Atherina atropatiensis, Carnevale & Haghfarshi & Abbasi & Alimohammadian & Reichenbacher, 2011

Atherina atropatiensis sp. nov.

Figs. 2 View Fig , 3 View Fig , Table 1.

Etymology: Atropates is the ancient founder of the Atropatene State, which is today Azerbaijan. The name of the new species refers to the province Azerbaijan of Iran, the country of the type locality.

Type material: Holotype: BSPG 2010 View Materials XXI−4, a well preserved articulated skeleton ( Fig. 3A View Fig 1 View Fig , A 2 View Fig ) . Paratypes: BSPG 2010 View Materials XXI−5, a well preserved articulated skeleton ; BSPG 2010 View Materials XXI−6 and −7, two well preserved heads ; BSPG 2010 View Materials XXI−8 and −9, two moderately to well preserved articulated skeletons ; BSPG 2010 View Materials XXI−10, a well preserved skeletal fragment ; BSPG 2010 View Materials XXI−11, a poorly preserved articulated skeleton .

http://dx.doi.org/10.4202/app.2011.0003

Type locality: Baghmisheh−Marzdaran , Tabriz Basin, NW Iran ( Fig. 2 View Fig ) . Type horizon: Lignite Beds , Upper Miocene .

Diagnosis.—A species of Atherina with 42 (20+22) vertebrae; first dorsal fin with five or six spines; second dorsal fin with one slender spine plus 11 to 13 soft rays; anal fin with one spine and 16 to 18 soft rays; anterior haemal arches and spines thin and slender; ascending process of the premaxilla spatulate and slightly shorter than the alveolar (ascending/alveolar ratio = 0.83); dorsolateral process of the basipterygium closely associated to the seventh or eighth pleural rib.

Differential diagnosis.— Atherina atropatiensis differs from other fossil Atherina species by its unique combination of meristic characters and anterior haemal arches and spines that are simple and not enlarged. It differs from the Recent Atherina boyeri by the relative development of the ascending and alveolar processes of the premaxilla, and the mutual relationship between pleural ribs and dorsolateral process of the basipterygium.

Measurements (based on the holotype).—Standard length: 43.2 mm; total length: 51.7 mm. Other measurements as percentage of standard length: Head length: 20.6; head depth: 15.3; maximum body depth: 11.6; predorsal (1 st fin) distance: 42.8; predorsal (2 nd fin) distance: 58.6; prepelvic distance: 37.7; prepectoral distance: 22.9; caudal peduncle length:> 18.

Geographic and stratigraphic range.—Known from the type locality and horizon.

Description

The body is elongate, slender and laterally compressed, with a relatively long caudal peduncle ( Fig. 3A View Fig 1 View Fig ). The head is moderately elongate and pointed, its length is contained approximately five times the SL. The orbit is large, located approximately at the middle of head length. Mouth terminal. The gape is slightly oblique, relatively short, extending posteriorly up to the level of the anterior margin of the orbit ( Fig. 4 View Fig ).

The neurocranium is moderately deep with a large orbital region ( Fig. 4 View Fig ). The dorsal surface of the skull is smooth, devoid of crests. The frontals are the largest bones of the skull roof. The prominent supraorbital sensory canal is particularly well exposed in specimens BSPG 2010 XXI−6 and −7 ( Fig. 4 View Fig ). The frontals broadly extend backwards replacing the parietals, which appear to be absent. The ethmoid region is rather short. The rostral ethmoid region appears to be excavated dorsally, forming a fossa for the ascending process of the premaxilla and its originally associated rostral cartilage. The lateral ethmoid is columnar and characterized by a posterior laminar wing. The mesethmoid is short and thickened. The vomer is not exposed in the specimens examined. A straight and robust parasphenoid is visible in the lower third of the orbit. A small basisphenoid is also present. The otic and occipital sectors of the neurocranium are extremely damaged and unclear in all the specimens examined, except for what appears to be a short lateral wing of the sphenotic, which appears to be present in specimens BSPG 2010 XXI−5 and −6.

