Big Meg's secret is out!

A quarter of a century after the historic discovery of the 300-million-year-old fossil, it transpires that Megarachne servinei was never a spider.

She was 'Big Meg,' the largest of all spiders that ever strode the Earth.

The 300-million-year-old fossil was so famous that plaster casts of her body are on display in numerous museums and copies can be purchased over the Internet for hundreds of dollars apiece.

As for the original, it was carefully locked away from public view. It was so precious that it was placed in a bank vault pending the outcome of an ownership squabble.

Alas... a quarter of a century after the historic find, it transpires that Megarachne servinei was never a spider, but a rather odd-looking and certainly less exotic species of sea scorpion.

'Big Meg' unleashed huge excitement after her find in Argentina's San Luis province.

No other creature like it had been found.

The assumption was that it was a giant spider, the mother of tarantulas, with a body length of 33.9 centimetres, a leg span of 50cm and goggly eyes more than one centimetre wide.

The chance of a second look came with the discovery of another 'Megarachne' in the same stratum in the province's Bajo de Veliz rock formation.

Megarachne is a 'bizarre eurypterid,' or sea scorpion, similar to a species called Woodwardopterus that was first discovered in 1959, according to the new study, published online on Wednesday in Biology Letters, a journal of Britain's Royal Society.

Eurypterid fossils of this kind have been found in Scotland and South Africa.

One of the three authors is Mario Huenicken of the Regional Centre for Scientific Investigation and Technological Transfer in Anillaco, Argentina.

Huenicken was among the team that described 'Big Meg' back in 1980 and now seeks to set the scientific record straight.