Pseudohilsa nikosi Kevrekidis, Arratia, and Reichenbacher, 2021

Kevrekidis, Charalampos, Arratia, Gloria, Bacharidis, Nikos & Reichenbacher, Bettina, 2021, A new clupeid fish from the upper Miocene of Greece: A possible Hilsa relative from the Mediterranean, Acta Palaeontologica Polonica 66 (3), pp. 605-621 : 607-614

publication ID	https://doi.org/ 10.4202/app.00871.2020
persistent identifier	https://treatment.plazi.org/id/814CBA51-1339-F67D-A630-F97EFE801D63
treatment provided by	Felipe
scientific name	Pseudohilsa nikosi Kevrekidis, Arratia, and Reichenbacher
status	sp. nov.

Treatment

Pseudohilsa nikosi Kevrekidis, Arratia, and Reichenbacher sp. nov.

Figs. 2–6 View Fig View Fig View Fig View Fig View Fig , Tables 1, 2.

Etymology: Named in honor of Nikos Bacharidis, the private research- er, fossil collector and visual artist who discovered the locality and collected the material presented here, in recognition of his longstanding contribution to the development of palaeontology in Greece.

Type material: Holotype LGPUT ADS 001 . Paratypes LGPUT ADS 002–005 , all from the type locality .

Type locality: Site “Aidonochori A” (40°50’ 29” N, 23°43’ 12” E) next to the village Aidonochori near Serres, Central Macedonia, Greece GoogleMaps .

Type horizon: Upper Miocene .

Material.— Type material only.

Diagnosis.—Distinguished from the only other species in Pseudohilsa , P. brevicauda ( Lednev, 1914) , by the possession of more dorsal fin rays (17 vs. 10–13 in P. brevicauda ), a smaller eye (ca. 25% of HL vs. ca. 33%) and the insertion of the pelvic fin under the anterior half of the dorsal fin’s base (vs. the posterior half).

Distinguished from all other extant and fossil clupeid species by the following combination of features: relatively small size (up to ca. 150 mm) with maximum depth at dorsal fin origin 24–33% SL; HL ca. 32% of SL; two pairs of bullae; at least six striae on the parietal and postparietal bones = frontoparietal striae in older literature, see description below); mouth terminal; two supramaxillae; lower jaw articulation at about middle of orbit; five branchiostegal rays; 10 supraneurals; 40–42 vertebrae; ratio of pleural ribs to total vertebrae 0.57–0.6; pectoral fin with 15 rays; pelvic fin with eight or nine rays; anal fin with 16–19 rays; two epurals; transverse row with ca. 40 scales, no dorsal scutes; strong ventral scutes (four at gular region, 11 or 12 prepelvic associated with ribs, 10 or 11 postpelvic).

Description.—Slender-bodied fish, with a triangular head Fig. 2 View Fig ). The dorsal outline is almost straight and the ventral outline is convex. In the following sections, the emphasis is placed on features or bones which are clearly discernible; structures that are badly preserved are noted as such or omitted from the description.

Neurocranium: The neurocranium is approximately triangular in lateral and ventral view. Some bones, particularly those behind the orbit, are crushed and their precise borders are difficult to discern. The basioccipital and first vertebra are preserved in anatomical connection. The small supraoccipital forms the posterodorsal angle of the neurocranium. Anteriorly to that is the postparietal (see Schultze 2008 and Teng et al. 2019; termed “parietal” in the traditional literature), which is separated by the oval temporal foramen from the parietal bone (see Schultze 2008 and Teng et al. 2019; termed “frontal” in the traditional literature). The temporal foramen seems not to be overlain by any flange. The parietals are large and have an almost straight to slightly convex profile in lateral view ( Fig. 3A View Fig ). Posteriorly, on their dorsal surface, they are ornamented with more than six prominent and reticulate striae, best seen in the specimen LGPUT ADS 005 ( Fig. 3B View Fig ). The parietals connect anteriorly with the mesethmoid and ventrally to the latter is the vomer, which appears toothless. Posteriorly to the vomer is the straight and slender parasphenoid. The parasphenoid projects at about the lower third of the orbit and apparently lacks teeth. It can be discerned that the orbit is limited anterodorsally by the parietal bones and lateral ethmoids, and posterodorsally by the parietal bones and sphenotics. Posteriorly to the sphenotics, the pterotics bear the pterotic bullae. Anteroventrally to the pterotic bullae, the larger prootic bullae are well preserved. Both pairs of bullae are easy to recognize owing to their round shape and their characteristic, glassy and perforated texture ( Fig. 3C View Fig ).

