Bollettino della Società Paleontologica Italiana, 53 (1), 2014, 27-37. Modena

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

Solmaz Afsari, Mehdi Yazdi, Ali Bahrami & Giorgio Carnevale

S. Afsari, Department of Geology, Science and Research Branch, Islamic Azad University, Tehran, Iran; [email protected] M. Yazdi, Department of Geology, University of Isfahan, Isfahan, Iran; [email protected] A. Bahrami, Department of Geology, University of Isfahan, Isfahan, Iran; [email protected] G. Carnevale, Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, I-10125 Torino, Italia; [email protected]; corresponding author

KEY WORDS - Stomiiformes, Sternoptychidae, deep-sea hatchetfish,Eosternoptyx discoidalis n. gen. et sp., Eocene, Ilam, Zagros Basin.

ABSTRACT - 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 generaArgyropelecus , Discosternon, Horbatshia, Polypnoides, and Sternoptyx.

RIASSUNTO - [Un nuovo pesce accetta (Teleostei: Stomiiformes: Sternoptychidae) nell’Eocene di Ilam, Bacino di Zagros, Iran] - Il clade dei pesci accetta comprende alcuni tra gli stomiiformi morfologicamente più bizzarri, caratterizzati da un corpo molto elevato e fortemente compresso con fianchi argentati appiattiti lateralmente. Questo peculiare gruppo di pesci mesopelagici è costituito da tre generi viventi e tre generi estinti che comparvero nel registro paleontologico a partire dall’Eocene Medio. Eosternoptyx discoidalis n. gen. et sp., un nuovo pesce accetta proveniente dai depositi dell’Eocene Medio-Superiore della Formazione Pabdeh, affioranti pochi chilometri ad Est della città di Ilam, nel Bacino di Zagros, Iran, viene descritto sulla base di un esemplare parzialmente completo e caratterizzato da una morfologia spiccatamente discoide e da una combinazione di caratteri molto particolare, che comprende la presenza di 28 (13+15) vertebre, otto paia di coste pleurali, due coste pleurali posteriori più corte che non raggiungono il margine ventrale della chiglia addominale, sei supraneurali, lamina dorsale formata da supraneurali modificati assente, pinna dorsale costituita da 18 raggi, pterigioforo anteriore della pinna dorsale molto sviluppato e protrudente oltre il margine dorsale del corpo, due spine preanali, origine della pinna anale opposta a quella della pinna dorsale, posttemporale separato dal supracleitro, spina posttemporale assente, e corpo privo di copertura scagliosa. L’analisi osteologica comparata sembra suggerire che Eosternoptyx n. gen. occupi una posizione filogeneticamente intermedia tra il genere basalePolyipnus e i generi di pesce accetta più derivati Argyropelecus, Discosternon, Horbatshia, Polypnoides e Sternoptyx.

INTRODUCTION including species belonging to groups today occurring in the oceanic realm, for example berycids, bythitids, The existence of exceptionally well-preserved teleost bregmacerotids, gempylids, gonostomatids, lophotids, skeletal remains from the Pabdeh Formation cropping nomeids, phosichthyids, and sternoptychids. Based on out in the territory surrounding the city of Ilam was Arambourg’s (1967) study of the fish fauna at Ilam, the reported for the first time by Priem (1908), who described family Sternoptychidae is exclusively represented by the a small fossil assemblage based on material discovered species Maurolicus morgani Arambourg, 1967. A few by Roland de Mecquenem around 1904 in the course of years after the publication of the monographic study by his geological exploration of Iran. In agreement with the Arambourg (1967), Haghipour & Brants (1971) published hypothesis formulated by de Mecquenem (1908) based a short report on a few fishes collected from the Pabdeh on stratigraphic data, Priem (1908) suggested that the fish Formation in another site in the surroundings of Ilam assemblage was Cretaceous in age. Towards the end of and included a few biostratigraphic data allowing a more 1938 and at the beginning of 1939, Camille Arambourg precise age assignment. More recently, Jafarian et al. visited the locality discovered more than 30 years earlier (1999) provided a short overview of the fish assemblage by Roland de Mecquenem and carried out extensive of Ilam. excavations that resulted in the accumulation of a vast The goal of this paper is to describe another collection of articulated skeletons of teleost fishes, many sternoptychid from the Ilam, representing the first deep of which were described in great detail in his monograph bodied member of this family, commonly known as deep- devoted to this fauna published about 30 years later sea hatchetfishes, recognized from this locality. (Arambourg, 1967). Arambourg (1967) documented The deep-sea hatchetfishes are among the most the existence of slightly fewer than 30 taxa from Ilam, bizarre members of the order Stomiiformes being

