A redescription of Omiodon cabassii Bassani 1888, a putative chlorophthalmoid fish (Teleostei: Aulopiformes) from the lower Eocene of northeastern Italy
Article number: 29.3.a34
https://doi.org/10.26879/1710
Copyright Paleontological Society, September 2026
Author biographies
Plain-language and multi-lingual abstracts
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Submission: 13 May 2026. Acceptance: 17 August 2026.
ABSTRACT
Omiodon cabassii Bassani, 1888 is a poorly known teleost originally described in the late nineteenth century based on a single articulated specimen recovered from Eocene deposits of the Friuli-Venezia Giulia region (northeastern Italy). Despite its marine paleoenvironmental context, the specimen was initially assigned to Characiformes by Francesco Bassani and later referred to the Myctophidae. Owing to the outdated nature of the original anatomical description and the problematic taxonomic placement of the species, we conducted a comprehensive re-examination of the type specimen long-considered lost together with a stratigraphic reassessment based on micropaleontological analysis of the enclosing matrix of the fossil. New anatomical evidence supports assignment of this taxon to Aulopiformes, and we tentatively interpret it as a member of Chlorophthalmoidei. However, the poor preservation of the specimen precludes a more precise determination of its phylogenetic position within Chlorophthalmoidei. Notwithstanding these limitations, Omiodon cabassii represents the oldest possible fossil record of this clade.
Pietro Calzoni. Department of Geosciences, University of Padova, Via Gradenigo, 6-35131 Padova, Italy. [email protected]
Jacopo Amalfitano. Department of Geosciences, University of Padova, Via Gradenigo, 6-35131 Padova, Italy. [email protected]
Eliana Fornaciari. Department of Geosciences, University of Padova, Via Gradenigo, 6-35131 Padova, Italy. [email protected]
Luca Giusberti. Department of Geosciences, University of Padova, Via Gradenigo, 6-35131 Padova, Italy. [email protected]
Giorgio Carnevale. Department of Earth Sciences, University of Torino, Via Valperga Caluso, 35 I-10125 Torino, Italy. [email protected]
Keywords: Aulopiformes; Eocene; Chlorophthalmoidei; Paleogene; Italy
Final citation: Calzoni, Pietro, Amalfitano, Jacopo, Fornaciari, Eliana, Giusberti, Luca, and Carnevale, Giorgio. 2026. A redescription of Omiodon cabassii Bassani 1888, a putative chlorophthalmoid fish (Teleostei: Aulopiformes) from the lower Eocene of northeastern Italy. Palaeontologia Electronica, 29(3):a34.
https://doi.org/10.26879/1707
palaeo-electronica.org/content/2026/5949-a-redescription-of-omiodon-cabassii-lower-eocene
Copyright: September 2026 Paleontological Society
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INTRODUCTION
In 1888, in a very concise paper, Francesco Bassani described the fossil fish Omiodon cabassii Bassani 1888, based on a single partially complete articulated skeleton collected from the Eocene deposits exposed near Rosazzo, south-east of the city of Udine, Friuli-Venezia Giulia, north-eastern Italy (Figure 1A). The fossil was originally part of the collection of engineer G.B. Cabassi at Corno di Rosazzo (Udine province), to whom Bassani dedicated the new species. Bassani (1888) interpreted the fossil as a characiform closely related to certain Neotropical lineages and provided only a cursory description. Based on its provenance, he assigned the fossil to the “San Giovanni Ilarione stage” (=Lutetian, middle Eocene). Some years after its first description, Omiodon was doubtfully reassigned to the family “Scopelidae” (=Myctophidae) (Woodward, 1901) and only sporadically listed as such in the literature (e.g., Boulenger, 1904; D’Erasmo, 1922; Jordan, 1923; Carroll, 1988). After years of oblivion, Dalla Vecchia (2008) briefly discussed the fossil as the “most significant” among the scarce Eocene vertebrate remains from the Friuli-Venezia Giulia region, stating that no trace of the fossil remained in the collections of the Friulian Museum of Natural History of Udine, into which Cabassi’s paleontological collection was incorporated in the 1950s after it was donated by Secondo Cabassi to the Stazione Chimico-Agraria sperimentale di Udine. Dalla Vecchia (2008) hypothesized that, due to the fragile nature of the matrix in which the fossil was preserved (a cinereous marl, as reported by Bassani, 1888), it had most likely been destroyed. At the beginning of 2020, however, one of us (LG) fortuitously located the specimen (Figure 1B) in the paleoichthyological collection of the Museum of Geology and Paleontology of the University of Padova (now a section of the Museum of Nature and Humankind of the University of Padova). No documentation associated with the fossil has been identified; consequently, the timing and circumstances of its incorporation into the paleontological collections of the University of Padova remain uncertain. Since June 2023, the fossil has been displayed in the Section of Geology and Paleontology of the Museum of Nature and Humankind of the University of Padova. The goal of this paper is to redescribe the single available specimen of the long forgotten Omiodon cabassii , to provide new data about its age and geological context, and to discuss its possible affinities within the teleosts.
