On April 25, 1977, the Japanese fishing trawler Zuiyo-maru found a rotting carcass 10 meters long entangled in their net as they were trawling in the South Pacific, east of New Zealand. They pulled in the carcass to release it outside the net area and discovered they weren't sure what kind of animal it was. A section chief photographed the carcass, and took samples of flesh from a fin before it was thrown overboard. The film was developed in Japan, and turned over to a professor of ichthyology at Tokyo University, Dr. Fujio Yasuda.
Since two pictures showed it to be very like what a carcass of a plesiosaur would possibly look like, the Taiyo Fishery Company announced the discovery in a press conference on July 20, 1977. If it were a discovery of a recently living plesiosaur it would be a major scientific find, and start a systematic search for living plesiosaurs.
However, as scientists weighed the evidence, it came down strongly in favor of a basking shark carcass, though there were some inconsistencies. Some evidences were: rib length indicated a shark, shark fin processors identified the carcass as a shark, the pelvic fins reported could be male shark clasper fins, and amino acid analysis of the samples matched basking shark elastoidin. Counter indications were the lack of ammonia smell such as a shark would have, eyewitness certainty about the presence of posterior fins, the photographed fatty adipocere that could indicate blubber (and imply that it could not have been a shark), and the photographed presence of red muscle in a limited area.
Malcolm Bowden,  in his investigation of the reports concludes that the creature was not a plesiosaur, but was definitely not a basking shark. He believes the evidence points to a previously unknown swimming mammal.
John Goertzen points out that the identification relies on the presumption that the dorsal fin slipped around into the wrong place as a result of decomposition. He presents the difficulties with this view and notes that the eyewitness testimony was to a pair of fins, and gives further supporting evidence. Goertzen's conclusion is that it was likely a marine tetropod, saying "A Sauropterygia identification thus remains viable."
This is an interesting debate, but is doubtful enough that it should not be used as an argument in favor of recent finds of ancient animals (see Arguments Creationists should NOT use. As is often the case, the rule of very strong evidence needed for very new or controversial topics should be applied here. It would of course be a different story if a live plesiosaur were found.
- Letter to the Editor: Zuiyo-maru Carcass Revisited: Plesiosaur or Basking Shark? by Todd Wood. CRSQ 33(4):292-295. March 1997.
- Malcolm Bowden`s notes on the japanese reports
- Live plesiosaurs: weighing the evidence
- Letting rotting sharks lie by by Pierre Jerlström and Bev Elliott. Technical Journal 13(2):83–87. November 1999
- New Zuiyo Maru Cryptid Observations-Strong Indications It Was a Marine Tetrapod by John Goertzen - CRSQ 38:(1):19-29. June 2001.