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Abstract.-A detailed description of the caudal skeleton of the Paleogene nasine surgeonfish Arambourgthurus scombrurus is provided herein. The compound (urostylar) centrum is fused with a large, nearly triangular, thick hypural plate, formed by consolidated hypurals 1-4. The compound centrum supports a large autogenous parhypural bearing a thick and blunt hypurapophysis. Hypural 5 is a small triangular bone. There is a single elongate autogenous uroneural and three epurals, the first of which is remarkably stout and elongate, with its proximal end embraced by the open neural arch of the second preural vertebra. The haemal spines of the second and third preural vertebrae are fused to their respective centra. The phylogenetic relationships of Arambourgthurus are discussed. Arambourgthurus shares some features with the Eocene Sorbinithurus and the Miocene Marosichthys, and together they constitute a monophyletic group that represents the sister group of the pair formed by the extant genus Naso and the fossil genus Eonaso. The overall structure of the caudal propulsive unit bears a strong resemblance to that of Luvarus and some of its closely related fossil taxa, and this suggests that Arambourgthurus scombrurus most likely had an oceanic habitat in the epipelagic zone.
Keywords: Arambourgthurus scombrurus, caudal-fin skeleton, Acanthuridae, Nasinae, phylogeny, paleoecology
The surgeonfish Arambourgthurus scombrurus was described by Camille Arambourg (1967) based upon two almost complete specimens from Istehbanat, south-central Iran, and was placed in Naseus (=Naso). These fossils were part of a large collection of fishes derived from extensive exploration of the Paleogene Pabdeh Formation. The precise age of the fish-bearing strata of the Pabdeh Formation is problematic. Arambourg (1967) considered them to be of early Oligocene age (Rupelian) based upon the taxonomic composition of the ichthyofauna, whereas Haghipour and Brants (1971) assigned these fossiliferous deposits to a generic Middle to Late Eocene age based upon the presence of planktonic foraminiferans of the genus Hantkenina (see also Afsari et al. 2014). A preliminary analysis of the calcareous nannoplankton content seems to suggest a latest Eocene (Priabonian) age (E. Fornaciari, pers. comm.); however, additional and much more detailed biostratigraphic analyses will be necessary to conclusively determine the precise age of these fossils.
Arambourg (1967) described the fossil surgeonfish from the Pabdeh Formation as a peculiar member of the family Acanthuridae characterized by a distintive...