GEOLOGICAL AND STRATIGRAPHICAL SETTING
Bassani (1888), without providing further details on the circumstances of the fossil’s discovery, reported that the holotype of Omiodon cabassii comes from the hill of Rosazzo, approximately 20 km south-east of Udine (Figure 2B), in the southern Julian Prealps of the Friuli-Venezia Giulia region. In this area, a lower-middle Eocene turbiditic succession crops out, belonging to the Cormons Flysch (Martinis, 1962; Venturini and Tunis, 1992). This formation represents the upper portion of the thick turbiditic fill (> 4000 m) of the late Campanian-middle Eocene “Julian Basin” (eastern Friuli-western Slovenia), a WNW-ESE-elongated basin bounded to the SW and S by a shallow-water, partly emergent carbonate platform (the Friuli Platform), to the NE by the Dinaric deformational wedge, and to the N by the embryonic Alpine system (Tunis and Venturini, 1992; Ogata et al., 2014; Venturini et al., 2023). The final infilling of this long lasting turbidite basin culminated in the rapid progradation in the early Lutetian of deltaic deposits, which characterize the upper portion of the Cormons Flysch and have been interpreted as reflecting a tectonic uplift event (Venturini and Tunis, 1992; Venturini et al., 2023). The upper portion of the Cormons Flysch, cropping out in several localities of the southern Julian Prealps, is renowned in literature since the second half of the nineteenth century for its rich paleontological content (e.g., Dainelli, 1915; Maddaleni and Tunis, 1993; Maddaleni, 1997; Bosellini et al., 2022; Benedetti et al., 2024). In the area of the hills of Rosazzo and Rocca Bernarda, the Cormons Flysch is severely affected by tectonic disturbance (Figure 2C): nevertheless, Venturini and Tunis (1992) reconstructed a composite stratigraphic section more than 700 m thick recording a shallowing upward trend from middle bathyal turbiditic deposits to a prodelta and deltaic plain setting in the uppermost portion of the sequence. Based on planktic foraminiferal content, they ascribed the succession to the Ypresian p.p.-lower Lutetian, locating the base of the Lutetian at the base of the Acarinina bullbrooki zone at ca. 500 m from the base (Venturini and Tunis, 1992). Based on what was originally reported in Bassani (1888), Dalla Vecchia (2008) proposed that the type specimen of Omiodon cabassii originated from the upper portion of the Cormons Flysch, cropping out in the hill of Rosazzo.
MATERIAL AND METHODS
The examined material consists of a single partially complete articulated skeleton housed in the collections of the Museum of Nature and Humankind of the University of Padova (Italy), with catalogue number MGP-PD 27136. The photos of the specimen were taken using a Sony α7R3 camera mounting a Sony FE 2.8/90 mm macro-G OSS lens. The specimen was photographed under both natural and UV light (Figure 3), to better emphasize the osteological details by creating a distinct contrast in color with the surrounding matrix. UV-A (peak emission at 368 nm), UV-B (peak emission at 318 nm), and UV-C (peak emission at 254 nm) wavelengths were produced with a 95 W discharge lamp from WayTooCool LLC. The specimen was photographed with a combination of the three different wavelengths to have a greater contrast between the fossil and the matrix. The lamp was used with a custom design to better photograph the specimen; it was leaned on a wooden box perpendicular to the specimen and placed on a cardboard frame, obscured by different black fabrics to keep out external light, to avoid the reflectiveness on the inside of the box and to protect the operator from any UV radiation. The specimen was studied with a stereomicroscope Leica M80 equipped with a camera lucida drawing arm. Measurements were taken directly on the specimens using a digital caliper.
Calcareous nannofossil analysis. One sample of matrix of the fish was removed with a scalpel after HCl immersion of the blade to avoid any contamination. To analyze the calcareous nannofossil content, the sample was processed according to the smearing technique (Bown and Young, 1998). Analysis was conducted under a polarized light microscope with a magnification of 1250X. Sample MGP-PD 27136 was initially analyzed qualitatively to assess both the calcareous nannofossil content and the state of preservation. Subsequently, a semi-quantitative analysis was performed, partially modifying the methodologies of Backman and Shackleton (1983) and Gardin and Monechi (1998). Specifically, the smear slide was studied over approximately 6-7 µm² (equivalent to three vertical traverses) under parallel nicols for Discoaster and Tribrachiatus , and over approximately 2 µm² (equivalent to one vertical traverse) under crossed nicols for the main index taxa (e.g., Toweius , Noelaerhabdaceae, and Sphenolithus radians ).