ISSN 0375-7633 doi:10.4435/BSPI.2014.03 28 Bollettino della Società Paleontologica Italiana, 53 (1), 2014 characterized by a deep and strongly compressed body, The age of the ichthyolitiferous layers of the with vertically flattened silvery sides used to reflect the Pabdeh Formation exposed in the surroundings of incident light and appear invisible at all angles of view. Ilam was erroneously assigned to the Rupelian by These mesopelagic fishes primarily feed on amphipods, Arambourg (1967) based on the presence of certain copepods, euphasiids and ostracods and usually occur at taxa as Bregmaceros filamentosus (Priem, 1908) and depths below 200 meters, exhibiting a diurnal vertical Palaeorhynchus, taxa which, at that time, were known migration toward the surface (e.g., Hopkins & Baird, exclusively from the Oligocene of the Rhine Basin, 1985). The group of deep-sea hatchetfishes includes three Switzerland, the Carpathians and North Caucasus; both extant genera, Argyropelecus, Polyipnus and Sternoptyx, these taxa have been subsequently documented in Eocene plus three fossil genera - Polyipnoides from the Middle deposits of Europe (e.g., Bannikov, 2010). Haghipour & Eocene of Georgia, Horbatshia from the Oligocene of the Brants (1971) proposed a Middle to Late Eocene age based Carpathians, and Discosternon from the Middle Miocene on the presence of planktonic foraminiferans belonging of Italy (Prokofiev, 2005; Carnevale, 2008; Colombero et to the genus Hantkenina in the fish-bearing layers (see al., 2013) - forming a well-defined and cohesive clade also Coxall et al., 2003). A recent analysis of the foraminiferan known as (subfamily) Sternoptychinae. content of strata placed about 300 meters above the fish- A detailed morphoanatomical analysis of the single bearing layers revealed the presence of several benthic available specimen of deep-sea hatchetfish from forms that seem to indicate a Middle to Late Eocene age, Ilam revealed that it represents a new genus of the including Bolivinoides draco (Marsson, 1878), Elphidium Sternoptychinae, thereby constituting the seventh genus sp., Laevidentalina communis (d’Orbigny, 1826), Lagena of the subfamily and one of the earliest members of this sp., Marginulina wetherelli Jones, 1854, Nodosaria group. spp., Operculina sp., Stilostomella sp., Tritaxia aspera (Cushman, 1926), and Uvigerina spinicostata Cushman & Jarvis, 1929. STRATIGRAPHIC NOTES

The Pabdeh Formation originated in the northwestern MATERIALS AND METHODS and southwestern parts of the Zagros Basin between the late Paleocene and the early Oligocene and consists of The specimen described herein was collected in 2011 thin to thick-bedded limestone intercalated with gray during summer field activities east of the city of Ilam shales with abundant planktonic microfauna (e.g., James (33°37’36’’N, 46°26’24’’E), western Iran (Fig. 1). It is & Wynd, 1965; Sampò, 1969). Overall, the Pabdeh currently housed in the paleontological collection of the Formation accumulated under oxygen-depleted conditions Museum of the Department of Geology, University of in the deeper parts of intrashelf basins formed in a Isfahan (IUV). The fossil was examined using a Leica M80 foreland suite (see, e.g., Ala et al., 1980; Murris, 1980). stereomicroscope with attached camera lucida drawing In a recent study, Mohseni et al. (2011) concluded that arm. Measurements were taked with a dial caliper, to the the depositional environment of the Pabdeh Formation nearest 0.1 mm. The specimen required matrix removal was a distally steepened ramp ending in intrashelf basin before examination in order to allow observation of its where organic matter-rich facies were deposited. In the anatomical structure in as much detail as possible; this Ilam territory (Fig. 1), the Pabdeh Formation is about was achieved using entomological mounting needles. 800 m thick and mostly consists of marls and gray shale Comparative data were derived from the literature, mainly alternated with thin-bedded limestone. Baird (1971), Weitzman (1974), Harold (1993, 1994), Carnevale (2003, 2007, 2008), and Prokofiev (2005).

Abbreviations AB, abdominal photophores; AN, anal photophores; bsp, basisphenoid; cl, cleithrum; cor, coracoid; ecp, ectopterygoid; enp, endopterygoid; epi, epioccipital; f, frontal; h, hyomandibula; io, infraorbital bone; IUV, Museum of the Department of Geology, University of Isfahan, Isfahan, Iran; le, lateral ethmoid; MGPA, Museo Geo-Paleontologico dell’Alto Aventino, Palena (CH), Italy; mtp, metapterygoid; op, opercle; pa, parietal; pal, palatine; PAN, preanal photophores; pas, parasphenoid; pfr, pectoral-fin rays; PIN, Borisyak Paleontological Institute, Russian Academy of Sciences, Moscow, Russia; pop, preopercle; pro, prootic; pto, pterotic; pts, pterosphenoid; ptt, posttemporal; q, quadrate; rad, pectoral-fin radials; SAB, supra-abdominal photophores; SC, subcaudal photophores; sca, scapula; scl, supracleithrum; soc, supraoccipital; spn, first supraneural; spo, sphenotic; USNM, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA; Fig. 1 - Sketch map of Iran showing the location of Ilam. sym, symplectic; vo, vomer. S. Afsari et alii - Eocene deep-sea hatchetfish from Zagros Basin, Iran 29

