The tracks left by a giant water scorpion as it dragged its great bulk across a beach 330 million years ago have been discovered in Scottish rock.
The six-legged beast, known as Hibbertopterus, would have been about 1.6m (5.2ft) long and 1m wide.
The markings, which have a central line made by a heavy, plated tail, represent one of the largest invertebrate trackways found in the fossil record.
Dr Martin Whyte reports his discovery in central Scotland in Nature magazine.
"I knew it was a trackway as soon as I saw it - my main work is on dinosaur footprints - but it wasn't immediately obvious to me what sort of trackway it was," the Sheffield University researcher said.
"I could rule out reptiles and amphibians because I could see whatever it was, it had six legs," he told the BBC News Website.
"I've worked through the possibilities and I think it can only be Hibbertopterus."
Hibbertopterus may have been big but it was not a ferocious predator
The trackway is preserved in sandstone. What would once have been a beach surface has been tilted to about 45 degrees by geological movement.
The length of track preserved, 6m (20ft), is remarkable. The stride pattern, too, is huge - 27cm (11in).
Fragmentary fossils of Hibbertopterus are well known from Scottish Lower Carboniferous rocks and were first described from West Lothian in 1831.
The creature did not have the big pincers or carry its tail in the air like the land scorpions we know today, and it did not have a sting, either; but these animal groups are nonetheless distantly related, scientists believe.
What is interesting about this trackway is that is shows Hibbertopterus could move out of its usual water habitat.
"There has been debate about whether it was restricted to water or could come out on land. I believe this trackway shows it could come out for short periods," explained Dr Whyte.
"It may have been taking a shortcut - from one body of water to another. People have asked about spawning but there's just no evidence to back that up."
Although the prospect of a man-sized scorpion might be scary to modern eyes, this particular beast was hardly a ferocious predator.
"It wouldn't have been chasing after an amphibian or reptile for a meal. It had comb structures on some of the anterior appendages, and it swept these through the water and caught small organisms, such as small worms and water bugs," Dr Whyte explained.