The specimen LGPUT ADS 003 is an isolated neurocranium in ventral view ( Fig. 3D View Fig ). In this specimen, the posterior part of the basioccipital, situated between the exoccipitals, is discernible. The parasphenoid is missing and seems to have broken off. Anterolaterally to the exoccipitals, the pterotics bear the pterotic bullae. Anteriorly to the exoccipitals and the basioccipital are the prootics with the much larger prootic bullae. Anterolaterally to the prootics, the sphenotics are recognizable. Anteriorly to the sphenotics and prootics are the large, broadly triangular parietal bones, whose anterior portion is missing.

Circumorbital series: The orbit is rounded and occupies ca. 25% of the HL. Five infraorbitals are discernible and traces of the infraorbital canal are visible on infraorbital 3 ( Fig. 3A View Fig ). Anteriorly, the first infraorbital is elongate, the second is shorter and the third is the largest of the series. The fourth and fifth infraorbitals form the postero-dorsal margin of the orbit, and they have a broadly rectangular shape ( Fig. 3A View Fig ). It is not clear if a sixth infraorbital (dermosphenotic) is present or not. A semi-circular sclerotic bone is preserved in the upper half of the orbit.

Jaws: There is no sign of teeth in any of the jaws. The premaxilla is subtriangular and well ossified ( Fig. 3A View Fig ). The maxilla has a robust, rod-like anterior articular process and a flattened and curved posterior blade; the angle between these two parts is about 150°. Of all the bones of the upper jaw, the maxilla extends farthest posteriorly, overlapping with about the anterior third of the orbit. The posterior supramaxilla is paddle-shaped and the anterior supramaxilla is smaller and elongate. There is no sign of a hypomaxilla.

The dentary is broad and robust, with a well-developed ventral arm. In specimen LGPUT ADS 004 the lateral surface of the dentary is visible; it possesses a fossa near its anterodorsal tip. The mandibular canal is positioned longitudinally near the ventral margin of the bone. The anguloarticular is preserved in anatomical connection with the dentary ( Fig. 3A View Fig ). The retroarticular is not recognizable. The articulation of the lower jaw with the suspensorium is positioned approximately at the level of the middle of the orbit.

Suspensorium and palatoquadrate: The dorsal part of the hyomandibula is expanded; ventrally the hyomandibular shaft is long and almost straight, and its ventral tip is separated from the quadrate by a space occupied by the symplectic, which is not preserved ( Fig. 3A View Fig ). The quadrate is subtriangular, with approximately equal sides. The dorsoposterior process is robust and pointed, and dorsally the quadrate’s margin is slightly concave, lacking an incision. The ectopterygoid, which appears toothless, is slender and the dorsal and ventral arms form an oblique angle (ca. 120°).

Opercular series: The opercle has an almost straight anterior margin and a slightly convex ventral margin. It is smooth and bears no striations. The subopercle is ovoid to broadly triangular, with a short and pointed anterodorsal process. The preopercle is wide, L-shaped, and its upper arm is longer than the lower arm, which is tapering and rounded anteriorly ( Fig. 3A View Fig ). The posterior and ventral margins of the preopercle form a slightly acute angle (ca. 80°). The preopercular canal runs near the anterior margin of the preopercle. The interopercle seems to be wider posteriorly and narrower anteriorly. Its anterior tip is almost as long as the lower arm of the preopercle.

