While commonly referred to by fans simply as the "Giant Mosasaur," the creature's name is revealed as Liopleurodon in Shinji Nishikawa: Drawing Book of Godzilla. As its name suggests, the Liopleurodon is likely to be based on the real world pliosaur genus of the same name, which lived during the Middle Jurassic period.
The Liopleurodon's concept art depicts it with a smooth, white underbody and a darker-colored pattern on the top of its body. It had a row of sharp teeth, as well as large, crocodilian jaws, and four fins.
The Liopleurodon would have washed ashore, killed by Godzilla. The carcass would be investigated by the AMF and it would be determined that Godzilla had killed it.
During the planning of Godzilla: Tokyo S.O.S., Toho decided to replace the Liopleurodon with an existing kaiju rather than introduce a new creature just for it to appear as a corpse. Director Masaaki Tezuka requested Anguirus, but producer Shogo Tomiyama relented, wanting to keep the monster alive for possible use in a sequel. Tezuka's follow-up proposal was Gamera, to which Toho countered with the only turtle kaiju they had the rights to: Kamoebas from Space Amoeba.
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