Pentecopterus

Pentecopterus Temporal range: Darriwilian PreЄ Є O S D C P T J K Pg N
Artist's reconstruction
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Merostomata
Order:	†Eurypterida
Family:	†Megalograptidae
Genus:	†Pentecopterus Lamsdell et al., 2015
Species:	†P. decorahensis
Binomial name
Pentecopterus decorahensis Lamsdell et al., 2015

Pentecopterus decorahensis is an extinct sea scorpion that lived during the Middle Ordovician period, as early as 467.3 million years ago. It is the oldest described eurypterid, and, at an estimated length of up to 1.83 metres (6 ft 0 in), one of the largest known arthropods ever discovered, similar in size to the famous millipede-like Arthropleura. Other notable sea scorpions include the more recent Silurian-aged Acutiramus, and Devonian Jaekelopterus, the only arthropod currently known to surpass P. decorahensis in size. According to researcher James Lamsdell of Yale University, P. decorahensis "is the first real big predator".

The generic name refers to the "penteconter", an ancient Greek warship, due to similarities in shape and predatory behavior. It is the oldest described eurypterid, living as early as 467.3 million years ago; at an estimated length of 1.7 metres (5 ft 7 in), or up to 1.83 metres (6 ft 0 in), it is also one of the largest known arthropods.

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.