SYSTEMATICS times in SL. The head is deep and antero-posteriorly compressed. The orbit is large. Part of the large eyeball is Subdivision Teleostei sensu Patterson & Rosen, 1977 preserved as a thin organic film. The interobital region was Order Stomiiformes sensu Harold & Weitzman, 1996 possibly relatively wide in origin. The caudal peduncle Infraorder Gonostomata sensu Harold, 1998 is short and deep. Family Sternoptychidae Duméril, 1806 The neurocranium is remarkably deep posteriorly and nearly triangular in outline (Fig. 2). The braincase is rather Genus Eosternoptyx n. gen. large. The ethmoid region is inadequately preserved in the single available specimen. The only partially recognizable Diagnosis - A deep-sea hatchetfish with discoid body; elements of the ethmoid region are the large and columnar body depth 79.3% of standard length; caudal peduncle lateral ethmoid and the vomer; scattered conical vomerine length 13.2% of standard length; 28 (13+15) vertebrae; teeth are also recognizable. The frontals are by far the eight pairs of pleural ribs; two posteriormost pleural largest bones of the skull roof. Each frontal bears a ribs shortened not reaching the ventral margin of the prominent longitudinal ridge characterized by a regularly abdominal keel; six supraneurals; dorsal blade absent; crenulated margin. The posteroventral sector of the frontals 18 dorsal-fin rays; first dorsal-fin pterygiophore very is partially covered by sparse irregular pits, exhibiting a large and protruding externally beyond the dorsal profile cancellous structure similar to that characteristic of certain of the body; two preanal spines; anal-fin origin opposite species of the genus Argyropelecus and Sternoptyx (see to dorsal-fin origin; posttemporal and supracleithrum not Weitzman, 1974; Carnevale, 2003). The parietal is rather fused; posttemporal spine absent; body naked. large and bears well-developed mid longitudinal and lateral ridges. The broad lateral and posterior extension Derivation of name - From the Greek Eos (dawn or of the parietal possibly resulted in the absence of a sunrise); and sternoptyx, from the Greek words sternon posttemporal fossa. The small supraoccipital has a pointed (chest) and ptyx (plate), referring to the remarkably deep distal end. What appears to be the epioccipital has a greatly and compressed body shape typical of the deep-sea concave posterior margin and a nearly smooth outer hatchetfishes; hence a ‘deep-sea hatchetfish from the surface. Exoccipital, basioccipital and intercalary are not Eocene’. accessible in the specimen. There is no evidence of the original presence of the otic bullae. The parasphenoid is bent at right angle; the horizontal arm constitutes most of Eosternoptyx discoidalis n. sp. the basicranium and is nearly straight while the vertical (Figs 2-4) arm gradually expands dorsally. The basisphenoid is tubular and roughly parallel to the vertical arm of the Diagnosis - As for the genus, only species. parasphenoid. Prootic and pterosphenoid are rather large; the ventral margin of these bones define the central and Holotype - IUV IL/S1, a partially complete articulated posterior portions of the dorsal border of the orbit. Both skeleton in part and counterpart (Fig. 2). the sphenotic and pterotic are of moderate size and appear to be nearly quadrangular in outline. Locality and age - East of Ilam, Ilam Region, western Fragmentary remains of relatively large and laminar Iran; Middle to early Late Eocene. bone(s) underlying the horizontal arm of the parasphenoid can be interpreted as element(s) of the infraorbital series. Derivation of name - The species name refers to the There is no evidence of the supraorbital (Fig. 3). almost discoid outline of the body. Both the upper and lower jaws are not preserved in the available specimen (Figs 2-3). Description - The holotype and only known specimen The structure of the suspensorium is, in large part, of Eosternoptyx discoidalis n. gen. et sp. consists of a relatively well exposed (Fig. 2). Overall, the suspensorium relatively well-preserved partially complete articulated is remarkably deep and the quadrate-mandibular joint was skeleton in part and counterpart lacking the jaws, hyoid apparently situated roughly under the midregion of the apparatus, part of the gill arches, and median and pelvic orbit. The spineless hyomandibula is slender and strongly fin rays (Fig. 2). elongate, reaching about four fifths of the neurocranial The specimen is relatively small, its standard length length; the articular head of the hyomandibula is flat and measuring 29.5 mm. Other measurements as percentage obliquely oriented and appears to be articulated with the of standard length (SL): body depth 79.3%; head length pterotic; the opercular process emerges along the posterior 33.5%; head depth 74.9%; orbit diameter 22.7%; caudal border of the hyomandibula within the upper one sixth of peduncle length 13.2%; caudal peduncle depth 15.6%; its length. The symplectic is short and stout, characterized dorsal-fin length 42.7%; anal-fin length 26.1%; predorsal by a weakly club-shaped distal termination. The quadrate is length 60.3%; preanal length 82.7%; prepectoral length rather large and appears to be roughly triangular in outline; 47.1%; prepelvic length 58.9%. the dorsal sector of the anterior border of the quadrate The body is nearly discoid in outline, extremely contacts the gently arched and laminar ectopterygoid. deep and remarkably compressed (Fig. 2). Like in Both the large endopterygoid and the metapterygoid are the Miocene Discosternon (see Carnevale, 2008), the incomplete; endopterygoid teeth are absent. The palatine, cephalic-abdominal portion of the body is rounded and which is partially recognizable in counterpart, articulates hypertrophied and the body trunk is deep and considerably posteriorly with the ectopterygoid and endopterygoid. shortened. The abdominal keel is contained about three Scattered palatine teeth appear to be present. 30 Bollettino della Società Paleontologica Italiana, 53 (1), 2014

Fig. 2 - Eosternoptyx discoidalis n. gen. et sp. from the Eocene of Ilam. (a) Holotype, IUV IL/S1, right lateral view; (b) holotype, IUV IL/ S1, counterpart, left lateral view; (c) reconstruction of the skeleton. Scale bars correspond to 10 mm.

The opercular series is only partially preserved posterior border of the dorsal limb of this bone is smooth. (Fig. 3). The exposed bones of the series are vertically The opercle is subquadrangular in outline, with a gently elongate. Of the preopercle, only the elongate dorsal limb curved upper border and an irregular ventral border; is clearly recognizable; the dorsal limb of the preopercle this bone is strengthened by a vertical longitudinal ridge encloses a laterosensory canal for most of its length; the originating at the level of the opercular-hyomandibular S. Afsari et alii - Eocene deep-sea hatchetfish from Zagros Basin, Iran 31