Hyoid and branchial arches: There are five branchiostegal rays attached to each ceratohyal ( Fig. 3A View Fig ); the border between the anterior and posterior ceratohyal is not discernible. Ceratobranchials and/or hypobranchials and epibranchials are preserved under the bones of the opercular series; they bear numerous gill rakers anteriorly. In the specimen LGPUT ADS 005, more than 20 gill rakers are recognizable in the second or third lower branchial arch, near the junction with the upper branchial arch.

Vertebral column and associated structures: There are 40–42 vertebrae, including preural centrum 1, and the caudal vertebrae start either at the 16 th or 17 th vertebra ( Table 1). The opercle overlies the first five or five and a half vertebrae; these are included in the total count of vertebrae. The first 10 vertebrae are almost square, while the rest are longer than high. The neural arches are fused to their respective vertebral centra in the abdominal and caudal regions. In some preural vertebrae it is possible to discern that the prezygapophyses are more developed than the postzygapophyses. There are 24 pairs of ribs, starting from the third vertebra and ending on the 26 th, which almost reach the ventral body margin. Therefore, the ratio of ribs to the total number of vertebrae is 0.57–0.6. Due to the state of preservation, the type of articulation of the ribs with the vertebrae and/or the parapophyses is not discernible.

Each vertebra seems to be associated laterally with an epineural (process) and an epipleural, each very thin and elongate; the epipleurals are all free. The epineurals are processes of the lateral walls of the neural arch until ca. the 20 th vertebra, and are disconnected from the wall of the neural arch posterior to that, continuing as free epineurals. In the most anterior vertebrae these structures are usually hard to discern. In the caudal region, the epineurals and epipleurals become progressively more strongly inclined towards the horizontal axis. There are 10 posterodorsally inclined supraneurals, shaped as slender wedges with an expanded dorsal tip, which become slenderer posteriorly.

Pectoral and pelvic girdles and fins: The posttemporal has a broad body and slender, rod-like dorsal and ventral arms, the dorsal arm being longer than the ventral ( Fig. 3A View Fig ). The supracleithrum is broad dorsally and tapers ventrally. The cleithrum is long and robust, curving anteriorly at its ventral portion. Close to the cleithrum and ventrally to the supracleithrum there are two elongate postcleithra, the upper one is short and broad, and the lower one is long, slender, and almost reaches the ventral margin of the body. Ventrally to the cleithrum and attached to it is the coracoid, which is flat, thin and highly perforated ( Fig. 3A View Fig ). The anterior and ventral margins of the coracoid are convex, and posteroventrally the coracoid is pointed. Posterodorsally, between the cleithrum and the coracoid, is the scapula. The scapula, which is perforated by the scapular foramen, is followed posteriorly by the pectoral radials, which are crushed in our specimens. The pectoral fin is situated close to the ventral margin and has 15 rays.

The pelvic bones are long and triangular, reaching forward the length of four or five vertebrae. The number of pelvic fin rays is eight or nine. The pelvic fin originates approximately under the 18 th or 19 th vertebra and under the anterior third of the base of the dorsal fin.

Dorsal and anal fins: Both median fins have the following characters in common. They are subtriangular in shape ( Fig. 2 View Fig ), their pterygiophores decrease in size posteriorly and their fin rays increase in length until the third to fifth element and then progressively decrease in lenght ( Fig. 4 View Fig ). The pterygiophores are formed by the proximal radials, which are interpreted as possibly fused to the middle radials (see e.g., Grande 1985: 338, 347; Fig. 4 View Fig ); it is not clear if the distal radials are present or fused with some other element. The first two procurrent rays are distally unbranched and the second ray may or may not be segmented. The third fin ray is segmented and unbranched = (first principal ray), and the rest of the rays are segmented and distally branched.

The dorsal fin origin is located near the middle of the body. It has 17 rays and an equal number of pterygiophores (the last two rays are borne by the same pterygiophore and are therefore counted as one). The pterygiophores of the extremities are modified; the anteriormost one is a flattened and deep, anteroventrally-facing keel, and the posteriormost pterygiophore is a slender horizontally-oriented stay ( Fig. 4A View Fig ). The first pterygiophore is associated with the neural spine of vertebra 9 or 10.

