The Stronsay Beast: a 55-foot carcass off Rothiesholm Head, Orkney, 1808

In 1808 Stronsay was spelled Stronsa. It is one of the Orkney islands, and Rothiesholm Head marks its "South western extremity", on land then belonging to Gilbert Laing Meason. In Edinburgh, the Wernerian Natural History Society had been founded that January. Before the year was out, it would hear about Stronsay.

25 September 1808, or the 26th, depending on the account. A storm had passed in the night. John Peace was fishing off Rothiesholm Head when he noticed seabirds circling something draped over a submerged rock. He rowed closer. From the shore, the farmer George Sherar watched.

About ten days later, strong winds drove the remains ashore. Peace, Sherar and Thomas Fotheringhame, a house carpenter from Kirkwall, each measured it. Each got 55 feet. And part of the tail was missing.

It was 4 feet wide and about 10 feet around. Serpent-like, almost eel-like, with a long neck and a small head. One account likens the head to a sheep's, with eyes larger than a seal's. Three pairs of appendages, which the witnesses called paws or wings. Five or six toes on each foot.

The skin was grey and rough. Smooth when stroked from head to tail, rough the other way. Bristles edged the fins, and a mane of bristles ran from the shoulders to the tail. When wet, it glowed in the dark. Inside, the animal was red.

In November 1808 the witnesses gave sworn statements before two justices of the peace, Dr Robert Groat and Malcolm Laing. Fotheringhame's came first, on 10 November. A clerk recorded them in the third person, and the historian Bill Jenkins notes they do not always agree. A Mr Petrie drew the creature in chalk on a table, then redrew it. Sherar agreed a sketch was "an exact resemblance", but Fotheringhame said the neck was too long.

On 19 November 1808 the society's secretary, Patrick Neill, presented the case. The society could not identify the animal and judged it a new species, "probably a sea serpent". On 14 January 1809 Neill named it Halsydrus pontoppidani, Pontoppidan's sea-snake. The anatomist Dr John Barclay called the remains the first solid scientific evidence for the "great sea snake".

In London, Sir Everard Home examined the vertebrae and skull that Sherar had salvaged. They were cartilaginous, with star-shaped calcification. Only sharks have that. His verdict was a decayed basking shark, about 30 feet long by one account, about 36 by another. John Goodsir agreed in 1849, and Dr Geoff Swinney's analysis in the 1980s supported it.

As a basking shark rots, the gills and jaws fall away, leaving a small skull on an exposed spine. Fin rays and decaying fibres look like a mane. Fins look like legs, and a male's claspers add a third pair. The result is often called a pseudo-plesiosaur. Ivan T. Sanderson's word "globster" is now applied to this case.

Yet the size still nags, because the largest reliably measured basking shark was 40 ft 3 in. Karl Shuker defends the 55 feet, measured independently by three men, and allows for an unknown larger relative. Ken Gerhard suggests one of the last giant basking sharks, from before overfishing. Dr Yvonne Simpson hoped DNA would settle it, but the preserving chemicals destroy DNA. Three vertebrae survive in Edinburgh, and partial remains have gone on show at the Stromness Museum, back in Orkney.