Vascularization of the osteostracan and antiarch (Placodermi) pectoral fin: similarities, and implications for placoderm relationships

Phylogenetic analyses frequently resolve the extinct group Placodermi at the base of the clade of jawed fishes (traditionally known as the Gnathostomata), with the jawless fish group Osteostraci as sister taxon to this clade. Both gnathostomes and osteostracans possess pectoral fins supported by a radial(s) articulating on a cartilaginous scapulocoracoid. Blood vessels and nerves pass by or through the scapulocoracoid to supply the musculature of the pectoral fin, and in the Osteostraci also pass through the postbranchial lamina backing the gill chamber before reaching the scapulocoracoid. This course also characterizes the placoderm group Antiarchi. Other placoderms retain the condition typical of other jawed fishes in which the scapulocoracoid, as well as the subclavian veins and arteries, are entirely posterior to the back wall of the gill chamber, lying within the internal region of the trunkshield. These observations suggest that these placoderm groups are more closely related to other jawed fishes than are the Antiarchi, challenging the monophyly of the Placodermi.