About 260 million years ago, a giant water scorpion well over two metres long made its way slowly over the sea floor, about 100m to 200m below the surface of the water.
This huge, ancient creature may have been making sweeping, brush-like movements with its right feet, collecting small animals like worms and crustaceans from the sediment which it then "combed" with its left feet, pushing the most desirable prey items towards its mouth.
As the water scorpion, or eurypterid as its now known to palaeontologists, moved across the sea bottom, it left complex footprints or tracks of its activity in the mud.
Gradually, the mud was covered by more and more layers of sediment and the thick ash of hundreds of massive volcanic eruptions.
Eventually, over aeons, the sea itself disappeared, and the land which had surrounded it broke and divided into new continents which drifted apart.
Millions of years later, while travelling with a group of friends towards the end of November last year, Cape Town-based British palaeontologist John Almond glanced up at a crumbling cliff near Laingsburg in the Karoo and noticed a double set of strange blob-like markings in the rock, starkly outlined in the cross-light of the late afternoon sun.
Miraculously, the blobs were the tracks left by the water scorpion, perfectly preserved as fossilised rock despite the passing of millions of years and the huge re-arrangement of Earth's surface involving massive geological forces like volcanoes and earthquakes.
The find is of major scientific significance, because it is the largest invertebrate trackway known in the world, and the eurypterid which made it is the largest arthropod ever recorded.
(Invertebrates are animals without a backbone. Arthropods are invertebrates with a segmented exoskeleton and numerous paired, jointed appendages, or legs, and include modern crustaceans, insects, spiders and their relatives).
The new discovery is also a rare example of a trace fossil - that is, fossilised behaviour of living organism such as tracks, trails, burrows or feeding marks, which can be confidently attributed to a specific group of animals.
"Most trace fossils cannot be assigned to particular animal, though we can interpret them in terms of what behaviour was involved," said Almond.
"Trace fossils record the activity of animals while they were still alive and where they actually lived. Body fossils - shells, skeletons and so on - represent dead organisms, and may be transported away from the habitat of the living animal."
The trackway occurs in what geologists call the Ecca Group of sediments of the Great Karoo.
These sediments were laid down in an extensive sea which covered large areas of what was then the supercontinent Pangaea - a single continent comprising all the land mass on Earth - for about 25 million years during the early- to mid-Permian Period (around 280-to-255 million years ago).
"At this time, southern Africa was situated about 50-70 degrees south of the contemporary equator," Almond said.
Geochemical evidence suggests that when the new trackway was formed, the Ecca seas ranged from brackish to freshwater, and the climate was cold to temperate and highly seasonal.
The Laingsburg area is well-known to geologists and palaeontologists, and is often visited to study the Ecca Group of ancient marine sediments.
So it's something of a paradox that so many knowledgeable people have been through the exact area without noticing the trackway before, said Almond.
"However, on this occasion, by pure chance - the right time of day, the right season of the year - the late afternoon sunlight on the beds of rock was at just the right angle to highlight a clear double series of large blobs on a bedding plane high up on a cliff-like outcrop.
"Although we were in a hurry to move on, a quick look through binoculars convinced me the suspicious-looking blobs would be worth a closer look.
"My first impression was that they were very complex impressions of some sort which appeared to form some fossil trackway.
"But, if so, it was clearly not made by a tetrapod - a four-legged vertebrate - and it was huge!"
It was only later when the photographs were developed that Almond realised his find was of exceptional scientific interest and that it should be studied further as soon as possible.
When he and friends revisited the area shortly after Christmas, they found incontrovertible evidence that the traces were a fossil trackway that extended much further than previously realised.
Almond explained that eurypterids are a fascinating group of extinct aquatic arthropods from the Palaeozoic Era, which lived some 480 to 260 million years ago.
"In particular, eurypterids are close relatives, and direct ancestors, of the living and almost exclusively terrestrial arachnids: scorpions, spiders and their kin.
"Like these, eurypterids were almost exclusively predatory in habits, feeding on living prey such as other arthropods, soft-bodied invertebrates, and fish.
"The front appendages were often specialised as huge grasping claws or pincers, or bore cage-like arrays of spines for capturing prey.
"In at least some forms, there may have been a venomous sting at the tip of the tail."
This spectacular trackway, which consists of two parallel series of complex footprints or tracks, is about one metre wide and extends at least 7m across the surface of a single bed.
"The eurypterid which made it must have been enormous, probably about 2,5m long, but there is good reason to think that it was not a fearsome predator like many of its smaller terrestrial and aquatic relatives," said Almond.
Well-preserved details of the newly discovered tracks show that while the animal was walking along the sea bed, it raked through the soft bottom muds, almost certainly foraging for food - probably small worms and crustaceans - using specialised comb-like structures on its limbs.
These combs are preserved in a much older giant eurypterid specimen collected near Prince Albert in the 1980s.
The Laingsburg trackway is unusually well-preserved.
"The excellent preservation of details of the eurypterid tracks is probably due to the fact that they were impressed by the limbs into a viscous muddy substrate through a thin overlying layer of volcanic ash," said Almond.
"The tracks were then infilled from above by ash as soon as the animal walked on.
"The ash layer subsequently protected the foot prints in the underlying muddy layer from erosion because the latter was no longer exposed at the surface of the sea bed."
The Ecca trackway is more complex and interesting than all those previously found in that it was formed by a combination of both locomotion and feeding activities, he said.
Before the trackway can be properly studied scientifically, the priority is to stabilise and cast the specimen in the field.
"The bed on which the trace fossil is preserved is cracking up into numerous small blocks of rock and is in danger of disintegrating through erosion, with the resulting loss of this unique specimen," said Almond.
"There's a fair chance that some of it will be gone after just one more winter."
A cast is being made this week with the assistance of staff of Iziko-SA Museum.
"Thereafter, ways of permanently preserving the specimen for posterity will have to be seriously considered."