250,000,000 B.C.

In the real world

 * The Permian–Triassic extinction event, the most severe extinction event in Earth's history, takes place, killing up to 96% of all marine species and 70% of terrestrial vertebrate species.

In the Kaiju Multiverse

 * A Shinomura looking for food unknowingly lands near Godzilla and begins eating a large marine creature before being blasted by Godzilla's atomic breath. A piece of Shinomura falls down a crevice and lay dormant until 1946. Just then, a meteor collides with Earth, diminishing the atmospheric radiation and forcing large creatures that fed on radiation like Godzilla and the M.U.T.O.s to look for it near the Earth's core deep underground or underwater. (Godzilla: Awakening)