Pentecopterus

Pentecopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Darriwilian age of the Middle Ordovician period, as early as 467.3 million years ago. The genus contains only one species, P. decorahensis, that was previously considered the oldest known eurypterid, surpassing all other Ordovician eurypterids known at the time, such as Brachypterus, in age by almost 9 million years; however, newer discoveries have yielded evidence for eurypterids that were older still (?Carcinosoma aurorae dated back to the Tremadocian). The generic name derives from the penteconter, a warship from ancient Greece, and the suffix -pterus, which means "wing" and is often used in other genus of eurypterids. The specific name refers to Decorah, Iowa, where Pentecopterus was discovered.

The genus is classified as part of the Megalograptidae family of eurypterids, a family differentiated from other eurypterids by the possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of almost all spines and the large exoskeletons with ovate to triangular scales. It is estimated that Pentecopterus had a length of , turning it into one of the largest eurypterids or arthropods ever discovered. However, Pentecopterus was overtaken by other eurypterids such as Jaekelopterus rhenaniae, the largest known arthropod with .