Paleontologists say that mass extinctions on Earth of land animals, including amphibians, reptiles, mammals and birds, occur on a cycle of 27 million years.
Interestingly, this mass extinction cycle also coincides with the previously discovered mass extinction of oceanic life. Research has shown that mass extinctions coincide with significant asteroid strikes and volcanic eruptions on the planet.
There are five major mass extinction events in the fossil record. Each falls at the end of a particular epoch – the Ordovician 443 million years ago, the Denovician 360 million years ago, the Permian 250 million years ago, the Triassic 201 million years ago and the Cretaceous 65 million years ago.
Statistical analysis from a study of extinctions of terrestrial species suggests that events occur every 27.5 million years. The authors compared these dates to the occurrence of impact craters created by asteroids colliding with the Earth, or massive volcanic eruptions, or a series of eruptions that cover large areas of land and release greenhouse gases into the atmosphere.
The new results support the idea that periodic global catastrophes can serve as a trigger for extinction. In addition to asteroid and comet impacts, the researchers also suggest that mass extinctions on land and in the oceans coincided with eruptions and floods.
The study offers no definitive explanation for what controls periodic mass extinction events. One controversial theory is that an undiscovered companion star to the Sun in a massive orbit may excite cosmic debris, pushing it towards Earth every 26 million years. The resulting basalt eruptions every 25 to 30 million years may be the result of long plume-rising cycles in the Earth’s mantle.