Holosteus esocinus, AGASSIZ, 1835

† HOLOSTEUS ESOCINUS AGASSIZ, 1835

( FIGS 1–6 View Figure 1 View Figure 2 View Figure 4 View Figure 5 View Figure 6 )

† Holosteus esocinus Agassiz, 1835, p. 306 ; Agassiz, 1844: 85, pl. 43, fig. 5; Bronn, 1856: 683, pl. 52, fig. 8; de Zigno, 1874: 140; Woodward, 1901: 270; Eastman, 1904: 28; Eastman, 1905:14–15; Leriche, 1906: 381; von Zittel, 1932: 464; Harry, 1953a: 244; Blot, 1980: 353.

† Xiphopterus falcatus ( Volta, 1796) : Frickhinger, 1991: 910; Caltran, Zorzin & Lazzarin, 1998: 81.

Holotype: MNHN F.Bol 175, nearly complete articulated skeleton ( Fig. 1A View Figure 1 ), 347.2 mm SL.

Referred material: MCSNV IG.23601 and IG.23602, nearly complete articulated skeleton, in part and counterpart ( Fig. 1B, C View Figure 1 ), 499.4 mm SL; MGPUP 11611/2, nearly complete articulated skeleton, in part and counterpart ( Fig. 1D, E View Figure 1 ), 541.0 mm SL; MCSNV B5/ T 98, incomplete articulated specimen lacking of part of the abdominal and cranial regions, in part and counterpart ( Fig. 2A, B View Figure 2 ); MCSNV VI.N41, partially complete articulated specimen ( Fig. 2C View Figure 2 ), 644.1 mm SL.

Type locality and horizon: Monte Bolca locality, Pesciara site; early Eocene, late Ypresian, middle Cuisian, SBZ 11, Alveolina dainelli Zone (see Papazzoni et al., 2014).

Diagnosis: As for the genus.

Description: Counts and measurements for † H. esocinus are provided in Tables 1 and 2. The body is laterally compressed and considerably elongated and slender ( Figs 1 View Figure 1 , 2 View Figure 2 ); it is characterized by a remarkably high fineness ratio (15.0–16.0), which is lower only than that of the extremely elongated Stemonosudis . The head is elongated and nearly triangular in lateral outline, and its length is contained between five to six times in SL. The snout is pointed and long, measuring c. 50% of the head length. The mouth is large, terminal, with a lower jaw projecting well anteriorly to the tip of the upper jaw through a non-ossified prominent and horizontally directed pointed projection. Dorsal and anal fins are displaced to the posterior half of the body, opposite to each other; the dorsal-fin origin is located slightly behind the vertical through the anal-fin origin. The adipose fin appears to be absent. The caudal peduncle is narrow and short (about 9–12% SL). The caudal fin is large, symmetrical and forked, with a concave posterior margin. † Holosteus esocinus is totally naked; lateral-line scales seem to be also absent.

The description of the skeletal structures of the neurocranium is primarily based on the specimens MCSNV IG.23601/2 and MGPUP 11611/2, in which this region is largely exposed and well preserved (Figs 3A, B, 4A, B), except for the otic region that is largely incomplete and difficult to interpret. The neurocranium is extremely elongated and nearly triangular in outline. The paired frontals occupy about half of the skull roof length; each frontal articulates anteriorly with the mesethmoid, ventrally with the orbitosphenoid, anteroventrally with the lateral ethmoid, posteriorly with the parietal and posteroventrally with the pterosphenotic and the sphenotic. The parietals are irregular in shape. The supraoccipital forms the posteromedial end of the neurocranium. As in extant paralepidids ( Rofen, 1966), the parietals are not fully separated by the supraoccipital ( Fig. 4A, B View Figure 4 ). The sphenotic is subrectangular in outline, devoid of an anterior process. The orbitosphenoid and pterosphenoid form the dorsal and posterior walls of the orbit, respectively. The epioccipital occupies the dorsolateral part of the posterior surface of the neurocranium. The exoccipitals are badly crushed in all the examined specimens, and their morphology is not clearly recognizable. The parasphenoid is slender and almost

Note: Due to the inadequate preservation of the fossil, lacking most of the cranial and abdominal portions of the body, the data for the specimen MCSNV B5/ T 98 are not reported in the table.

