Most Godzilla incarnations are specialized, semi-aquatic dinosaurs that are each similar species, with the some of the millennium Godzilla's being subspecies of the original Godzillas, and Godzillasaurus being a more land living offshoot of this evolutionary tree.
Legendary Goji's kind is an early ancestor to this Godzillasaur-group.
Showa Godzilla fought Rodan's kind at some point in the past.
Godzilla 1954 and Showa G are from separate nests of the same group who met as hatchlings and became minor rivals. G54 was the dominant male of his group, while Showa Godzilla was an intellectual loner, who only got angry when G54 & others bullied him.
Godzilla's and Titanosaurus's species fought for territory and the Godzillas won due to intelligence, but both became rare as a result.
Anguirus 1955 is different from the Showa Anguirus. Torn neck + nuclear grade flames bathing entire body+regen that isn't strong enough=ded
The Space hunter aliens were actually going to begin their plans before GvsG but postponed them until Hedorah was killed.
The Xillian who tried to "escape to the future" had a time machine similar to the futuranians of the Heisei timeline, but his ship exploded before the machine could work.
All the aliens in the Showa universe are actual aliens, the only ones who had anything to do with earth in the past were the space hunter roaches, who helped the Seatopians live underground as well as tell them where to place a dormant prehistoric beetle to create Megalon.
Ghidorah is the reason earth is one of the only inhabitable planets in the galaxy.
In the Showa and other Godzilla universes, there was a prehistoric "Age of Kaiju" which began around the Carboniferous period:
Ordovician extinction (gamma ray burst) causes mutations, some creatures mutate to absorb radiation and thrive in the increased radiation.
around 300 million years ago Megaguirus & their Meganulon swarms become the first Kaiju to roam the earth.
Mutos and the first "Godzillasauroforms" appear, including Legendary Goji.
The Permian extinction causes their decline, and around 200 million years ago, Rodans appear and becomes the dominant aerial predator.
150 million years ago Varan evolves, either from dinosaurs or from varanidae.
In the Cretaceous Gorosaurus evolves from Allosaurus, while the Showa Godzilla and Anguirus species evolve as well.
Titanosaurus and baragon evolve just before the extinction of the dinosaurs and end of the kaiju age.
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Also Showa Godzilla is the Godzilla in DAM since he showed no signs of getting any weaker in ToMG, there is no reason to assume he died, plus the odds of yet another SoG turn of events happening and creating a new Minya are slim to none.