Landowner Nick Freeman stumbled across the remains of the one-metre long eurypterid on his family property and had the honour of it being named after him: Woodwardopterus freemanorum.

It is not only the last sea scorpion known to have walked the Earth, fossil evidence of its existence is also the first of sea scorpions having lived in Queensland.

Its discovery would, perhaps, have been a source of personal vindication for one man.

While premier of the sunshine state a century ago, Ted Theodore fought long and hard for the Dawson River to be dammed so water could be deviated to irrigate the arid Banana shire.

No funding materialised and the ambitious plans were abandoned.

It was in this area - in the town of Theodore, pre-emptively named after the man who had promised the "model garden city" - that the discovery of the eurypterid was made: proof it was once a lush wetland, like he promised it could be.

Even if that was 252 million years earlier.

The important discovery has international significance, says the scientist who identified the species, Queensland Museum Principal Curator Geosciences and CQUniversity Adjunct Associate Professor Andrew Rozefelds.

"This new tantalising fossil helps fill the gap in our knowledge of this group of animals in Australia and indeed worldwide," Dr Rozefelds said.

Freeman made the discovery in 1990 but the fragmented remains were only brought to the attention of the museum in 2013.

The palaeontology team knew it was special but couldn't even determine which group of animals it belonged to.

One unexpected perk of COVID lockdown was it gave Dr Rozefelds time to fully investigate this intriguing 'cold case'.

"When the fragmentary specimen came into our collection, it was initially placed in the too hard basket but the closures provided the opportunity to study and reassess some of our fossil collection and this particular fossil had always intrigued me," he said.

He concluded it had to be an arthropod and possibly a eurypterid - a species most often found overseas but also in Victoria.

Given how rare specimens are, he invited Dr Markus Poschmann - a German expert on this group of animals - to assist in studying and describing it.

Sea scorpions are an extinct group of invertebrates related to modern day scorpions and spiders. Some grew to two metres in length and some, like Freeman's discovery, made the transition from marine to freshwater.

The scientists accurately dated the remains as 252 million years old using volcanic sediments preserved in the coal surrounding the fossilised remains.

The recently-discovered sea scorpion would have been among the largest predators in the lakes and rivers of the Theodore area at this time.

"After extensive research this particular fossil turned out to be the last eurypterid known from anywhere in the world," Dr Rozefelds said.

"This is just before the end-Permian extinction event. The eurypterids disappeared, along with other groups of animals, at this time."

Australian Associated Press