The origin of pterygotid eurypterids (Chelicerata: Eurypterida)

The Silurian and Lower Devonian pterygotid eurypterids were the largest of all arthropods, but their origin and early evolution are poorly understood. Pterygotus? ventricosus from Kokomo, Indiana, is shown to be the most basal of the pterygotids and constitutes the sister taxon to the rest of the clade. P.? ventricosus has walking appendages similar to those of Slimonia, which is the sister taxon of pterygotids. There is no evidence in P.? ventricosus of the dorsal median pretelson carina characteristic of other pterygotids, but the taxon does have the typical pterygotid undivided genital appendage. The status of the iconic pterygotid character (and presumed synapomorphy), the enlargement of the chelicerae, is uncertain in the Kokomo pterygotid. All the diagnostic characters of pterygotids did not develop simultaneously. A new genus Ciurcopterus is erected to accommodate the Kokomo pterygotid and Pterygotus? sarlei Ciurca and Tetlie.