Zuiyo-maru carcass

Professor Tokio Shikama from Yokohama National University was convinced the remains were of a supposedly extinct plesiosaur. Dr. Fujiro Yasuda from Tokyo University of Marine Science and Technology agreed with Shikama, "the photographs show the remains of a prehistoric animal."[5] Dr. Fujiro Yasuda summarizes his observations in newsreel from the time of the discovery.[7] Yasuda and Taki, icthyologists, disputed that the cryptid could be a shark which would necessitate Michihiko Yano was mistaken when he claimed to observe two dorsal fins.[8]

However, other scientists were more skeptical. According to Bengt Sjögren,[5] Swedish paleontologist Hans-Christian Bjerring was soon interviewed by Swedish news agency Tidningarnas Telegrambyrå, and said:

"If it's true that the Japanese collected samples of fins and skin, it would be possible to conclude from a microscope what it is. If it would be shown to be a hitherto unknown animal from the sea, it is as big of a sensation as the discovery of the coelacanth in 1938… but there is reason to be suspicious of the claims of plesiosaurs, for example, as the marine environment and fauna changed drastically since the age of the plesiosaurs on earth."

Another Swedish scientist, Ove Persson, was also critical of the plesiosaur interpretation. He recalled other discoveries of similar dead marine creatures resembling plesiosaurs that on closer inspection revealed them to be just decomposed, unusually large sharks. He also added,[5] "The discovery of the coelacanth was not as strange as if a plesiosaur would be discovered. The plesiosaur is much bigger and breathes with lungs. It seems incredible that it would manage to remain hidden."

A team of Japanese scientists, Tadayoshi Sasaki and Shigeru Kimura from the Tokyo University of Marine Science and Technology, Ikuo Obata from the National Museum of Nature and Science, and Toshio Ikuya from the Atmosphere and Ocean Research Institute at the University of Tokyo, jointly concluded, while the identity of the carcass could not be determined with certainty, the carcass was most likely that of a basking shark or closely related species.

On July 28, 1977, the Zuiyō Maru carcass was commented upon in the international science magazine New Scientist. A scientist from the Natural History Museum in London had the same opinion as Bjerring and Persson, that the remains were not from a plesiosaur. The decomposition pattern of a basking shark, whose spine and brain case is relatively highly calcified for a cartilaginous fish, can be expected to produce a similar shape to a plesiosaur; the first parts that fall off during decomposition are the lower jaw, the gill area, and the dorsal and caudal fins. Of the view that the carcass was explained as a plesiosaur, Sjögren concluded,[5] "it was the infamous old 'Stronsay Beast' that once again haunted like on innumerable other occasions. The scholars in Japan went into the same easy trap as the Scottish naturalists did in the 19th century."

The identity as a basking shark was further supported by the proportions given by Michihiko Yano when he measured the carcass before it was put back to sea. His dimensions giving a much longer tail than neck, proportions of the hind and forward fins, and size of the "head" do not closely correspond to any known Plesiosaur or Pliosaur, but did perfectly match the expected body dimensions for a very large basking shark. Closer review of the photographs taken of the carcass also do show the presence of a dorsal fin, which was speculated to have been overlooked due to decomposition. The presence of cartilaginous fibers visible in the fin are only found in fish and are unknown in any tetrapod.[3]