I’ve got several different pet theories about Godzilla. In the canon of Mecha Kaiju Sensō Tai!!! – my half-developed Savage Worlds kaiju setting – he’s a weapon created by beings from the future. In another theory, Godzilla is a mutated dinosaur from Skull Island. Sometimes he’s both at once.
In the latter two versions, Skull Island did not sink into the sea following the events of Son of Kong, but was instead quarantined by the U.S. government. During WWII, Skull Island became the battleground for DC Comics’ The War That Time Forgot. American forces “tested” the atom bomb by dropping one on Skull Island during the final days of the war.
Few of the animal inhabitants survived. The Son of Kong began to suffer the gigantism that early atomic weapons infected so many of their victims with and swam away from the island to find a life elsewhere, but he was suffering from radiation sickness and would eventually succumb to his disease. The most successful of the survivors was the semi-aquatic carnosaur species later known as godzillasaurus.
The godzillasaurus was – like the spinosaurus known from the fossil record – a semi-aquatic bipedal predator. Unlike the spinosaurus (and the more familiar tyrannosaurus), it was a carnosaur – a sub-group of dinosaurs less directly related to birds – which accounts for its more distinctly reptilian appearance. That exposure to an atomic blast mutated the godzillasaurus into a radiation-absorbing monster of mass destruction suggests the population of Skull Island might have been seeded with nano-biological enhancements by parties unknown.
Godzilla was attracted into northern waters by the atomic testing of the early ‘50s and eventually attacked Japan. In the early ‘60s, the mutated Son of Kong was discovered and the furious battle seen in King Kong vs. Godzilla occurred. Godzilla was, of course, unharmed by the fall off the cliff and into the ocean.
Apes are notoriously bad swimmers. Godzilla is an aquatic predator. The Son of Kong’s survival after the events of the film is doubtful.
There was evidently a small population of godzillasaurs in residence on Skull Island. While it has been assumed that Godzilla is male, eggs and young of his species have hatched several times since the 1960s. These may have been laid by deceased females of the species prior to Skull Island’s destruction.
Huh, I just noticed I changed tones partway through from a personal collection into a Wold Newtonian treatise. Ha!
And this all was a plot point in the ongoing Victorian adventure campaign...