The largest living scorpion we know of is about 20 centimetres long. While this is quite large, it is dwarfed by the size of a fossil of an ancient scorpion recently found. Pentecopterus decorahensis, which was found in Winneshiek Shale in Iowa, is a massive five and a half feet long.
The fossil of the giant scorpion was found in 2010 along with about 5000 other fossilized specimens. It was actually found in several segments and the team was able to assemble the pieces to get a picture of what the creature looked like. The fossil is estimated to be about 460 million years old; at this time, Iowa was under water. The team of researchers has given it the name Pentecopterus decorahensis because of its resemblance to the ancient Greek warship the pentaconter.
The fossils were found in the Upper Iowa River and were about 18 metres deep. To examine the fossils, the material around them was first dissolved by water and probes were used to chisel it away. Each of the fossils was then photographed which made them easier to examine.
Pentecopterus decorahensis is part of order Eurypterid, which is an extinct group of mainly aquatic arthropods closing related to arachnids. They are nicknamed the ‘sea scorpions’. Records of fossils of Eurypterids from the Ordovician period, which lasted from 485-483 million years ago, are scarce and P. decorahensis is one of the eleven found.
P. decorahensis had several appendages including paddle shaped legs that would have made it an efficient swimmer and predator. The second and third appendages were angled forward which, according to the team, would probably have been used for capturing prey. Most of the other limbs were shorter and were likely used for locomotion. P. decorahensis probably walked on six legs and not eight like its arachnid cousins.
P. decorahensis is very unique. Its paddle appendages and wide head are unlike any features that have been observed in other Eurypterid species.