Anatomical abbreviations. achy: anterior ceratohyal; br: branchiostegal rays; cc: compound centrum; den: dentary; ect: ectopterygoid; end: endopterygoid; ep: epural; fr: frontal; h1-6: hypurals; hpu2: haemal spine of the second preural centrum; hyo: hyomandibula; IO2-5: infraorbitals; lac: lachrymal; let: lateral ethmoid; mes: mesethmoid; mtp: metapterygoid; mx: maxilla; na: nasal; op: opercle; pa: parietal; pal: palatine; pchy: posterior ceratohyal; phy: parhypural; pmx: premaxilla; pop: preopercle; psp: parasphenoid; pte: pterosphenoid; pto: pterotic; ptt: posttemporal; pu2: second preural centrum; q: quadrate; soc: supraoccipital; sop: subopercle; sph: sphenotic; sym: symplectic; un1-2: uroneural; vom: vomer.
Measurement abbreviations. BD: body depth; CPD: caudal peduncle depth; denL: length of the dentary; DFL: length of the dorsal-fin base; DRL: length of the longest dorsal-fin ray; HL: head length; mxL: length of the maxilla; O: orbit diameter; PD: predorsal length; pmxL; length of the premaxilla; PP: prepectoral length; PV: prepelvic length; PVL: length of the longest pelvic-fin ray; SL: standard length; SNL: snout length; TL: total length.
Institution abbreviations. MGP-PD: Museum of Nature and Humankind of the University of Padova (Italy).
AGE DETERMINATION AND STRATIGRAPHIC PLACEMENT OF OMIODON CABASSII
The sample of matrix removed from MGP-PD 27136 is characterized by sparse calcareous nannofossils in a generally moderate state of preservation. Overall, the assemblage presents abundant or common sphenoliths, Toweius , and Coccolithus , while Noelaerhabdaceae are absent. Other minor components of the assemblage include Zygrhablithus bijugatus and Thoracosphaera . The genera Discoaster and Tribrachiatus are very rare. Within the genus Sphenolithus, Sphenolithus radians are common, while within Tribrachiatus , a single specimen of T. orthostylus was identified. The few observed Discoaster specimens are not significant, except for one specimen tentatively attributed to Discoaster lodoensis , though identification is uncertain due to poor preservation. Based on the presence of Tribrachiatus orthostylus, Sphenolithus radians, and the absence of Noelaerhabdaceae, the sample can be attributed to the combined Zones CNE3-CNE4 (middle part) of Agnini et al. (2014) or NP11-NP12 Zones of Martini (1971). The absence of specimens that can be confidently attributed to Discoaster lodoensis precludes a more precise assignment to either biozone CNE3 (absence of D. lodoensis ) or CNE4 (presence of D. lodoensis ). Nevertheless, the specimen can be confidently assigned to the Ypresian rather than to the Lutetian, as previously reported. In the absence of calcareous nannofossil data from the Rosazzo-Rocca Bernarda section of Venturini and Tunis (1992), the approximate stratigraphic provenance of MGP-PD 27136 can be cautiously inferred by comparison with an integrated calcareous nannofossil and planktic foraminiferal biozonation derived from a deep-sea succession deposited in a comparable deep-water setting. For this purpose, the original data of planktic foraminifera reported in Venturini and Tunis (1992) have been herein tentatively reinterpreted (Figure 2D), based on the more recent zonal scheme of Wade et al. (2011): the section of Rosazzo-Rocca Bernarda spans the E5-E8 planktic foraminiferal zones (ca. lower-middleYpresian to lower Lutetian). Based on the integrated biozonation established for the bathyal Possagno section (Veneto region) by Luciani and Giusberti (2014), the NP11/NP12 (or CNE3/CNE4) zonal boundary falls within the lower portion of the planktic foraminiferal Zone E5 of Wade et al. (2011). This implies that the holotype of Omiodon cabassii was most likely recovered from a bed within the bathyal basal portion of the Cormons Flysch at Rosazzo-Rocca Bernarda spanning the lower-middle Ypresian E5 zone (Figure 2D), rather than from the Lutetian strata of the upper part of the formation, which are characterized by prodelta to deltaic facies.
SYSTEMATIC PALEONTOLOGY
Order AULOPIFORMES Rosen, 1973
Superfamily CHLOROPHTHALMOIDEA sensu Davis, 2010
Genus OMIODON Bassani, 1888
1888 Omiodon Bass.; Bassani, p. 2.
1904 Omiodon ; Boulenger, p. 611.
1923 Omiodon Bassani, 1888; Jordan, p. 155.
1988 Omniodon (sic); Carroll, p. 607.
Type species (by monotypy). Omiodon cabassii Bassani, 1888
Original diagnosis (our translation from the Italian). Maxilla edentulous. Very small conical teeth uniform in size on the premaxilla and dentary. Caudal fin forked.
