The structure and relationships of a new osteolepiform fish from the late devonian of Victoria, Australia

A new osteolepiform, Marsdenichthys longioccipitus gen. et sp. nov., is described from the Frasnian lacustrine shales (Avon River Group) near Mt Howitt, central Victoria. Because it has thin cycloidal scales with large overlap areas and lacks cosmine in the dermal skeleton Marsdenichthys gen. nov. is regarded as the sister taxon to eusthenopterids. In possessing an extratemporal bone in the skull roof, Marsdenichthys gen. nov. is more primitive than other eusthenopterids. The Osteolepiformes are assumed to be monophyletic by possession of a unique complex of cheek bones, large dermal anocleithrum, scapulocoracoid structure and basal scutes on the fins. Other characters unique to Eusthenopteron such as a long basihyal, fused first infra-and suprapharyngobranchials and large vestibular fontanelles on the braincase may be useful synapomorphies of the Osteolepiformes when more is known of the anatomy of other genera. Problems in defining osteolepidids and eusthenopterids are discussed.