Fig. 3 - Eosternoptyx discoidalis n. gen. et sp. from the Eocene of Ilam. Reconstruction of the skull, right lateral view. See text for abbreviations. 32 Bollettino della Società Paleontologica Italiana, 53 (1), 2014 joint. Subopercle and interopercle are not recognizable second hypurals only at its base. What appears to be the in the specimen. proximal portion of an uroneural can be recognized. The The architecture of the gill arches is difficult to interpret neural spine of the second preural centrum is flattened and due to the extensive lateral compression that modified the anteroposteriorly expanded, while the haemal spine seems original configuration of these complex structures so to be autogenous. The caudal fin is not recognizable in the that the mutual relationships between the bones cannot single available specimen. be properly recognized. Nevertheless, hypobranchials, The long-based dorsal fin is exclusively represented ceratobranchials, epibranchials and pharyngobranchials by its endoskeletal portion (Fig. 2). The fin is preceded by can be recognized evidently displaced from their original six laterally flattened and distally expanded supraneurals, position. Elongate gill rakers bearing small denticles and the anterior inserting in the first interneural space; possibly belonging to the hypobranchial or ceratobranchial all the supraneurals are similar to each other and not of the first branchial arch are also partially exposed. The especially modified into a dorsal blade. There are 18 third and fourth pharyngobranchials are irregular in shape proximal+middle dorsal-fin pterygiophores, some of and bear strong conical teeth with recurved tips. Overall, which are poorly preserved and mostly represented as the morphology of the individual bones of the gill arches impression only; the distal pterygiophores of the dorsal of Eosternoptyx discoidalis appears to be similar to those fin are not preserved. The first dorsal-fin pterygiophore of Polyipnus asteroides figured by Weitzman (1974). originates in the seventh interneural space; it is the largest The vertebral column is compact and sigmoid with element of the series and protrudes from the continuous the abdominal portion bent in a remarkably kyphotic dorsal profile of the body due to a spatulate process curve (Fig. 2). The vertebral column consists of 28 arising from the main axis of bone. The second dorsal-fin vertebrae. The vertebral centra are anteroposteriorly pterygiophore also inserts in the seventh interneural space compressed, higher than long and laterally ornamented and resembles the first element of the pterygiophore series. with shallow fossae and ridges. According to Weitzman Overall, the dorsal-fin pterygiophore series includes a (1974), the abdominal vertebrae of sternoptychids variety of morphologies. Considering that the first dorsal- include those elements that do not bear a long, single fin pterygiophore of extant deep-sea hatchetfishes supports haemal spine and/or are not directly associated with the two rays and that the last one bears no rays (see Weitzman, anal-fin pterygiophores, with the anterior haemal spine 1974), it is reasonable to hypothesize that the dorsal fin that is invariably associated with the anterior anal-fin of Eosternoptyx contained 18 rays in origin. ptergiophore. Based on these considerations, Eosternoptyx The anal-fin rays are not preserved (Fig. 2). Of has 13 abdominal and 15 caudal vertebrae. All the neural the anal fin only the peculiar 12 pterygiophores are and haemal arches appear to be fused with their respective recognizable. There is not a definite ratio between dorsal- centra. The anterior six anterior neural spines remain fin pterygiophores and rays in extant deep-sea hatchetfishes unfused medially, completely separated from each other and for this reason it is not possible to provide a reasonable up to their distal tips. The neural arches of the anterior hypothesis about the original number of anal-fin rays nine vertebrae are anteroposteriorly expanded. The neural in Eosternoptyx. All the anal-fin pterygiophores are in spines of the abdominal and caudal vertebrae are elongate some ways anteroposteriorly expanded, characterized and robust; the neural spines of the vertebrae 11 to 23 are by a central nearly cylindrical axis that supports anterior very robust and vertically oriented. The haemal spines of and posterior bony laminae of different morphology and the anterior caudal vertebrae are expanded, as well as the size; the anterior and posterior borders of these bones haemal arches of the three posterior abdominal vertebrae. are parallel and closely associated to each other for most The haemal spine of the 18th vertebra is less developed of their length. The two anterior anal-fin pterygiophores that those of the preceding and successive elements, are greatly elongate and bear an external spine (preanal resulting in the origin of a moderately developed anal- spines in the sense of Schultz, 1961) that project beyond fin hiatus (see Harold, 1994); the vertebrae 23 to 25 are the ventral profile of the body. The anal-fin hiatus is notably shortened in order to accommodate the enlarged primarily developed between the sixth and seventh anal- subcaudal photophore mass. The third through eighth fin pterygiophores; the proximal shaft of the fifth and vertebrae bear thick pleural ribs that extend ventrally to eighth anal-fin pterygiophores are also slightly modified the ventral profile of the body; the two posterior elongate to accommodate the deep structures of anal photophores. pleural ribs are associated with the pelvic girdle. The ninth The shaft of the last anal-fin pterygiophore is opposite to and tenth vertebrae support small pleural ribs. The pleural that of the dorsal fin, both lying in the interneural space ribs insert on the parapophyses and their bases are notably between the vertebrae 21 and 22. expanded anteroposteriorly. Epineural bones are present The pectoral girdle is very high due to the discoid on the bases of the neural arches of the second through morphology of the body (Figs 2-3). The outer surface of ninth vertebrae. some of the bones of the pectoral girdle, including the The caudal is scarcely preserved and difficult to posttemporal, supracleithrum and cleithrum is densely interpret (Fig. 2c). The first preural centrum is fused with ornamented. The posttemporal is a robust L-shaped bone the ural centra. The first and second hypurals are fused with irregular dorsal border that is completely separate together into a single autogenous plate; the third, fourth from the supracleithrum but articulates with it. The and fifth hypurals are mostly preserved as impression only supracleithrum is very large, elongate and blade like; it and appear to be co-ossified into an autogenous plate; a broadly overlaps the upper limb of the cleithrum. The small autogenous sixth hypural was possibly present in cleithrum is roughly sigmoid in shape and has a long and origin. The parhypural is a cylindrical and rather elongate slender upper limb and a relatively short anteroventral bone; it appears to be co-ossified with the fused first and limb; the ventral border of the anteroventral limb of the S. Afsari et alii - Eocene deep-sea hatchetfish from Zagros Basin, Iran 33

