An article in the journal “BMC Evolutionary Biology” describes the discovery of fossils of Pentecopterus decorahensis, a sea scorpion that lived in the Ordovician period, about 467 million years ago, in Iowa. It was huge by arthropods standards because it’s estimated that its length could reach 180 cm (almost 6 feet). It’s in fact one of the largest arthropods ever lived and according to James Lamsdell, the lead author of this study, was part of a group of important predators in the Paleozoic era.

The name Pentecopterus decorahensis derives for its first part from the penteconter, a type of galley of ancient Greece, and for its second part of Decorah, the name of the town near the place where the fossils were found. It’s the oldest fossil belonging to the group of eurypterids (Eurypterida), also known as sea scorpions.

Today eurypterids are extinct and, although they’re also known as sea scorpions, actually it’s a misnomer. They were part of the group of chelicerates so they were related to scorpions but for their characteristics form a separate group, closer to the so-called horseshoe crabs of the Limulidae family. Probably they developed in the Cambrian, about 510 million years ago, to become extinct in the Permian, about 260 million years ago.

Most eurypterids were as small as modern insects and arachnids but some species reached a size impossible in the current environmental conditions, with less oxygen than it was present in the past in the atmosphere. Some such as those belonging to the Pterygotus genus could even exceed two meters in length.

The Pentecopterus decorahensis were close to those cousins in their size. They had a long armored head, a slim body and large limbs adapted to grasp prey. Even the legs they used to swim like paddles have a unique shape. Finding these fossils was very lucky because they were discovered in the crater created by a meteorite.

Researchers from Yale and the University of Iowa worked to analyze the fossils of Pentecopterus decorahensis. They both belonged to adult and juvenile specimen and their preservation is outstanding. This resulted in a great deal of information on this species with the chance to examine its characteristics and its development.

These fossils are allowing researchers to get a better idea of ​​the evolution of the Ordovician eurypterids and overall of the ecosystem they lived in. Today arthropods that size don’t exist anymore but back then, long before the dinosaurs developed, they had to be deadly predators.