Figure 3. † Holosteus esocinus Agassiz, 1835 . Reconstructions of the head based on the paratype. (A) MCSNV IG.23602; (B) MCSNV IG.23601. Scale bars, 10 mm. Abbreviations: aa, anguloarticular; ac, anterior ceratohyal; br, branchiostegal rays; cl, cleithrum; co, coracoid; de, dentary; ect, ectopterygoid; end, endopterygoid; epo, epioccipital; fe, premaxillary fenestra; fr, frontal; hym, hyomandibula; iop, interopercle; le, lateral ethmoid; me, mesethmoid; mpt, metapterygoid; mx, maxilla; op, opercle; orb, orbitosphenoid; pa, parietal; pas, parasphenoid; pc, posterior ceratohyal; pcl, postcleithrum; pl, palatine; pmx, premaxilla; pop, preopercle; pr, non-ossified projection of dentary; pts, pterosphenoid; q, quadrate; ra, retroarticular; scl, supracleithrum; soc, supraoccipital; sop, subopercle; sph, sphenotic; sr, sclerotic ring; str, cleithral strut; vhy, ventral hypohyal.

straight, extending for most of the basicranial length. The vomer is not clearly visible. The mesethmoid is thin and elongated; it extends posteriorly and partially overlies the anterior portion of the frontals. The lateral ethmoids are large and fan-shaped, with a broad dorsal portion.

The specimen MGPUP 11611/2 shows a complete and well-preserved infraorbital series ( Fig. 4A, B View Figure 4 ) comprising eight elements. The lachrymal is very large and irregular in shape. As in extant paralepidids, this bone is located horizontally on the snout, anterior to the orbit, and extends along the upper border of the maxilla (see Baldwin & Johnson, 1996); it is ornamented with a few radial grooves along its dorsal margin. The second and third infraorbitals are small and triangular. The posterior process of the posterior five infraorbitals is well developed with a lanceolate or subquadrangular outline. The antorbital is small, is irregular in shape and lies dorsal to the lachrymal. The nasal is subrectangular in outline and anteroposteriorly elongated. A unique subtrapezoid supraorbital is also recognizable in MGPUP 11611/2. The sclerotic ring is partially visible in MCSNV IG.23601/2 (Fig. 3A).

The premaxilla is elongated, not protractile and closely attached to the maxilla throughout its length, thereby excluding it from the mouth gape (Figs 3B, 4A, B). It has a moderately developed and posteriorly directed ascending process with a wide ovoid premaxillary fenestra. Anteriorly, the premaxilla bears small and pointed retrorse teeth. The edentulous maxilla is thin, anteroposteriorly elongated, with a broad posterior margin. A single short supramaxilla closely associated with the posterodorsal margin of maxilla is well expanded in specimen MGPUP 1161/2 ( Fig. 4A, B View Figure 4 ). The dentary is slender with a nearly straight dorsal margin; three to four foramina of the laterosensory system are visible in the anterior portion of the dentary ( Fig. 4A, B View Figure 4 ). The lower jaw teeth are large, without a corrugate pattern or serrate margins. † Holosteus esocinus is characterized by a vertical, moderately developed symphysis, probably accommodated between the two contralateral premaxillae when the mouth was closed (see Harry, 1953a). Anterior to it, there is a long, non-ossified and horizontally directed extension, projecting well beyond the anterior tip of the upper jaw (Fig. 3A, B); a similar structure characterizes many derived alepisauroids ( Harry, 1953a, b), although it is extremely elongated in † H. esocinus with its length being contained about five to six times in head length. The anguloarticular is robust, triangular in shape and penetrates the dentary for about the half of its length. The retroarticular is small and curved.

