The first major extinction that has to be reviewed occurred during the Hirnantian Age (approximately 445 million years ago). The Ordovician-Silurian extinction can be considered a global event because it eliminated up to 85% of all species living during the Ordovician period.
The extinction became known for affecting a large sample of marine families that could not avoid the extinction. For example, numerous families of trilobites disappeared completely as a result of the Ordovician-Silurian extinction while also causing quite a few graptolites to come close to complete extinction.
It can be argued that this wave of extinctions happened due to crucial climate change events, such as a constantly decreasing daily average temperature. The second wave of the Ordovician-Silurian extinction occurred closer to the end of the ice age when the climate became significantly warmer.
The second important extinction is the Cretaceous-Tertiary extinction, also known as the K-T or the Cretaceous-Paleogene extinction. According to Hull et al., it was a global event that caused almost 80% of all species to disappear completely from the face of the Earth. The extinction occurred approximately 66 million years ago due to a meteorite hitting the planet and eliminating numerous species.
The most important organisms to have been eliminated by the K-T extinction were marine invertebrates and dinosaurs. Overall, the K-T extinction can be ranked third in terms of its severity compared to other major extinction episodes from different geologic eras.