Emended diagnosis. A putative chlorophthalmoid fish characterized by the following set of traits: body slender and elongate; head length slightly less than one third of SL; infraorbital bones with smooth posterior border; jaws bearing numerous minute and conical teeth arranged in multiple rows; premaxilla with curved alveolar process and distally placed postmaxillary process; maxilla toothless; supramaxillae absent; palatine bearing several minute teeth; at least seven branchiostegal rays; 34 (16+18) vertebrae; epineural and epipleural series extending throughout the vertebral column; caudal skeleton with six autogenous hypurals, first hypural proximally consolidated with parhypural, diastema between hypaxial and epaxial hypural plates, two autogenous uroneurals, at least two epurals, second preural vertebra bearing fully developed neural spine and autogenous haemal spine; caudal fin forked, containing 19 principal rays; a single supraneural; dorsal fin with three spinous rays plus nine soft rays; pectoral fin with at least 12 soft rays; pelvic fins abdominal, containing seven rays; body naked.
Omiodon cabassii Bassani, 1888
Figure 1, Figure 3, Figure 4, Figure 5
+1888 Omiodon Cabassii; Bassani, p. 2, pl. 1.
1901 Omiodon cabassii; Woodward, p. 270.
1922 Omiodon Cabassii Bassani; D’Erasmo, p. 80.
2008 Omiodon cabassii; Dalla Vecchia, p. 241, fig. 235 (after Bassani, 1888).
Holotype (monotype). MGP-PD 27136, a nearly complete articulated skeleton, 186.1 mm SL.
Diagnosis. As for the genus.
Range. lower-middle Ypresian.
Occurrence. Rosazzo, Udine, Friuli-Venezia Giulia region, Italy. Cormons Flysch, lower-middle Ypresian, lower Eocene.
Description. The body is moderately slender and elongate, with a laterally compressed caudal portion (see Table 1). The head is rather large, representing slightly less than one third of SL (Table 1). The snout is relatively short and moderately depressed. The orbit is large; its diameter is comprised less than four times in HL. The mouth is terminal, obliquely oriented, and was rather large in origin, extending back to the posterior half of the large and rounded orbit. The caudal fin is forked. The pectoral fin inserts near the ventral body wall. The pelvic fin is abdominal and originates behind the dorsal-fin insertion.
Neurocranium and circumorbital bones. The neurocranium is almost triangular in outline and only partially preserved, most of its bones not being properly recognizable. The frontals are the largest bones of the skull roof; these are quadrangular in outline, ending abruptly anteriorly. What appears to be the parietal is polygonal and reduced in size, separated from its antimere by the supraoccipital. The supraoccipital crest is feebly developed. The lateral ethmoid is thick and columnar, while the mesethmoid is quadrangular in outline. The vomer is stout, ovoid in lateral view and toothless. The sphenotic is almost triangular in outline. The pterotic is elongate and largely hidden by the overlying hyomandibula. The occipital and otic regions of the neurocranium are scarcely exposed and difficult to interpret. The pterosphenoid is poorly preserved. The parasphenoid is straight and slender.
The nasals are poorly preserved. The bones of the left infraorbital series are exposed in medial view. The lachrymal is approximately triangular in outline with an expanded anterior end (Figure 4). The second, third and fourth infraorbital bones are tubular; the fifth infraorbital is conical, recognizable at the posterodorsal corner of the orbit (Figure 4). The dermosphenotic is not clearly exposed. All the infraorbitals have a canal that extends through the bone.
Jaws and suspensorium. The jaws and suspensorium are exposed in medial view. The premaxilla has a curved and thick alveolar process, an expanded articular process and a relatively short ascending process; there is a moderately expanded postmaxillary process at the distal end of the premaxilla. There are numerous minute conical teeth throughout the alveolar surface of the premaxilla (Figure 4). The maxilla is slightly longer than the premaxilla, only distally was probably involved in the gape (Table 1); it is slender and toothless, with a linear dorsal margin and an expanded distal end and bears a well-developed articular head characterized by a long median process and a moderately large cranial condyle, lacking a palatinad facet (see Stiassny, 1986). There is no evidence of the supramaxillae, although their original presence cannot be excluded. The mandible is slightly longer than the upper jaw (Table 1). The dentary is robust, with thickened toothed and ventral margins, almost triangular in outline. There are numerous minute conical teeth similar to those of the upper jaw (Figure 4); the teeth are also present on the outer surface of the anterior tip of the dentary as an additional patch. The dentary portion of the mandibular sensory canal runs in an open groove throughout its length, except for a short bridge in the anterior portion of the dentary. The anguloarticular is not preserved, being completely disarticulated from the dentary.
