A new deep-sea hatchetfish (Teleostei: Stomiiformes: Sternoptychidae) from the Eocene of Ilam, Zagros Basin, Iran

The deep-sea hatchetfish clade includes some of the most bizarre stomiiforms, which are characterized by a deep and strongly compressed body, with vertically flattened silvery sides. This peculiar group of mesopelagic fishes consists of three extant and three extinct genera that appeared in the record in the Middle Eocene. A new genus and species of deep-sea hatchetfish, Eosternoptyx discoidalis n. gen. et sp., is described from the Middle to Late Eocene deposits of the Pabdeh Formation exposed a few kilometres east of the city of Ilam, Zagros Basin, Iran. The new deep-sea hatchetfish taxon from Ilam is based on a partially complete specimen in part and counterpart characterized by a remarkably discoid physiognomy of the body and a unique combination of features, including: 28 (13+15) vertebrae; eight pairs of pleural ribs; two posteriormost pleural ribs shortened not reaching the ventral margin of the abdominal keel; six supraneurals; dorsal blade absent; dorsal fin contains 18 rays; first dorsal-fin pterygiophore very large and protruding externally beyond the dorsal profile of the body; possession of two preanal spines; anal-fin origin opposite to dorsal-fin origin; posttemporal and supracleithrum not fused; posttemporal spine absent; body naked. The comparative osteological analysis suggests that Eosternoptyx n. gen. appears to be intermediate between the basal genus Polyipnus and the more derived deep-sea hatchetfish genera Argyropelecus, Discosternon, Horbatshia, Polypnoides, and Sternoptyx.