Ciurcopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Ciurcopterus have been discovered in deposits of Late Silurian age in North America. Classified as part of the family Pterygotidae, the genus contains two species, C. sarlei from Pittsford, New York and C. ventricosus from Kokomo, Indiana. The genus is named in honor of Samuel J. Ciurca, Jr., who has contributed significantly to eurypterid research by discovering a large amount of eurypterid specimens, including the four specimens used to describe Ciurcopterus itself.
Ciurcopterus is the most basal (primitive) known member of the Pterygotidae, and combined characteristics of more derived members of the family with features of close relatives of the group, such as Slimonia. Measuring 70 centimetres (28 inches) in length, Ciurcopterus was relatively large though smaller than many of the later members of its family, which would grow to become the largest known arthropods to have ever lived.
Description
Ciurcopterus was a eurypterid of medium size, with C. ventricosus measuring approximately 70 centimetres (28 in) in length and C. sarlei measuring 50 centimetres (20 in). Though this is large relative to most modern day arthropods, Ciurcopterus was dwarfed by many of the members of its family (the Pterygotidae), such as Jaekelopterus rhenaniae at 2.5 metres (8 ft, the largest known arthropod) and Acutiramus bohemicus at 2.1 metres (7 ft).
Ciurcopterus possessed walking legs that were similar to those of Slimonia in bearing distal serrations. The telson (the posteriormost segment of its body) was wide and possessed dorsal median carinae. The type A genital appendage (one of the morphs of eurypterid genital appendages, equipped with clasping organs) was undivided and the pretelson (the segment immediately preceding the telson), lacking dorsal median carina (keels running down the center of the dorsal side), is laterally expanded.
Other than the type species C. ventricosus, defined by its quadrate (square-shaped) pretelson and the narrow and elongated telson, one other species has been assigned to the genus; C. sarlei. The telson of C. sarlei is similar to that of C. ventricosus but the pretelson is shorter and wider. Both of these species have in the past been assigned to the larger and more derived pterygotid Pterygotus. Some authors have speculated that if the chelicerae of Ciurcopterus are large (they are at present unknown from the genus), many fragmentary pterygotid specimens and species known only from chelicerae could be reassigned to Ciurcopterus.
Ciurcopterus is classified as part of the pterygotid family of eurypterids, a group of highly derived eurypterids of the Silurian to Devonian periods that differ from other groups by a number of features, perhaps most prominently in the chelicerae (the first pair of limbs) and the telson. The chelicerae of the Pterygotidae were enlargened and robust, clearly adapted to be used for active prey capture and more similar to the claws of some modern crustaceans, with well developed teeth on the claws, than to the chelicerae of other eurypterid groups. Another feature distinguishing the group from other eurypterid groups were their flattened and expanded telsons, likely used as rudders when swimming. Their walking legs were small and slender, without spines, and they were likely not capable of walking on land.
History of research
Ciurcopterus was first described as a species of Pterygotus, P. ventricosus, by Erik. N. Kjellesvig-Waering in 1948. This species was represented by the dorsal impression of a single incomplete fossilised individual discovered near Kokomo, Indiana. The specimen (USNM 88130, currently housed at the U.S. National Museum in Washington) preserves most of the body, excluding parts of the appendages and the end of the telson. This individual would have measured approximately 29 cm (11 in) in length in life. Kjellesvig-Waering noted that the species did not resemble any of the other North American species of Pterygotus that had been described but that it did share some similarities with the British P. anglicus, from which it could still be differentiated by the different shape of the carapace, differences in the sixth appendage and P. ventricosus exhibiting a greater gibbosity.