The palatine is robust and elongated; as in all the alepisauroid fishes, the palatine supports a remarkable series of well-developed pointed teeth (Fig. 3A); the palatine teeth are long, without serrations or corrugate pattern and increase in size posteriorly in the series. As in other paralepidids (see Baldwin & Johnson, 1996), the palatine terminates anteriorly with a long process articulating with the premaxilla. The quadrate is fan-shaped, with a slight concavity along the dorsal margin; the quadrate–articular joint is located just below the posterior margin of the orbit, in a position that significantly differs from that characteristic of the Oligocene † P. mariae in which this articulation lies anterior to the orbit (see Prokofiev, 2005). The symplectic is not clearly recognizable. The metapterygoid is subtriangular in shape, with rounded margins; it does not extend anteriorly as in synodontids ( Baldwin & Johnson, 1996), and its posterior margin articulates with the hyomandibula. The ectopterygoid is slender and gently curved. The endopterygoid is anteroposteriorly elongated. There is no trace of pterygoid teeth. The hyomandibula is short, robust and oriented vertically; it bears two dorsal articular heads and a distinct opercular process emerging dorsally from its posterior margin.

The preopercle is large and crescent-shaped, with the vertical arm slightly longer than the ventral one; the corner formed by the junction of the two arms of the preopercle is ornamented with several radial striae. The opercle is the largest bone of the opercular series; it is subrectangular in outline, about 1.5 times broader than deep, with a robust condyle for the articulation with the hyomandibula. The subopercle is long, with a rounded posterior margin. The interopercle is long and slender.

The hyoid apparatus is located just under the orbital region (Fig. 3A, B). The anterior ceratohyal is slender, is subrectangular in shape, constricted in the middle and characterized by an indented ventral margin. The posterior ceratohyal is about the same length as the anterior ceratohyal and robust. There are eight branchiostegal rays, four of which articulate with the anterior ceratohyal; the first three branchiostegals are closely associated with each other along the anterior side of the indentation. The posterior two branchiostegals associated with the posterior ceratohyal are very close to each other and insert on the posteroventral corner of the bone. The ventral hypohyal is small and subquadrangular in shape. The dorsal hypohyal as well as the gill arches are not recognizable in the available specimens.

The vertebral column consists of 106–112 vertebrae, of which 75–78 are abdominal and 31–34 are caudal; caudal vertebrae represent about 30% of the total number. The vertebral centra are subquadrangular, slightly higher than long. Pre- and postzygapophyses are weakly developed throughout the entire vertebral column. Neural and haemal spines of the posterior 25–28 caudal vertebrae are thick and robust with a proximal broad laminar expansion, especially on the posteriormost 10–12 caudal vertebrae. There are about 75 pairs of well-ossified pleural ribs, the first of which seems to originate on the fourth vertebra.

Intermuscular bones are well developed and extend from the abdominal to the caudal region. Epineurals originate laterally on the neural arches; it is unclear whether these are fused to the arches on the anterior vertebrae or not. The epineurals are forked proximally (most of them are bifid and in a few cases trifid; Fig. 5A View Figure 5 ) from vertebrae 12–15 to the end of the abdominal region, thereby resembling the condition of certain primitive ipnopoids and chlorophthalmoids (see Patterson & Johnson, 1995; Baldwin & Johnson, 1996); most of the branches of the forked epineurals appear to be autogenous, unattached to the axial skeleton. Epineurals are long and thin, reaching the dorsal margin of the body; those of the caudal region are more robust and thick. Like in extant paralepidids, epipleurals seem to originate on the first vertebra, and at least the two anterior are autogenous ( Fig. 4A, B View Figure 4 ). † Holosteus esocinus shows an extended series of epipleurals associated with all the abdominal vertebrae and most of the caudal vertebrae, resembling the condition of Paralepis and Arctozenus ( Baldwin & Johnson, 1996) . The epipleurals of the abdominal region are thin and long, reaching the ventral margin of the body. All the epipleurals are attached to the axial skeleton and are not forked distally or proximally. As in extant paralepidids, there are no epicentrals in † H. esocinus .

