occurred about 445 million years ago and killed about 85% of all species.
Human-induced climate change is currently causing a sixth mass extinction, according to some researchers.
Other mass extinctions
The Devonian mass extinction: This occurred about 375 million years ago and wiped out about 75% of the world’s species.
The Permian mass extinction: It occurred about 250 million years ago and also known as the Great Dying that caused the extinction of over 95% of all species.
The Triassic mass extinction occurred about 200 million years ago and eliminated about 80% of Earth’s species, including some dinosaurs.
Till now it is believed that global warming caused oceans to lose dissolved oxygen, resulting in destruction of entire ecosystem.
Other evidences have suggested that oxygen levels also dropped in cooling climates. This was confirmed by measuring iodine concentration in rocks from that period.
A computer based simulation also pointed out that climate cooling was likely responsible for the Late Ordovician mass extinction.
There are evidences that climate cooling contributed to changes in nutrient cycling, primary producer communities which ultimately drove the Late Ordovician mass extinction.
Mass extinction is an event in which species vanish much faster than they are replaced. More than 75% of the species are lost in a short span.
The total number of mass extinction events range from 5 to more than 20 based on the number of species extinct.