At an estimated length of 1.83 metres (6 feet), Pentecopterus is also one of the largest arthropods ever discovered, similar in size to the famous millipede-like Arthropleura. Other large eurypterids include Pterygotus, Hibbertopterus and the enormous Jaekelopterus, which is the only known arthropod to surpass P. decorahensis in size. Researcher James Lamsdell of Yale University described P. decorahensis as the "first real big predator".
Pentecopterus is among the largest known arthropods at an estimated length of 1.83 metres (6 feet). The genus is named after the penteconter, an early galley from ancient Greece and one of the first true warships, due to some similarities in shape and that the taxon represents an early predator. The epithet -pterus means "wing" and is commonly applied to eurypterid genera. The species name decorahensis refers to Decorah in Iowa, where the fossils were discovered.
The large amount of fragmentary specimens recovered of Pentecopterus, including juveniles and exuviae, means that its morphology is relatively well understood. It bears some striking similarities with Megalograptus, but can be distinguished from it and other megalograptid eurypterids in that it retains a single pair of spines on the third podomere of the third prosomal appendages, that appendage V is short and has a serrated distal margin of podomeres, that the prosomal ventral paltes widen anteriorly, that the podomeres VI-7 and VI-8 has small serrations and VI-7 has a rounded projection anteriorly, that the pretelson lacks posterolateral expansion, that the telson is xiphous and that the margin is laterally ornamented with scales.
Pentecopterus has some features unique among the eurypterids, notably the shape of its carapace and the unusual shape of the sixth podomere of appendage VI. Pentecopterus also possesses lateral scales on the telson, something otherwise only seen in pterygotid eurypterids. This feature likely arose through convergent evolution.
Scientists from the Iowa Geological Survey and Yale University discovered, as early as 2005 but mostly in 2010, 150 fossil pieces, from at least 30 individuals, about 60 feet below the Upper Iowa River, within the Decorah crater, an ancient meteorite impact crater.