A new species of Atlantidosteus Lelièvre, 1984 (Placodermi, Arthrodira, Brachythoraci) from the Middle Devonian of the Broken River area (Queensland, Australia)

A new brachythoracid arthrodire, Atlantidosteus pacifica n. sp., is described from the Middle Devonian (Eifelian Broken River Group; costatus-australis conodont zone) of Queensland (Australia). This is the first record of this genus outside its type area (Early Devonian of Morocco), and adds to evidence of a similar fish fauna in shallow tropical to subtropical marine environments along the eastern and northern margins of the Gondwana supercontinent. The new species is based on a large suborbital plate of unusual shape, from possibly the largest arthrodire recorded so far from the Australian Devonian, with an estimated skull length of 50 cm. It closely resembles the suborbital of the type species (A. hollardi Lelièvre, 1984), with a high postorbital lamina and distinctive sensory groove pattern, but differs in the pointed anterior margin to the unornamented mesially directed suborbital lamina, and the absence of a dorsal angle on the curved dorsal margin of the postorbital lamina. Atlantidosteus Lelièvre, 1984 is provisionally placed as a sister group to Homostius Asmuss, 1856, with which it shares a suborbital plate with an elongate internal suborbital lamina.