The caudal skeleton is consistent with that of most extant paralepidids ( Fig. 6A, B View Figure 6 ), although some unique features can be recognized. The first preural and first ural centra appear to be fused to each other. The second ural centrum is small and subtriangular in shape. There are six autogenous hypurals, of which the first and the second are fused into a single plate, resembling the condition observed in Arctozenus and Lestrolepis (see Fujita, 1990; Baldwin & Johnson, 1996). The autogenous parhypural is long and slender. Two autogenous uroneurals are recognizable, of which the anterior one is the largest. There are two thin and slender epurals. There are no urodermals. The caudal fin is large and externally symmetrical, with a nearly concave posterior margin; it contains 19–20 caudal-fin rays with segmentation beginning on the proximal half of each ray; the proximal portion of the principal caudal-fin rays is not modified in any way. The proximal ends of most of the caudal-fin rays of the upper lobe extensively overlap the associated hypurals, resulting in a sort of epaxial hypurostegy ( Fig. 6A, B View Figure 6 ). There are about 10–12 upper and 10–12 lower procurrent rays. There are two well-developed bony fulcral scales, nearly ovoid in shape located just in front of the upper and ventral caudal-fin lobes, resembling the condition characteristic of certain aulopoids and chlorophthalmoids ( Sulak, 1977; Russell, 1999).

The number and morphology of supraneurals is unclear. The dorsal and anal fins are displaced to the posterior half of the body, just in front of the caudal-fin ( Figs 1 View Figure 1 , 2 View Figure 2 , 5B View Figure 5 ). The dorsal-fin origin is located slightly behind that of the anal fin; the dorsal fin is triangular in shape and contains 18–20 distally segmented rays decreasing in size posteriorly. There is no trace of an adipose fin between the dorsal fin and caudal fin. The anal fin has a long base and contains 19–22 distally segmented rays; as in all alepisauroids, its outer margin is deeply indented anteriorly. Dorsal- and anal-fin rays are supported by a similar number of pterygiophores; the posterior pterygiophores of both the dorsal and anal fins do not appear to be fused or proximally modified.

The post-temporal is not recognizable in the available material. The supracleithrum is spatulate, laminar and subtriangular in shape. The cleithrum is crescent-shaped with a broad ventral arm. It bears a distinctive paddle-shaped projection (=cleithral strut of Baldwin & Johnson, 1996) along its posterior margin; the presence of such a projection typically characterizes the most derived paralepidids ( Davis, 2010; Fig. 3A, B; Table 2). The number of postcleithra is unclear, but at least one is visible along the posterior margin of the supracleithrum; its position and size, as well as its subcircular shape, are reminiscent of the first postcleithrum of Paralepis atlantica and Stenomosudis rothschildi (see Rofen, 1966; Baldwin & Johnson, 1996). The coracoid is robust and large. The cleithrum–coracoid articulation is located near the anteroventral end of the cleithrum. At least three or four proximal radials can be recognized. The pectoral fin sits low on the body flanks and contains 16–18 rays, with their proximal portions (=spurs of Sato & Nakabo, 2002) being almost equal in size; the first pectoral-fin ray is the longest of the series. As in extant paralepidids, the pectoral-fin base seems to be horizontally oriented and inserts along the ventrolateral surface of the body.

The pelvic fins are abdominal and displaced slightly behind the mid-length of the body, well anterior to the dorsal fin origin. There are 10–12 pelvic-fin rays. The basipterygia are elongated, joined to each other medially and without any trace of posterior or lateral pelvic processes.

The body is totally naked without any evidence of body or lateral-line scales. Because of the lack of body squamation, the pigmented peritoneal membrane is clearly visible in well-preserved specimens (e.g. MCSNV IG.23601/2). † Holosteus esocinus has a broad, uniform, mid-dorsal brown-pigmented band along the dorsum, extending from the head to the caudal fin, similar to the body pigmentation pattern of the naked genera like Lestidium ( Harry, 1953b) . There is no evidence of pigmentation along the ventral surface of the body. There are no traces of preserved chromatophores.