Article: Structure and function of the pectoral joint and operculum in antiarchs, Devonian placoderm fishes
G. C. Young and Zhang Guorui
The structure and function of the pectoral fin joint and operculum in antiarchs (Devonian placoderm fishes) are re-examined in the light of new evidence from primitive Early Devonian representatives from South China, and acid-prepared material from the Late Devonian of Western Australia. The characteristic dermal brachial process of advanced antiarchs is absent in Yunnanolepis, and rudimentary in Procondylolepis. The latter displays a perichondrally ossified articular surface of the scapulocoracoid surrounded by three large foramina, which may be homologized with similar structures in primitive anrthrodires. It is proposed that during the evolution of the complex brachial articulation of advanced antiarchs the posterior two foramina fused to form the axillary foramen, which carried nerves and vessels to the fin, but not a muscle. Procondylolepis is reinterpreted to have had two proximal dermal articulations on its pectoral fin, which was rotated by an abductor muscle inserting posteroventral and an adductor muscle inserting posterodorsal to the scapulocoracoid articulation. A similar interpretation is applicable to other antiarchs. Manipulation of the fin in acid-prepared Bothriolepis indicates that an oar-like swimming function was most unlikely. The same specimen reveals a small groove and opening adjacent to the anterior articulation of the submarginal plate (dermal operculum), which probably contained the spiracular tube.