Fig. 4 - Eosternoptyx discoidalis n. gen. et sp. from the Eocene of Ilam. Ventral part of the body showing the photophore clusters. Holotype, IUV IL/S1, right lateral view. Scale bar corresponds to 10 mm. See text for abbreviations. cleithrum is characterized by a regularly serrated posterior by a black globular film developed in the anal-fin hiatus sector and ends anteriorly with a ventrally directed stout and under the 10th to 12th caudal vertebrae, respectively spine (preabdominal spine in the sense of Schultz, 1961). (Fig. 4). Coracoid and scapula are poorly preserved. Two short cylindrical pectoral-fin radials can be recognized. Of the pectoral fin, remains of seven rays are clearly exposed DISCUSSION in the fossil. The pelvic girdle is dorsally oriented as in other deep- The morphological analysis of the specimen sea hatchetfishes (Fig. 2c). The relatively short pubic documented herein has revealed a variety of features process is preserved mostly as impression. A ventrally that unquestionably support its recognition as a new genus directed iliac spine is stout and well developed, protruding and species of deep-sea hatchetfish. That of the deep- beyond the ventral profile of the body. What appears to sea hatchetfishes is a derived group of the stomiiform be a second iliac spine is preserved as impression only. family Sternoptychidae. According to Weitzman The ischial process is posterodorsally oriented. The pubic (1974), the Sternoptychidae includes ten extant genera process is closely associated with the pleural ribs of the (Araiophos, Argyripnus, Argyropelecus, Danaphos, seventh and eighth vertebrae. Ischial and pubic processes Maurolicus, Polyipnus, Sonoda, Sternoptyx, Thorophos, are separated by an angle of about 45°. There is a stout Valenciennellus) sharing several synapomorphies, and pointed anterodorsally directed process emerging including: the presence of type Alpha photophores and along the anterior margin of the basipterygium, which their occurrence in glandular clusters, possession of possibly is homologous of the anterior blade-like process three branchiostegal rays articulated with the posterior characteristic of the genus Argyropelecus (see Harold, ceratohyal, parietals separated by the supraoccipital, and 1993). Pelvic-fin rays are not preserved. absence of basihyal and endopterygoid teeth. In a more There are no traces of the original squamation, recent phylogenetic analysis of the stomiiform fishes, including the specialized and thick scales associated Harold & Weitzman (1996) recognized more than 20 to photophores typical of Argyropelecus, Horbatshia, characters that corroborated the monophyly of the family; Polyipnoides and Polyipnus (see Weitzman, 1974; however, Harold & Weitzman (1996) also pointed out Prokofiev, 2005), thereby suggesting that the body was that only two of the more than 20 features that define probably naked (Fig. 2). The absence of the stiff bony the Sternoptychidae are unique and unreversed (parietals scales in the abdominal keel suggests a condition similar separated by the supraoccipital, and photophore arranged to that characteristic of Sternoptyx with a membranous in clusters as a result of their development by budding; keel constituted by strong connective tissue. see also Ahlstrom et al., 1984). As documented above, A thin organic film reveals the outlines of at least seven Eosternoptyx exhibits some of the diagnostic characters of photophores of the abdominal cluster situated along the the Sternoptychidae, including: absence of endopterygoid ventral margin of the abdominal keel, the four posterior teeth, hypurals 1+2 and 3+4+5 (apparently) fused, and partially separated from each other by the distal portions parhypural fused to the lower hypural plate. of the pre-pelvic pleural ribs (Fig. 4). The supra-abdominal Within the Sternoptychidae, the deep-sea cluster seems to be represented by two small photophores hatchetfishes form a well-defined monophyletic clade, vertically aligned in the lower third of the space between the Sternoptychinae (e.g., Baird, 1971; Weitzman, 1974), the fourth and fifth pleural ribs (Fig. 4). Of the preanal currently diagnosed by more than 30 synapomorphies cluster, three photophores are partially recognizable, while (Harold, 1993; Harold & Weitzman, 1996). The the anal and subcaudal clusters are exclusively represented assignment of Eosternoptyx to the Sternoptychinae 34 Bollettino della Società Paleontologica Italiana, 53 (1), 2014

Fig. 5 - Representatives of fossil and extant deep-sea hatchetfish genera. (a)Horbatshia brevis (Horbatsh, 1961) from the Oligocene Menilite Formation, Ukrainian Carpathians, holotype, PIN4841/1, left lateral view; (b) Discosternon federicae Carnevale, 2008 from the Miocene of Tufillo Unit, Gessopalena, central Italy, holotype, MGPA GES001, left lateral view; (c) Polypnoides laevis Daniltshenko, 1962 from the Eocene Dabakhan Formation, Georgia, holotype, PIN 1413/62, right lateral view; (d) Polyipnus tridentifer McCulloch, 1914, paratype, USNM 179851, left lateral view; (e) Argyropelecus lychnus Garman, 1899, USNM 274434, left lateral view; (f) Sternoptyx obscura Garman, 1899, paratype, USNM 120294, left lateral view. Scale bars correspond to 10 mm. (a) and (c) courtesy of Artem M. Prokofiev. (d), (e) and (f) courtesy of Sandra J. Raredon. is supported by a number of features (see Harold & ossified and well-developed, neural spine of the second Weitzman, 1996), including: body depth measuring preural centrum flat and broad, basipterygium vertically about 80% of SL, abdominal keel present, possession of developed and directly supported by pleural ribs, and prominent frontal crest, parietal crest present, sphenotic possession of iliac spine. of moderate size, symplectic with club-shaped distal Eosternoptyx is characterized by a unique combination termination, opercle elongate and subrectangular in of features that clearly indicate that it represents a new outline, hyomandibula greatly elongate, possession genus of the deep-sea hatchetfish clade. As pointed out of expanded neural and haemal spines, pleural ribs above, the sternoptychine clade (Fig. 5) consists of three enlarged, anal-fin hiatus present, posttemporal strongly extant (Argyropelecus, Polyipnus, Sternoptyx) and three S. Afsari et alii - Eocene deep-sea hatchetfish from Zagros Basin, Iran 35