The distance between the end of the dorsal fin and the beginning of the anal fin corresponds to five vertebrae on the horizontal level. There are 16–19 anal fin rays supported by 15–18 pterygiophores ( Fig. 4B View Fig ). The first pterygiophore of the anal fin is unmodified, slender and elongate, as are the rest of the pterygiophores, and it supports two procurrent rays, which are undivided distally. The same pterygiophore is associated with the haemal spine of vertebra 26 or 27. The last pterygiophore is modified to a slender horizontally-oriented stay and bears two fin rays, counted as one ( Fig. 4B View Fig ).

Caudal endoskeleton and fin: The caudal fin is forked and comprises ten principal and seven procurrent rays in the upper lobe and nine principal and six to seven procurrent rays in the lower lobe ( Fig. 5 View Fig ). The longest principal rays are almost three times the length of the shortest. The uppermost principal ray is associated with the sixth hypural and the lowermost principal ray reaches the haemal spine of preural vertebra 2. All except the anteriormost procurrent rays are segmented. Anteriorly to the procurrent rays of each lobe there is a caudal scute, which appears undivided medially and also longer and flatter relative to the procurrent rays ( Fig. 5 View Fig ). The basal segments of the two middle principal rays bear small processes. The caudal rays are additionally supported by the spines of the preural vertebrae 1–5 ( Fig. 5 View Fig ).

Preural vertebra 1 bears a neural arch and a short neural spine and dorsoposteriorly it is fused to the elongate first uroneural, known in clupeoids as well as in other otomorphs as the pleurostyle. Between the pleurostyle and the neural spine of preural vertebra 2 there are two moderately elongate epurals. The arch of the long parhypural articulates with, and is not fused to, the preural centrum 1. The haemal spine of preural centrum 2 is long and broad, and also the haemal spine of preural centrum 3 is more expanded and robust than the haemal spines of the preceding vertebrae. Their neural and haemal arches are fused to their respective centra.

There are six hypurals, of which the first and third are expanded, the rest are narrower. The proximal end of hypural 1 tapers to a hook-shaped process, which does not reach the ural centrum 2 P (polyural terminology, see Methods). Hypural 3 is posteroventrally notched, forming the hypural diastema. All hypurals are autogenous, except for hypural 2, which is fused to the ural centrum 2 P. Posteriorly to the pleurostyle there is the elongate uroneural 2 and posterodorsally to that the small rod-like uroneural 3 ( Fig. 5 View Fig ).

Squamation: The scales are visible only from their medial side, and therefore their anterior fields are exposed to the observer rather than the posterior fields that one would see in a live specimen. Consequently, the sculpture on the lateral surface of the scales is not visible, except for some patches that are transparent enough to allow one to see the fine circuli on the lateral surface of the anterior field of the scales Fig. 6A View Fig ). A transverse row along the body, from behind the head to the end of the hypural plates, comprises ca. 40 scales and there are ca. 11 horizontal scale rows over the pelvic fin. The scales are imbricate and of similar size ( Fig. 6A View Fig ), with the posterior scales being somewhat smaller than the anterior ones. There are no lateral line scales.

There is a series of scutes along the ventral midline of the fishes’ body. There are ca. four free prepelvic scutes along the gular region, 11 or 12 prepelvic scutes each associated with the ventral portion of a pair of ribs and 11 postpelvic scutes, also associated with an equal number of pairs of ribs. The scutes are robust, with a sharp median keel which deepens posteriorly ( Fig. 6B, C View Fig ). The pelvic scute, i.e. the scute which is directly in front of the insertion of the pelvic fin, is shaped similarly to the rest. All the scutes, with the possible exception of the one directly below the pelvic fin, bear ascending arms. These arms are robust and long; the arms of the rib-associated prepelvic scutes extend to about 40% or more of the body cavity and the pelvic scute has the most prominent arms. In general, the ascending arms are placed closer to the anterior end of the scute, except for the posteriormost postpelvic scutes in which the ascending arms are placed closer to the posterior end. Predorsal scutes are absent.

Stratigraphic and geographic range.—Upper Miocene, Serres Basin, Greece.