New information on the structure and relationships of Buchanosteus (Placodermi: Euarthrodira) from the Early Devonian of New South Wales

New material of the Early Devonian placoderm Buchanosteus confertituberculatus (Chapman) is described, and a revised diagnosis is presented. The new material permits a detailed account of endocranial morphology; a canal for the orbital artery is identified for the first time, the courses of the efferent pseudobranchial and ophthalmica magna arteries are reinterpreted, and new information is presented on the structure of the palatoquadrate and parasphenoid. The presence of an eyestalk is established. Interrelationships of euarthrodiran taxa are briefly discussed; Buchanosteus is referred to the suborder Brachythoraci, which is redefined on the structure of the cheek and elongation of the submarginal plate. Other phylogenetically significant characters for brachythoracid taxa are considered to be the reduction of the trunk-shield and the shape of central and preorbital plates in the skull-roof. Revised diagnoses are presented for the order Euarthrodira, suborder Brachythoraci, infraorder Phlyctaenioidei and family Buchanosteidae. Parabuchanosteus murrumbidgeensis (White) is considered synonymous with Buchanosteus confertituberculatus.