A badly damaged nasal is partially preserved in specimens BSPG 2010 XXI−6 and −7. The lachrymal is thin and roughly quadrangular in shape, with a shallow notch extending along its posterior border for the articulation with the lateral ethmoid (see Dyer and Chernoff 1996). The second and third infraorbital bones are thin, laminar, subrectangular in shape. The infraorbital and preopercular sensory canals are disconnected.

The general structure of the upper jaw indicates high protrusibility ( Fig. 4 View Fig ). The premaxilla has a moderately elongate alveolar process and a slightly shorter antero−posteriorly enlarged and spatulate ascending process (ascending/alveolar ratio: 0.83). The articular process of the premaxilla is stout and pointed, while the postmaxillary process is poorly developed. Premaxillary teeth are short, curved and pointed. The maxilla is elongate with a thick and rounded anterior articular head; this bone has an anterior shelf that partially overlaps the premaxilla (see Alexander 1967). The dentary is thick with a prominent coronoid process arising from the posterior sector of its alveolar arm. The dentary teeth are similar to those of the premaxilla. The angulo−articular is nearly triangular with a shallow concave anterior margin. The retroarticular is small and compact.

The suspensorium is elongate to span the orbit. The thick and straight hyomandibula is characterized by two scarcely separated articular heads. The symplectic is nearly rectangular in outline. The quadrate is a shallow triangular bone with a posteriorly thickened margin which terminates ventrally in a massive articular head. The metapterygoid is partially preserved in specimen BSPG 2010 XXI−5, showing a nearly quadrangular shape. The ectopterygoid is elongate and laminar, while the endopterygoid is relatively large and ovoid in shape. Pterygoid teeth are not visible but their presence in origin cannot be excluded. The palatine is short, lacking a dorsally directed anterior process (see Dyer and Chernoff 1996).

The hyoid bar is well preserved in specimens BSPG 2010 XXI−6 and −7 ( Fig. 4 View Fig ). The hypohyals are minute. The anterior ceratohyal is robust with a thick dorsal margin and a broad concavity occupying the anterior half of its ventral portion. The posterior ceratohyal is roughly triangular with a gently curved posteroventral margin. There are six sabre−like branchiostegal rays of which the four anterior articulate with the anterior ceratohyal. The urohyal is large, elongate and triangular in shape. The posterodorsal processes of the urohyal (see e.g., Chernoff 1986) are absent.

The gill arches are partially preserved in specimens BSPG 2010 XXI−5 to −7. The ceratobranchials are elongate and slightly curved. The epibranchials are slender, with short uncinate processes. The pharyngobranchials are crushed and difficult to recognize. A large nearly triangular (?upper) pharyngeal jaw with a broad alveolar surface is preserved in BSPG 2010 XXI−7. Thin and elongate branchiospines articulate along the posterior surface of the gill bones.

The preopercle is a L−shaped bone with vertical and horizontal arms of approximately the same length ( Fig. 4 View Fig ). The vertical arm has an anterior thickening, which extends also on the posterior portion of the horizontal arm. The interopercle is laminar and elongate, with a rounded ventral profile. The opercle is quadrangular with curved posterior and ventral margins. The articular process of the opercle is short with a slightly concave articular surface. The subopercle extends along the ventral margin of the opercle; it possesses a rounded ventral margin and is characterized by a relatively short and thin anterior vertical apophysis.

The vertebral column is slightly curved in its abdominal portion and consists of 42 (20+22) vertebrae ( Fig. 3A View Fig 1 View Fig ). The

http://dx.doi.org/10.4202/app.2011.0003

vertebral centra are rectangular, longer than high, slightly constricted in the middle, and characterized by one or two horizontal ridges along their lateral surface. The neural arches of vertebrae two to five with a large anteroposteriorly expanded plate; these neural arches appear to be not meeting in the midline to form a spine. The neural spines of vertebrae six to eight (or nine?) are spatulate distally. Well−developed neural prezygapophyses progressively reduced in size posteriorly arise from the anterior margin of the basal portion of the neural arches. The neural prezygapophyses of the anterior abdominal vertebrae are bifid. The neural spines are straight, thin and obliquely oriented, becoming thickened in the posterior portion of the caudal region. The haemal arches, spines and prezygapophyses are similar in morphology to their neural counterparts. Slender and elongate pleural ribs insert along the lower third of the abdominal centra, beginning on the third. Epineural bones thin.