The quadrate is fragmented and fan-shaped, only partially preserved. The symplectic is thin and splint-like. The hyomandibula is almost vertically oriented and has a rather thick ventral shaft and appears to have a large articular head (Figure 4). The ectopterygoid is curved, blade-like. The endopterygoid is especially large, oblong in outline; teeth appear to be absent on the endopterygoid. The metapterygoid is smaller than the endopterygoid, almost squared, its ventral border articulates with the quadrate. The palatine is not sutured to the ectopterygoid; like the ectopterygoid, the palatine exhibits the impression of minute conical teeth along its oral margin.
Opercular series. Like the jaws and suspensorium, the bones of the opercular series are exposed in medial view. The preopercle is crescent-shaped and ventrally expanded with smooth edges. The opercle is large and diamond-shaped, in large part preserved as an impression only, with a thick horizontal ridge and moderately developed dorsal projection. The subopercle is poorly preserved, showing an almost elliptic ventral profile.
Hyoid and branchial archs. The anterior and posterior ceratohyals appear to be sutured together. The anterior ceratohyal is deep posteriorly and shows a broadly concave ventral margin; there is no evidence of a beryciform foramen, and a shallow groove for the hyoidean artery is also recognizable. The posterior ceratohyal is triangular in outline and rather thick. There are seven sabre-like branchiostegal rays, although the original number was probably higher; the first branchiostegal inserts on the inner surface of the anterior ceratohyal, the second and third on the ventral margin of the anterior ceratohyal, the fourth and fifth on the outer surface of the anterior ceratohyal, the sixth on the outer surface between the anterior and posterior ceratohyals, and the seventh on the outer surface of the posterior ceratohyal. The heads of the branchiostegal rays are not preserved, thereby preventing the observation of possible anterior projections (see McAllister, 1968; Figure 3-Figure 4). The branchial skeleton is not preserved.
Vertebral column. The vertebral column is not completely exposed in the fossil; the three anterior-most vertebrae are covered by the overlying opercle, and their presence can be determined based on the slender neural spines and epineurals emerging from the dorsal and posterior edges of the opercle. Therefore, it is reasonable to define that the vertebral column comprises 34 (16+18) vertebrae, including the second ural centrum (Figure 3). The centra of the anterior four exposed abdominal vertebrae are subrectangular, longer than high, becoming more compact and almost squared in the rest of the abdominal region and throughout the caudal region. The centra are robust and well-ossified, exhibiting ridges and irregular pits on their lateral surface. The ventral prezygapophyses and the dorsal postzygapophyses are delicate and short. The two posterior abdominal vertebrae bear curved and distally pointed parapophyses; due to inadequate preservation, it is not possible to determine whether parapophyses were present on some of the anterior abdominal vertebrae or not. The neural spines of the anterior eight abdominal vertebrae are only slightly anteroposteriorly expanded compared to those of the posterior abdominal and caudal vertebrae, which are more slender, more obliquely oriented and distally pointed. The neural arches of the posterior four abdominal and of the caudal vertebrae are anteroposteriorly expanded and almost triangular in lateral view; these are pierced by a relatively large central foramen. The haemal spines are robust, straight and distally pointed, thicker than their neural counterparts. The haemal arches are similar to their opposite neural arches; a central rounded foramen can be observed on the lateral surface of some of the better-preserved haemal arches. The ribs are only partially preserved and only five pairs of thick and curved ribs can be recognized in the anterior part of the abdominal cavity; the preserved ribs likely inserted high on the lateral surface of the vertebral centra; the absence of ribs in the posterior portion of the abdominal cavity may indicate that these were not ossified in origin. The original pattern of the intermuscular bones is difficult to interpret. There are two series of intermuscular bones, of which the elements of the ventral one are interpreted herein as the epipleurals. The epineurals extend posteriorly up to the twenty-ninth vertebra. The proximal portion of the epineurals of the first four vertebrae is not exposed; the epineurals of the fifth to thirteenth vertebrae seem to articulate on the lateral surface of the neural arches, whereas those on the fourteenth to twenty-ninth vertebrae are forked anteriorly with an anteroventral branch attaching to the neural arch and a dorsal anteromedial branch extending anteriorly to the neural spine of the preceding vertebra. The epineurals are recognizable from the eighth abdominal vertebra extending posteriorly to the twenty-ninth vertebra. In the abdominal region, the epipleurals are rod-like and seem to originate ventrally on the lateral surface of the vertebral centra, whereas in the caudal region, they probably originate on the lateral surface of the haemal arches; at least the epipleurals from the fifth to tenth caudal vertebrae are forked anteriorly (Figure 3).
