Bones of the pectoral girdle skeleton in representatives of 48 genera in 41 families of eurypterygian fishes were studied. The structure and position of the pectoral girdle is relatively stable in eurypterygians. It consists of the extrascapula, posttemporal, supracleithrum, cleithrum, postcleithrum, scapula, coracoid, actinosts, and fin rays. An ectocoracoid is present in some Gasterosteiformes. Most eurypterygians have a posttemporal with distinct dorsal and ventromedial processes. The dorsal process ligamentously articulates with the epioccipital and the ventromedial process with the intercalary when present, and when absent, with the posteroventral margins of the pterotic and exoccipital. In most taxa, the supracleithrum bears a sensory canal and articulates with the posttemporal anteriorly and with the cleithrum posteriorly. The cleithrum is Y-shaped consisting of the anterodorsal, posterodorsal, and ventral rami. Dorsal and ventral postcleithra are present in some Eurypterygii. The scapula articulates with the cleithrum and bears a foramen which is complete or open anteriorly and bordered by the cleithral ventral ramus. Most taxa have four autogenous actinosts that increase in size gradually from the first to the fourth, but in some, the first actinost is fused. The dorsalmost ray is rudimentary in some taxa.
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