No. species Vertebrae Supraneurals Dorsal-fin rays Median fin position Argyropelecus 7 34-40 8 8-10 anal-fin origin opposite to last dorsal-fin ray Discosternon 1 28 3 26 anal-fin origin approximately opposite to dorsal-fin origin Eosternoptyx 1 28 6 18 anal-fin origin approximately opposite to dorsal-fin origin Horbatshia 1 27 3 8 anal-fin origin beneath the middle of dorsal fin Polyipnus 32 31-36 5-6 10-16 anal-fin origin usually beneath middle of dorsal fin Polypnoides 1 27 ? ? ? Sternoptyx 4 27-32 4 8-11 dorsal-fin origin behind dorsal-fin origin

Tab. 1 - Synopsis of selected meristic and morphological features of deep-sea hatchetfishes. Data from Baird (1971), Weitzman (1974), Badcock & Baird (1980), Ahlstrom et al. (1984), Harold (1993, 1994), Last & Harold (1994), Harold, Wessel & Johnson (1998), Prokofiev (2005), and Carnevale (2008). extinct (Discosternon, Horbatshia, Polypnoides) genera the cleithrum in Discosternon might be interpreted as one (see Weitzman, 1974; Prokofiev, 2002, 2005), thereby of the morphological traits that appeared in response to implying that Eosternoptyx represents the seventh known the miniaturization of the body. None of the apomorphic genus of this highly modified group of stomiiform fishes. features that diagnosed Argyropelecus (e.g., basisphenoid The genus Polyipnus is currently regarded as the absent; possession of a ventral, median keel-like structure sister group of all the other deep-sea hatchetfish genera of the parasphenoid; ossification of ventral limb of (see Weitzman, 1974; Harold, 1993; Harold & Weitzman, the posttemporal incomplete) and Sternoptyx (parietal 1996; Prokofiev, 2005; Carnevale, 2008). With more contacts the intercalary; epioccipital process spine-like than 30 described species, this monophyletic genus and slender; opercular spine as a strong lateral spiny (see Harold, 1994) is the most speciose of the deep-sea process; vomer absent; lateral ethmoid absent; epineurals hatchetfish genera (see Tab. 1). Only one of the uniquely absent) are present in Eosternoptyx (Harold, 1993; Harold derived features that characterize Polyipnus (see Harold & Weitzman, 1996). & Weitzman, 1996), presence of vomerine teeth, appears The Miocene Discosternon exhibits a set of reductive to be present in Eosternoptyx. However, Eosternoptyx features (e.g., 13 caudal vertebrae; slender neural and shares several other features with Polyipnus, including the haemal spines; three supraneurals present; loss of anal-fin possession of six distally spatulate supraneurals roughly hiatus; loss of abdominal keel; Fig. 5b) none of which was similar to each other in size and shape (with the consequent observed in Eosternoptyx. absence of the so-called dorsal blade characteristic of The osteological structure of Horbatshia and more advanced deep-sea hatchetfishes), possession of Polypnoides is largely unknown, documented by a modified first dorsal-fin pterygiophore that protrudes inadequately preserved material of problematic from the dorsal profile of the body, overall structure of the interpretation (see Prokofiev, 2002, 2005; Fig. 4a, c). ventral portion of the pectoral girdle, possession of two Anyway, Polypnoides differs from Eosternoptyx in a preanal spines, and pleural rib complement characterized number of morphological, morphometric and meristic by six very large ribs, of which the posterior two are features (see Tab. 1), among which the possession of a associated with the pelvic girdle, followed by two short crescent-shaped opercle and a caudal skeleton with four ribs that do not contribute to the abdominal keel (see separated hypurals appear to be the most relevant. As far Weitzman, 1974; Harold, 1994). Despite the presence of as Horbatshia is concerned, it differs from Eosternoptyx several features shared with the basal sternoptychine genus in several meristic and morphological features (see Polyipnus, Eosternoptyx shows substantial morphological Tab. 1), including the possession of three supraneurals, evidence suggesting that it should be regarded as more eight dorsal-fin rays, and posttemporal fused with the derived. supracleithrum. Eosternoptyx shares osteological characters with Overall, the compact and very deep body of Argyropelecus, Discosternon and Sternoptyx, including Eosternoptyx with the associated extremely shortened the absence of otic bullae and the extremely arched caudal region and opposite dorsal- and anal-fin origins parasphenoid. Weitzman (1974) suggested that the are also characteristic of Discosternon, Horbatshia and development of a parasphenoid that is strongly arched Sternoptyx (Fig. 5a, b, f). However, the phylogenetic or bent at a right angle, which is characteristic of the significance of these strongly correlated and possibly derived deep-sea hatchetfishes, is possibly constrained by homoplastic features is difficult to interpret. a combination of features, including the very deep head, In summary, the osteological evidence seems to presence of large eyes and lack of otic bullae. Moreover, indicate that Eosternoptyx might occupy an intermediate Eosternoptyx shares with all the extinct and extant deep- position between the basal genus Polyipnus and the sea hatchetfish genera but Polyipnus the possession other deep-sea hatchetfish genera, possibly representing of frontals with ornamented outer surface. Finally, it the sister group of the clade formed by Argyropelecus, shares with Argyropelecus, Horbatshia, Polypnoides and Discosternon, Horbatshia, Polypnoides and Sternoptyx. Sternoptyx the possession of cleithrum with extensively One of the most relevant features in the evaluation of the pitted lateral surface. Carnevale (2008) hypothesized that phylogenetic relationships of the deep-sea hatchetfishes is the absence of ornamentation along the lateral surface of the possession of the modified supraneurals that form the 36 Bollettino della Società Paleontologica Italiana, 53 (1), 2014 so-called dorsal blade (secondarily lost in Discosternon). Carnevale G. (2008). Miniature deep-sea hatchetfish (Teleostei: As discussed above, both Polyipnus and Eosternoptyx are Stomiiformes) from the Miocene of Italy. Geological Magazine, characterized by six unmodified supraneurals and by an 145: 73-84. enlarged anterior dorsal-fin pterygiophore that emerges Colombero S., Bonelli E., Kotsakis A., Pavia G., Pavia M., Villier B. & Carnevale G. (2013). Late Messinian rodents from Verduno from the dorsal profile of the body. Unfortunately, because (Piedmont, NW Italy): biochronological, paleoecological and of the incompleteness of the single available specimen paleobiogeographical implications. Géobios, 