Literature
The Bridge-Monster'verse Timeline
The Bridge: Monster'verse Timeline
Permian Era: At the end of the Permian era, a three-headed kaiju named Ghidorah arrived and caused a massive genocide and killed off most of life on the planet, very few life forms survived, including Gojira, Rodan, Mosura, and other MUTOs.
70,000 BC: Former Guardian, Reijuu, now Bagan begins attacking continent Mu causing the third global cataclysm on Terra. Mosura came in and confront Bagan only to be badly wounded and retreated to the Yucan Territory while Gojira lead the last Guardians; Anguirus, Raiga, Yongarry, Gamera, and Baragon to fight against the God of Extinction.
??? AD: Ghidorah returns in R