Caudal skeleton. The configuration of the caudal skeleton is extremely difficult to properly define since a fracture that crosses the proximal portion of the hypurals was repaired with abundant glue that pervasively expanded on both sides of the fracture, limiting the observation of certain structures (Figure 3). Therefore, a tentative interpretation of its structure is presented herein. The compound centrum articulates with the two hypaxial hypurals of which the ventral one is larger and proximally consolidated with the parhypural (Figure 5). There are four autogenous epaxial hypurals associated with a posterior ural centrum (Figure 5). There is no parhypurapophysis. In addition, there are two poorly preserved uroneurals and at least two epurals (Figure 5). The second preural vertebra has a fully developed neural spine and what appears to be an autogenous large haemal spine (Figure 5). The haemal spine of the third preural vertebra seems to be autogenous. The caudal fin is forked and contains 19 principal rays (I,9+8,I), plus nine dorsal and at least five ventral procurrent rays (Figure 3).
Median fins and supports. What appears to be the proximal shaft of a single supraneural can be observed just above the fourth abdominal vertebra very close to the epineurals of the third vertebra.
The dorsal fin originates just above the articulation between the abdominal vertebrae 10 and 11 (PD: 44% SL; Table 1), the first dorsal-fin pterygiophore inserts in the sixth interneural space. The dorsal and posterior margin of the dorsal fin appears to be gently rounded. The dorsal fin contains 12 rays, of which the anterior three are not segmented but bilaterally unfused throughout their lengths, resembling the spiny rays of certain aulopiforms and myctophiforms or the pseudospine of certain gadiforms (see Okamura, 1970; Rosen, 1973; Johnson and Patterson, 1993). Therefore, the dorsal fin contains three spinous rays plus nine segmented and distally bifurcated rays, supported by 11 pterygiophores. The first spinous ray is relatively short, representing half of the length of the second and third. The first two spinous rays are supernumerary on the first pterygiophore. The pterygiophores gradually decrease in size posteriorly in the series; the anterior six pterygiophores are gently curved. The first pterygiophore has a short anteriorly projecting procumbent spine. Autogenous middle radials can be recognized in the third, sixth, seventh, ninth, and tenth pterygiophores. The three anterior dorsal-fin rays are the longest.
The anal fin originates well behind the dorsal fin, and it is largely incomplete, solely represented by the distal portions of the six posterior rays.
There is no trace of an adipose dorsal fin.
Paired fins and girdles. The pectoral girdle is incomplete and poorly preserved. The posttemporal is partially preserved, showing a long and slender epioccipital limb, which is not tightly connected to the neurocranium (Figure 3). Of the supracleithrum only the ventral portion can be recognized. The cleithrum is large, with a thick posterior plate. The scapula and coracoid appear to be tightly sutured to each other. The scapular foramen lies in what appears to be the central portion of the scapula. There are two largely incomplete postcleithra. The dorsal postcleithrum is quadrangular and rather expanded and originates at the level of the posterior plate of the cleithrum. What is interpreted herein as the ventral postcleithrum terminates ventrally into an obliquely oriented spine that almost reaches the ventral body margin. Fragments of pectoral-fin radials seem to be recognizable, together with the proximal tips of at least 12 obliquely oriented rays.
The pelvic fin originates at the level of the first dorsal-fin ray and seems to contain seven rays, of which the fourth is the longest. The basipterygium is triangular in lateral view and bears a stout and pointed posterior process. (Figure 3). The basipterygium is dissociated from the cleithrum and appears to be closely associated with the distal tips of the ribs.
Squamation. There are no traces of the original squamation.
DISCUSSION
The Affinities of Omiodon
The inadequate preservation of the sole available specimen of Omiodon cabassii and the unusual combination of morphological features make it very problematic to clarify the phylogenetic relationships of this Eocene teleost fish. Bassani (1888) described Omiodon cabassii as a characiform fish intermediate between the Erythrinidae and Lebiasinidae. This taxonomic attribution was not supported by a comparative analysis in which the possible characiform features of Omiodon were discussed. We can speculate that the relative position of the paired fins as well as the extended series of intermuscular bones led Bassani to hypothesize the characiform affinities of Omiodon . Moreover, within characiforms, the apparent absence of an adipose fin was used to justify a certain degree of affinity with erythrinids and lebiasinids from which Omiodon differs due to the presence of a series of morphological features (maxilla toothless; oral jaw teeth small, conical, and uniform in size; caudal fin forked) that were listed in the diagnosis (Bassani, 1988). The descriptive analysis of the only available and moderately preserved specimen of Omiodon cabassii presented herein revealed a peculiar set of features that does not comprise substantial morphological evidence to confirm the taxonomic attribution hypothesized by Bassani (1888). In particular, it is sufficient to consider the presence of a large premaxilla with a long alveolar process associated with the absence of the Weberian apparatus to unquestionably exclude any possible alignment of Omiodon with the otophysan fishes and, consequently, with the Characiformes (see e.g., Fink and Fink, 1981, 1996; Wiley and Johnson, 2010).