The caudal fin is deeply forked and consists of 17 principal rays, plus eight dorsal and 12 ventral procurrent rays. The caudal skeleton consists of a robust autogenous parhypural, a large triangular hypaxial hypural plate (hypurals 1+2) fused to the last centrum, three autogenous epaxial hypurals, a large uroneural fused to the urostyle, and two (or three) epurals ( Fig. 3A View Fig 2 View Fig ). The neural spine of the penultimate vertebra is reduced to a short crest. The autogenous haemal spine of the penultimate vertebra is greatly enlarged. The haemal spine of the antepenultimate vertebra is also autogenous; this structure is bifid in the holotype, with the anterior spine preserved as impression only. Haemal arches of both the penultimate and antepenultimate vertebrae are greatly expanded into a crest characterized by delicate serrations along their anteroventral margin.

Supraneural bones absent. The first dorsal fin inserts at the level of the 14 th vertebra and consists of five or six spines supported by four or five pterygiophores. The second dorsal fin contains a single spine plus 11 to 13 soft rays. The two dorsal fins are separated by a large gap, which equals the length of eight vertebrae; in the endoskeleton, this gap is filled in part by rayless pterygiophores.

The anal fin inserts in advance to the second dorsal−fin origin ( Fig. 3C View Fig ). It consists of one spine plus 17–18 soft rays, supported by 16–17 pterygiophores.

The pectoral fin inserts high along the lateral side of the body and consists of ten rays. The cleithrum is a large, crescent−shaped bone with a thickened anterior margin, and a small laminar process emerging from its posterodorsal margin. The coracoids is a broad posteriorly curved lamina. The scapula and posttemporal are poorly preserved and difficult to recognize. A single postcleithrum is recognizable. There are four hourglass−shaped pectoral−fin radials.

The pelvic fin consists of one spine plus five rays. The basipterygia are strongly ossified elongate bones with a thickened central axis and an enlarged distal portion characterized by an irregular posterior margin ( Fig. 3B View Fig ). Medially, each basipterygium possesses a thin and pointed anteromedial spine, a stout posteromedial spine (sensu Stiassny 1990), and an irregular posterodorsally oriented dorsolateral process, the latter closely associated to the seventh or eighth pleural rib.

The body is covered by thin deciduous cycloid scales.

Discussion

Taxonomic interpretation.—The taxonomic interpretation of fossil atherinid fishes is problematic. This diverse group of tropical to temperate fishes contains a number of anatomically conservative species, which are distinguishable from each other by delicate skeletal structures, as well as by characters related to muscles and ligaments (see Dyer and Chernoff 1996), which are not normally adequately preserved. Nevertheless, our morpho−anatomical study indicates several features that indicates that the Iranian material represents a new species of the genus Atherina .

The specimens show a number of features typical of atherinomorphs ( Parenti 1993), including the absence of supraneurals, neural arches of vertebrae two through five that are anteroposteriorly expanded into a flat quadrangular plate, dorsolateral process of the basipterygia associated with the distal tips of pleural ribs, and reduced number of infraorbital bones.

The inclusion of the studied specimens within the “atherinoid” fishes is justified by several morphological features, including the general physiognomy, presence of six branchiostegal rays ( Rosen 1964), possession of small teeth ( Starks 1899), infraorbital series constituted by the three anterior elements only ( Chernoff 1986; Stiassny 1990), epibranchials uncinate (see Rosen and Parenti 1981), first pleural rib inserting on the third vertebra ( Rosen 1964), and basipterygia bearing anteromedial and posteromedial spines ( Stiassny 1990).

