During which geological period did a significant mass extinction occur?

The Permian period is notable for hosting the most significant mass extinction event in Earth's history, known as the Permian-Triassic extinction event. This dramatic event occurred around 252 million years ago and resulted in the extinction of approximately 90% of marine species and around 70% of terrestrial vertebrate species. The causes of this mass extinction are attributed to a combination of volcanic activity, climate change, and possibly ocean acidification that drastically altered habitats and ecosystems.

While the Triassic, Cretaceous, and Jurassic periods also experienced mass extinction events, such as the Triassic-Jurassic extinction which was less severe, and the Cretaceous-Paleogene extinction which famously led to the demise of the dinosaurs, none were as extensive in terms of species loss as the event that defined the Permian period. This makes the Permian period the correct choice for identifying a geological period during which a significant mass extinction occurred.