Despite the incompleteness and inadequate preservation of the sole available fossil, however, it is possible to determine the taxonomic affinities of Omiodon . In particular, the combined occurrence of a large non-protrusible premaxilla with a well-developed alveolar process and short ascending process, abdominal pelvic fins, 19 principal caudal-fin rays, two series of ossified intermusculars extended throughout the body, and lack of true fin spines concur to indicate an alignment with the Aulopiformes (e.g., Gosline et al., 1966; Rosen and Patterson, 1969; Gosline, 1971; Rosen, 1982; Patterson and Johnson, 1995). The order Aulopiformes was created by Rosen (1973) to include a diverse array of benthic and pelagic extant and fossil non-ctenosquamate eurypterygian fishes. Rosen (1973) recognized the monophyletic status of the Aulopiformes by solely using branchial characters. Several subsequent studies have been focused on the intrarelationships of aulopiform fishes (e.g., Sulak, 1977; Johnson, 1982; Rosen, 1985; Hartel and Stiassny, 1986; Baldwin and Johnson, 1996; Johnson et al., 1996; Sato and Nakabo, 2002) and, recently, Davis (2010) provided a new classification of this group based on a total evidence approach.
Within the aulopiform fishes, the overall physiognomy of Omiodon appears to be consistent with those of chlorophthalmoids and paraulopoids, since it shares several features with these groups, including body with a moderately slender outline, caudal fin forked, dorsal fin originating in the anterior half of the body, pelvic fin inserted posterior to the dorsal-fin origin, snout short, orbit large, mouth terminal and obliquely oriented, upper jaw extending posteriorly beyond the anterior edge of the orbit, and maxilla distally expanded (e.g., Mead, 1966; Hartel and Stiassny, 1986; Sato and Nakabo, 2002). The alignment of Omiodon with paraulopoid fishes, however, can be easily ruled out based on a set of features that are characteristic of paraulopoids (see Sato and Nakabo, 2002) and have not been observed in Omiodon , including presence of ossified epicentrals and of three supraneurals, caudal principal rays with proximal joint separating a small proximal segment from the rest of the ray (Johnson et al., 1996), presence of endopterygoid teeth, of a small urodermal and of large horizontal caudal scutes (fulcral scales of Sulak, 1977). It is interesting to note that Omiodon shares the absence of a supramaxilla with paraulopoid fishes (Sato and Nakabo, 2002), although this resemblance can be regarded as the result of convergence. On the other hand, Omiodon shares at least four of the chlorophthalmoid synapomorphies listed by Baldwin and Johnson (1996), Sato and Nakabo (2002), and Davis (2010), including maxilla lacking the palatinad facet (possibly convergent in paraulopoids; see Sato and Nakabo, 2002), dentary with tooth patch exposed on the outer surface of its anterior tip, hyomandibula with a single large articular head, and presence of anteriorly forked epineurals and epipleurals. The latter character is especially relevant in evaluating the possible affinities of Omiodon since it has been observed only in the Chlorophthalmidae (e.g., Baldwin and Johnson, 1996; Sato and Nakabo, 2002). According to Davis (2010), the family Chlorophthalmidae comprises the extant genera Chlorophthalmus and Parasudis , representing the sole members of the superfamily Chlorophthalmoidea. Omiodon exhibits a unique set of features that clearly separates it from the two extant chlorophthalmid genera, including supramaxilla absent (vs present), at least five branchiostegal rays on the anterior ceratohyal (vs four), 34 vertebrae (vs 38-48), two epurals (vs three), neural spine of the second preural centrum fully developed (vs reduced), and body (apparently) naked (vs covered with scales; see Table 2). The caudal skeleton of Omiodon appears to be similar to that of certain species of Chlorophthalmus by having proximally consolidated parhypural and first hypural (Sulak, 1977). Conversely, Omiodon shares with Parasudis a single supraneural (vs two in Chlorophthalmus ; Baldwin and Johnson, 1996), and less than 60% of the vertebrae as caudal vertebrae (vs > 60% in Chlorophthalmus ; Baldwin and Johnson, 1996). Summarizing this comparative discussion, based on the shared occurrence of several chlorophthalmoid synapomorphies (e.g., Davis, 2010), especially the presence of anteriorly forked epineurals and epipleurals, we tentatively interpret Omiodon as a member of this aulopiform lineage. Omiodon differs from the extant genera of the family Chlorophthalmidae by having a peculiar combination of features. In any case, additional, more complete and better-preserved material would be essential to unquestionably demonstrate the plausibility of this hypothesis.
