In the original Yamane only speculates what Gojira could be. The only solid evidence he had of the "ancient" origin is a Trilobite found near Gojira's footprint. Which is a moot point since Trilobites went extinct before the advent of dinosaurs, so if they actually didn't extinct then originally, before the H-bomb mutation, Gojira could be any marine animal from any period of time.... wasn't Godzilla '54 a mutated T-Rex type dinosaur as shown in Godzilla vs. King Ghidorah?
"Godzilla vs. King Ghidorah" was created in 90's. It's part of the Heisei reboot and has no relation to the original film.