46: 111-125. documented herein, it is not possible to properly define Coxall H.K., Huber B.T. & Pearson P.N. (2003). Origin and the phylogenetic position of Eosternoptyx within the morphology of the Eocene planktonic foraminifer Hantkenina. sternoptychine clade. Additional well-preserved material Journal of Foraminiferal Research, 33: 237-261. is certainly necessary to conclusively infer its phylogenetic Cushman J.A. (1926). The foraminifera of the Velasco Shale of the affinities. Moreover, a detailed morphological analysis of Tampico Embayment. American Association of the Petroleum Horbatshia and Polypnoides would be also desirable to Geologists, Bulletin, 10: 581-612. perform a comprehensive phylogenetic study of fossil and Cushman J.A. & Jarvis P.W. (1929). New foraminifera from Trinidad. Contributions of the Cushman Laboratory for extant deep-sea hatchetfishes. Foraminiferal Research, 5: 6-17. Daniltshenko P.G. (1962). Fishes of the Dabakhan Formation of ACKNOWLEDGEMENTS Georgia. Paleontologicheskyi Zhurnal, 5: 111-126. [in Russian] Duméril A.M.C. (1806). Zoologie analytique, ou méthode naturelle Many thanks go to Walter Landini (Dipartimento di Scienze de classification des animaux, rendue plus facile à l’aide de della Terra, Università di Pisa, Italy) for useful discussion. The late tableaux synoptiques. 344 pp. Allais, Paris. Erminio Di Carlo (MGPA) is thanked for permission to examine Garman S. (1899). Reports on an exploration off the coasts of material under his care and logistic support. Sandra J. Raredon Mexico, Central and South America, and off the Galapagos (USNM) provided radiographs of extant deep-sea hatchetfishes and Islands. Memoirs of the Museum of Comparative Zoology, Artem M. Prokofiev (Severtsov Institute of Ecology and Evolution, 24: 1-431. Russian Academy of Sciences, Moscow, Russia) provided pictures Haghipour A. & Brants A. (1971). Eocene remains from the Pabdeh of Horbatshia brevis and Polypnoides laevis. Thanks are also Formation north of Ilam. Geological Survey of Iran Report, due to Federica Giudice for improvement of the English and to 19: 81-107. Doriano Caputo for technical help. The manuscript has benefited Harold A.S. (1993). Phylogenetic relationships of the sternoptychid from the comments and suggestions made by Jean Gaudant Argyropelecus (Teleostei: Stomiiformes). Copeia, 1993: 123-133. (Muséum National d’Histoire Naturelle, Paris), Annalisa Ferretti Harold A.S. (1994). A taxonomic revision of the sternoptychid (Dipartimento di Scienze Chimiche e Geologiche, Università degli genus Polyipnus (Teleostei: Stomiiformes), with an analysis Studi di Modena e Reggio Emilia) and an anonymous reviewer. The of phylogenetic relationships. Bulletin of Marine Science, 54: authors thank the Department of Geology, University of Isfahan for 428-534. financial and logistic supports. Harold A.S. (1998). Phylogenetic relationships of the Gonostomatidae (Teleostei: Stomiiformes). Bulletin of Marine Science, 62: 715- 741. REFERENCES Harold A.S. & Weitzman S.H. (1996). Interrelationships of stomiiform fishes. In Stiassny M.L.J., Parenti L.R. & Johnson Ahlstrom E.H., Richards W.J. & Weitzman S.H. (1984). Families G.D. (eds), Interrelationships of fishes, Academic Press, San Gonostomatidae, Sternoptychidae, and associated stomiiform Diego: 333-353. groups: development and relationships. In Moser H.G., Harold A.S., Wessel J.H. & Johnson R.K. (1998). A new species of Richards W.J., Cohen D.M., Fahay M.P., Kendall Jr. A.W. & Polyipnus (Teleostei: Stomiiformes) from the western Indian Richardson S.L. (eds), Ontogeny and Systematics of Fishes. Ocean, with comments on sternoptychid ecology. Proceedings American Society of Ichthyologists and Herpetologists Special of the Biological Society of Washington, 111: 942-953. Publication, 1: 184-198. Hopkins T.L. & Baird R.C. (1985). Feeding ecology of four Ala M.A., Kinghorn R.R.F. & Rahman M. (1980). Organic hatchetfishes (Sternoptychidae) in the eastern Gulf of Mexico. geochemistry and source rock characteristics of the Zagros Bulletin of Marine Science, 36: 260-277. petroleum province, South West Iran. Journal of Petroleum Horbatsh L.P. (1961). Ostatki ryby semeystva Sternoptychidae iz Geology, 3: 61-89. melinitovych slantsev Karpat. Paleontologicheskiy Zhurnal, 4: Arambourg C. (1967). Résultats Scientifiques de la Mission C. 168-170. [in Russian] Arambourg en Syrie et en Iran (1938-1939). II. Les Poissons Jafarian M.A., Samani P.B. & Ghobadipour M. (1999). Investigation Oligocènes de l’Iran. Notes et Mémoires sur le Moyen-Orient, and introduction of some Eocene fish fossils in Zagros Basin. 8: 1-210. University of Isfahan Journal (Science), 13: 181-196. [in Farsi] Badcock J. & Baird R.C. (1980). Remarks on systematics, James G.A. & Wynd J.G. (1965). Stratigraphic nomenclature of development, and distribution of the hatchetfish genus Iranian Oil Consortium Agreement Area. American Association Sternoptyx (Pisces, Stomiatoidei). Fishery Bulletin, 77: 803-820. of Petroleum Geologists Bulletin, 49: 2182-2245. Baird R.C. (1971). The systematics, distribution, and zoogeography Jones T.R. (1854). Foraminifera. In Morris J. (ed.), A catalogue of the marine hatchetfishes (family Sternoptychidae). Bulletin of British fossils: comprising the genera and species hitherto of the Museum of Comparative Zoology, 142: 1-128. described; with reference to their geological distribution and to Bannikov A.F. (2010). Fossil Vertebrates of Russia and Adjacent the localities in which they have been found, London: 32-44. Countries. Fossil Acanthopterygian Fishes (Teleostei, Last P.R. & Harold A.S. (1994). Polyipnus latirastrus n. sp., a new Acanthopterygii). 244 pp. GEOS, Moscow. [in Russian] and unusual hatchetfish (Stomiiformes: Sternoptychidae) from Carnevale G. (2003). Redescription and phylogenetic relationships the Western Pacific.Copeia , 1994: 201-215. of Argyropelecus logearti (Teleostei: Stomiiformes: Marsson T. (1878). Die Foraminiferen der weissen Schreibkreide der Sternoptychidae), with a brief review of fossil Argyropelecus. Insel Ruegen. Mitteilungen des Naturwissenschaftlichen Vereins Rivista Italiana di Paleontologia e Stratigrafia, 109: 63-76. für Neu-Vorpommern und Rugen in Greifswald, 10: 115-196. Carnevale G. (2007). Fossil fishes from the Serravallian (Middle McCulloch A.R. (1914). Report on some fishes obtained by the Miocene) of Torricella Peligna, Italy. Palaeontographia Italica, F.I.S. ENDEAVOUR on the Coasts of Queensland, New 91: 3-69. South Wales, Victoria, Tasmania, and South and South-western S. Afsari et alii - Eocene deep-sea hatchetfish from Zagros Basin, Iran 37