Regardless of Omiodon , the fossil record of chlorophthalmoid fishes is very meager. Patterson (1993) assigned the genus Acrognathus from the Cenomanian and Turonian of England and Lebanon to the Chlorophthalmidae without any systematic analysis, and Gayet et al. (2003) added the Cretaceous genera Sardinioides and Volcichthys to this group. Although a detailed anatomical revision of Acrognathus would be desirable to define its affinities within aulopiforms, its attribution to the chlorophthalmoids should be ruled out due to its possession of large caudal scutes (Patterson, 1968), a character that has not been observed in the extant member of this lineage (Sato and Nakabo, 2002). The genus Sardinioides comprises several species (e.g., Goody, 1969; Rosen, 1973), the majority of which have been considered as myctophiforms (Dietze, 2009). Recent observations on the Cenomanian-Turonian Sardinioides illustrans from the English Chalk Group suggest that it is an aulopiform close to the aulopoids or paraulopoids (Beckett et al., 2018). Volcichthys was described by D’Erasmo (1946) from the Cenomanian of Comeno and originally referred to the Enchodontidae. Volcichthys has a long dorsal fin and a toothed maxilla that enters the gape. Goody (1969) compared this Cretaceous genus with the extant Aulopus , and Rosen (1973) hypothesized an affinity with synodontids. Davis and Fielitz (2010) estimated that the chlorophthalmoids originated around 100 Ma. However, as discussed above, the Cretaceous record of this group remains elusive and Omiodon possibly represents the only plausible record of the chlorophthalmoid fishes based on articulated skeletal remains.
CONCLUSIONS
The re-examination of the holotype of Omiodon cabassii Bassani, 1888 provides new and significant anatomical, stratigraphic, and paleobiological data on this poorly known Eocene teleost. The new anatomical evidence supports the reassignment of Omiodon cabassii from Characiformes - as originally proposed by Bassani (1888) - to the order Aulopiformes, and more specifically suggests a tentative placement within the superfamily Chlorophthalmoidea. This attribution is supported by several chlorophthalmoid synapomorphies, most notably the presence of anteriorly forked epineurals and epipleurals, a character otherwise restricted to the extant family Chlorophthalmidae. However, the incomplete and somewhat inadequate preservation of the holotype precludes a more precise determination of the phylogenetic position of Omiodon within Chlorophthalmoidei. Additional and better-preserved material would be essential to fully resolve the systematic affinities of this genus.
The micropaleontological analysis of calcareous nannofossils from the matrix enclosing the holotype allows us to confidently reassign the stratigraphic provenance of Omiodon cabassii to the lower-middle Ypresian, rather than the Lutetian (middle Eocene) as originally reported by Bassani (1888). This revised age places the specimen within the bathyal turbiditic facies of the lower portion of the Cormons Flysch, consistent with a deep-water depositional environment. The paleoecological implications of this finding are noteworthy. Extant chlorophthalmids ( Chlorophthalmus and Parasudis ) are mesobenthic to benthopelagic fishes, typically inhabiting the continental slope and upper bathyal zone at depths ranging from approximately 100 to over 1000 m (Mead, 1966; Hartel and Stiassny, 1986). The recovery of Omiodon cabassii from bathyal turbiditic deposits of the Cormons Flysch is therefore entirely consistent with the deep-water ecology characteristic of extant members of this lineage, suggesting that the preference for deep-water environments may represent an ancestral trait of Chlorophthalmoidei that has been conserved since at least the early Eocene. Beyond its paleoecological significance, Omiodon cabassii represents the oldest putative fossil record of Chlorophthalmoidei based on articulated skeletal remains. The previously proposed Cretaceous records of this lineage - including Acrognathus (Patterson, 1993) and Sardinioides (Gayet et al., 2003) - are either doubtful or have been reassigned to other aulopiform lineages (Beckett et al., 2018). The early-middle Ypresian age of Omiodon cabassii is younger than the molecular divergence time estimated for the family Chlorophthalmidae by Davis and Fielitz (2010). Moreover, this age is broadly concordant with the pattern documented by Davis and Fielitz (2010), whereby most extant aulopiform families are hypothesized to have appeared by the Late Cretaceous to the Eocene.
ACKNOWLEDGEMENTS
We are particularly obliged to M. Fornasiero (Museum of Nature and Humankind, University of Padova, Italy), for access to the specimen MGP-PD 27136 and the logistic support. We are grateful also to S. Castelli (Department of Geosciences, University of Padova, Italy) for his valuable help with the photographs and figures preparation. V. Luciani (University of Ferrara, Italy) is gratefully acknowledged for her support in converting the planktic foraminiferal zones adopted by Venturini and Tunis (1992) for the Rosazzo-Rocca Bernarda section area to those of Wade et al. (2011). We are also grateful to the two anonymous reviewers and to the editorial team, who, with their constructive comments, improved the quality of this paper.
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