The absence of neurocranial crests ( Gosline 1962), possession of a developed basisphenoid ( Starks 1899), absence of parietals (see Ivantsoff et al. 1987), presence of an articular notch along the posterior margin of the lachrymal ( Dyer and Chernoff 1996), as well as certain meristic values (vertebral number, fin formulae) support the assignment of the specimens to the family Atherinidae .

Finally, the attribution of the specimens to the genus Atherina is based on several diagnostic features typical of living species of this genus, such as the presence of a maxillary ventral shelf (e.g., Alexander 1967), preopercular and infraorbital sensory canals disconnected ( Dyer and Chernoff 1996), dorsally directed anterior palatine process absent ( Dyer and Chernoff 1996), and dorsolateral process of the basipterygium oriented posterodorsally ( Dyer and Chernoff 1996).

Comparisons.—The genus Atherina includes five extant species ( A. breviceps , A. boyeri , A. hepsetus , A. lopeziana , A. presbyter , see Table 1) that live in brackish and shallow marine waters of the eastern Atlantic, from Scandinavia to South Africa, Mediterranean Sea, Black Sea and Caspian Sea, plus seven skeleton−based extinct species ( A. cavalloi , A. impropria , A. prima , A. sarmatica , A. schelkovnikovi , A. suchovi , A. sumgaitica , see Table 1) from the Miocene of the Mediterranean (e.g., Gaudant 1981; Landini and Sorbini 1989) and Paratethys ( Gorjanović−Kramberger 1891; Switshenskaja 1973; Anđelković 1989). All these species show marked morphological variability (e.g., Kiener and Spillmann 1969; Boscolo 1970; Switshenskaja 1973; Bamber and Henderson 1985; Mistri and Colombo 1988) and can be distinguished from each other mostly based on meristic features ( Table 1), premaxillary and dentary configuration, and relative enlargement of the anterior haemal arches ( Borsieri 1904; Schultz 1948; Kiener and Spillmann 1969; Switshenskaja 1973). A comparative analysis of the potentially diagnostic features of Atherina atropatiensis sp. nov. has revealed that this form differs from all other congeners, except A. boyeri , in having a unique combination of meristic values ( Table 1) and anterior haemal arches and spines that are simple and not enlarged.

Atherina boyeri View in CoL is a highly heterogeneous and polymorphic species that displays a wide range of morphometric and meristic values, broadly overlapping those of other species, often related to various environmental conditions ( Kiener and Spillmann 1969, 1972); the morphology of the anterior haemal arches and spines is also highly variable in this eurytopic species, ranging from thick and chunky to thin and slender ( Kiener and Spillmann 1969). In spite of considerable morphological similarity, however, Atherina atropatiensis sp. nov. and A. boyeri View in CoL are readily distinguished by the different relative development of the ascending and alveolar processes of the premaxilla, with higher values of the ascending/alveolar ratio in the former than the latter (0.83 vs 0.58–0.69) ( Bamber and Henderson 1985), and the dorsolateral process of the basipterygium, which is closely associated with the seventh or eighth pleural rib in A. atropatiensis sp. nov., and the sixth in A. boyeri ( Kiener and Spillmann 1969) View in CoL .

Moreover, Bogachev (1962) described a new taxon Prolebias schelkovnikovi from the Neogene sediments near Tabriz that may be another Atherina View in CoL . The proportions and meristic values are similar to those of Atherina atropatiensis sp. nov., but it differs in having a lower number of abdominal vertebrae (17 rather than 20) and higher number of principal rays in the caudal fin (20 rather than 17). This may then be a second Atherina species from the Tabriz Basin: the name Prolebias schelkovnikovi is not available for this species because Bogachev (1936) described a different Atherina species with the same name in an earlier study (see also Table 1). Further fossils from the Tabriz Basin are required before a new name is provided for “ Prolebias ” [= Atherina View in CoL ] schelkovnikovi Bogachev, 1962 .