I personally think Godzillasaurs evolved from an unknown theropod dinosaur and tyrannosauridae just dosn't fit Godzilla's description your right but spinosauridae fits the bill to some extent no other group of theropods that we know of had such extravagant sails and the plates could be some evolved form of this i think we should really put him in his own group though due to his amphibious nature no known theropod spent anywhere near as much time in the water as Godzilla