Fossils in Scotland revealed information about the evolution of the Scorpion, an invertebrate arthropod arachnid – and how it left the ocean to live on land.
Most arthropods (insects, centipedes, and crustaceans, such as crabs) currently living on land are small. From the Scorpion fossils found in a rock quarry near Edinburgh in Scotland in 1984, scientists (palaeontologists) found large specimens, said Ian Rolfe from the Royal Museum of Scotland.
Complete Scorpion fossil specimens are rare, but fragments of their cuticle, the exo-skeleton (outer shell), are common in sedimentary rocks, says an article in New Scientist in June 1990. In these fragments, the eyes and sensory hairs are usually well preserved.
The palaeontologists found that the earliest Scorpions lived in the Silurian period around 430 million years ago on the bottom of shallow tropical oceans, said Andrew Jeram from the Geology Department of Manchester University in 1990. They could breathe underwater. They probably lived underwater due to gills hidden behind the abdominal (stomach) plates of the cuticle that could flap open to get oxygen from the water.
Modern land Scorpions have four plates on the underside of their abdomens, but no flaps. Instead, they have plates, known as sternites, with two holes, called stigmata, through which oxygen and carbon dioxide can diffuse. Above each hole is a chamber with many thin membranous folds, called lamellae, functioning a bit like lungs – often called ‘book-lungs.’
So, the ancient ocean Scorpions and the modern land Scorpions breathed differently. They also ate differently. Like all arachnids, such as spiders, the modern land Scorpion eats liquids which they suck out of their prey, usually insects. Ocean (aquatic) Silurian Scorpions probably fed on solid food because, in addition to pincers and stingers (like modern Scorpions), they had a visual (eye) system suitable for hunting – their eyes were close together at the front of their heads for good stereoscopic vision, able to judge distance.
They also walked differently. Silurian Scorpions were buoyant underwater and didn’t have strong legs.
By 400 million years ago, fossil Scorpions were found in rivers, suggesting that they moved from the sea into freshwater habitats. And some of them were estimated at 100 centimetres (39 inches) long.
Most fossils that were found were in rocks dating from 320 to 290 million years ago in the Upper Carboniferous period – a time of low-lying tropical forests. They were mostly terrestrial (land-living) and looked more like the modern Scorpions we have today. They had breathing structures halfway between gills and ‘book-lungs.’ They were also becoming nocturnal, active at night.
The fossil specimens from the Scottish quarry in 1984 were between 35-40 centimetres (14-16 inches) long – large compared with today’s Scorpions.
Therefore, the invertebrate ancestors of modern Scorpions left the sea to move onto land a short time before the vertebrates, which means that they were probably the first predators on land.
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM