Arambourgthurus, a new genus of hypurostegic surgeonfish (Acanthuridae) from the Oligocene of Iran, with a phylogeny of the Nasinae

The two specimens of an acanthurid fish from the Oligocene of Iran described by Arambourg (1967) as Naseus scombrurus are redescribed and illustrated. This species is unique among all acanthurid fishes, both fossil and Recent, in its scombroid-like extensive hypurostegy (bases of caudal-fin rays deeply overlapping the hypural plate) and in having three vacant interneural spaces. The new genus Arambourgthurus is established for this species. Arambourgthurus is shown to be a member of a clade (along with the Eocene Sorbinithurus and the Miocene Marosichthys) distinguished by the derived condition of having two dorsal-fin pterygiophores situated in the preneural space; this clade is shown to be the sister group of the clade composed of the Recent Naso and of Eonaso (the latter of unknown but probably less than Eocene age) on the basis of six synapomorphies defining the Nasinae.