Scientists are looking for funds to save the prints of the largest arthropod known to have existed on Earth
Belinda Beresford
Claw in claw on the edge of the sand, 260-million years ago giant water scorpions may have danced by the light of the moon on the borders of a primeval South African sea. Their nuptial dance over, they would have slipped under water to resume a solitary life trawling the sea-bed for food.
Munching its way through the mud, one of these behemoths wandered over a layer of ash spat out by volcanoes erupting in South America then just a few hundred kilometres away.
As the water scorpion, or eurypterid, tramped on, the tracks it impressed into the mud filled with the volcanic ash. This preserved them for the millions of years it took for the Cape Fold mountains to form and the so-called Ecca Sea basin to become the Karoo of today.
Last year a scientist walking near Laingsburg found his footprints dwarfed by those of the long-dead invertebrate. It was a groundbreaking find by Dr John Almond of Natura Viva: the first evidence that such large creatures had lived deep underwater in the Ecca Sea.
Extrapolating from the size and spacing of the prints, Almond estimates that the creature that made them was about 2,5m or longer, which would make it the largest arthropod (animals with a segmented exoskeleton, rather than an internal skeleton) known to have existed on Earth.
Unusual for a trace fossil (one preserving fossil behaviour rather than anatomy), the tracks can be confidently attributed to a particular animal group. Almond believes that the tracks show not only the animal moving along on five pairs of limbs, but also its sweep-feeding technique.
Alongside the claw marks are signs of raking, suggesting that as it walked along, the arthropod was using comb-like extensions on its legs to stir up the mud and find its prey.
Almond’s discovery came just in time. The tracks at about 1m wide and 7m long the largest ever found are cracking up and portions are in danger of falling off an eroding cliff.
Ideally the find should be preserved where it is so that further research can be done in context, but removing it to safer quarters such as a museum could be the only viable option. The cost of saving what looks to be a unique find worldwide is just a few thousand rand, and the scientists involved are urgently looking for funds.
Almond says the enormous aquatic scorpion that made the tracks probably represents the last of a group that flourished about 480-million to 260-million years ago.
No skeletal remains of these aquatic behemoths have yet been found, so palaeontologists are forced to draw conclusions about the animals from fossils of the same group that lived in different, earlier eras. One of the best comb-footed eurypterid fossils is 80-million years older than the animal that made the Laingsburg prints. Found near Prince Albert in the Karoo, this specimen is in such good condition that researchers have been able to look at the contents of its gut: small crustaceans, lots of mud and fragments of plant material.
Fortunately comb-footed eurypterids were probably a fairly conservative group. Their design did not change drastically over time. Probably the largest animals living in their cold, dark world 200m below the surface of the sea, they are likely to have been rare, solitary creatures, avoiding each other when not in the mood for mating.
It is likely that eurypterids led a dual life: they could breathe both in and out of water. It is likely that one lure brought them to the shore sex.
Scientists believe that the similarities between the anatomy of the prehistoric eurypterids and their modern-day terrestrial relatives correlates with similar behaviour patterns. Solitary predators such as modern scorpions indulge in nuptial dances during pairing: knowing the right moves allows the amorous pair to convince each other that they are partners of the same species rather than potential prey.
Eurypterid experts such as Dr Simon Braddy of the University of Bristol in the United Kingdom, who is working with Almond on the Laingsburg tracks, believes that eurypterids did the same.
But for now, until someone finds the fossils of an unfortunate amorous pair stricken in mid-performance, it is just conjecture that giant eurypterids once danced together in the moonlight on the shores of their long-gone sea.