The Godzillasaurus is different from other theropods because its posture is more upright like a human, and it drags its tail on the ground, much like older theropod reconstructions. At 12 meters in height, it is much larger than any other theropod species, probably due to its erect posture. It has a smaller head than a real theropod, and has a double-row of teeth, much like a shark. The Godzillasaurus species is also different from other theropod dinosaurs due to the fact that it contains a second, or "sacral", brain located in its hip to control lower body functions.
As is shown in Godzilla vs. MechaGodzilla 2, Godzilla seem to be telepathic, as Baby Godzilla can contact his father. However, it is unknown if an unmutated Godzillasaurus would have this ability.
The Godzillasaurus lived out in Lagos Island in Godzilla vs. King Ghidorah, and was attacked by soldiers during World War II. This was observed by Futurians that had traveled back in time to remove Godzilla from history. They transported the wounded Godzillasaurus to the Bering Sea, thinking it would die. However, the Godzillasaurus was mutated by a nuclear submarine crash without them knowing.
The back side of the box of Jakks Pacific's Giant Godzilla figure states Godzilla was an amphibious reptile evolved while Earth had ten times more radiation. It is currently unknown if Godzilla was mutated further than that in the film.
In "Godzilla: A History," a book about Godzilla made by Kenichiro Terasawa, the main character of Godzilla vs. King Ghidorah, one of Terasawa's theories about how Godzilla came to be was that he was mutated from a whale, or kujira, that had Godzilla's scutes.
An actual species of dinosaur was named the Gojirasaurus, in homage to Godzilla himself. However, this "Godzillasaurus" looks nothing like the ones displayed in the movies.
It is possible that Godzillasaurus is related to Therzinosaurus as it may be also a plant eater despite having sharp teeth (this is proven as of Baby Godzilla's diet). It is also possible that it is not a theropod at all and is instead a species of neo-prosauropod. This would explain its upright stance (some semi-bipedal prosauropods stood like this), its small head, its sacral brain and that it seems to be at least partially herbivorous.
However, in deleted scenes from Gojira, Godzilla vs. Biollante and Godzilla vs. Destoroyah, Godzilla and Godzilla Junior were seen eating meat (cows, the unmade monster Deutalios, and whales, respectively). This could be an indication that the diet of Godzillasaurus is omnivorous or even completely carnivorous, though this could be because of them being mutated and thus needing an appropriate change in their diets.
In Legendary Pictures' Godzilla, Godzillas were giant amphibious reptiles that evolved when Earth had ten times more radiation than it has today.