Australia. Pt. 2. Zoological Results of the F.I.S. ENDEAVOUR Sampò M. (1969). Microfacies and microfossils of the Zagros 1909-10, 2: 78-87. Area, Southwestern Iran (from pre-Permian to Miocene). 102 de Mecquenem R. (1908). Note sur le Crétacé supérieur pp. E.J. Brill, Leiden. d’Husseinabad (Poucht-e-Kouh). In Priem F., Poissons fossiles Schultz L.P. (1961). Revision of the marine silver hatchetfishes de Perse. Annales d’Histoire Naturelle, Paléontologie, 1: 1-3. (family Sternoptychidae). Proceedings of the United States Mohseni H., Behbahani R., Khodabakhsh S. & Atashmard Z. (2011). National Museum, 112: 587-649. Depositional environments and trace fossil assemblage in the Weitzman S.H. (1974). Osteology and evolutionary relationships Pabdeh Formation (Paleogene), Zagros Basin, Iran. Neues of the Sternoptychidae, with a new classification of stomiatoid Jahrbuch für Geologie und Paläontologie, Abhandlungen, families. Bulletin of the American Museum of Natural History, 262: 59-77. 153: 327-478. Murris R.J. (1980). Middle East: stratigraphic evolution and oil habitat. American Association of Petroleum Geologists Bulletin, 4: 597-618. d’Orbigny A. (1826). Tableau méthodique de la classe des Céphalopodes. Annales des Sciences Naturelles, 7: 245-314. Patterson C. & Rosen D.E. (1977). Review of ichthyodectiform and other Mesozoic teleost fishes and the theory and practice of classifying fossils. Bulletin of the American Museum of Natural History, 158: 81-172. Priem F. (1908). Poissons fossiles de Perse. Annales d’Histoire Naturelle, Paléontologie, 1: 1-25. Prokofiev A.M. (2002). Sternoptychidae (Stomiiformes) in Eocene Manuscript received 24 June 2013 to Lower Oligocene of Caucasus and Ukrainian Carpathian Manuscript accepted 10 March 2014 Mountains. Journal of Ichthyology, 42: 19-27. Zoobank registration number urn:lsid:zoobank.org:pub:2D47CE58- Prokofiev A.M. (2005). Systematics and phylogeny of the 6BD1-4D61-96A7-2F1ACA99F395 stomiiform fishes (Neoteleostei: Stomiiformes) from the Zoobank registration date 31 March 2014 Paleogene-Neogene of Russia and adjacent regions. Journal Published online 9 April 2014 of Ichthyology, 45: S89-S162. Editor Annalisa Ferretti