Squaloraja sp.
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publication ID |
https://doi.org/ 10.4081/nhs.2023.642 |
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persistent identifier |
https://treatment.plazi.org/id/03DBB32C-FFF7-FB6E-EE21-020EFB9BFDFA |
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treatment provided by |
Felipe |
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scientific name |
Squaloraja sp. |
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Figs. 6 View Fig , 8-11 View Fig View Fig View Fig View Fig
Note: Although the studied specimen was previously reported and partially figured in several papers as Squaloraja polyspondyla (Arduini et al., 1982: pl. 8, fig. 1; Duffin & Patterson, 1993: fig. 11; Stahl, 1999: figs. 34 H, 106B, 107; Tang, 2002: fig. 14.6; Garassino & Teruzzi, 2015: figs. 64-66), nobody has never formal- ly described the specimen ( Figs. 6 View Fig , 8 View Fig ). Based upon the short diagnosis provided by Patterson (1965) (focused on the male clasper shape only), it is impossible to as- sign the studied specimen to S. polyspondyla . The only specimen reasonably confidently identified as pertaining to a female is that housed in the NHMUK (Woodward, 1886: 529, pl. 55, figs. 3, 7; catalogue number NHMUK PV P.3184; Fig. 7A View Fig ), but it is too incomplete to allow a close comparison with the studied specimen. Woodward (1886: 529) suggested that a second, previously unfig- ured (apart from a section through a vertebra; Woodward 1886 pl. LV fig. 8) specimen might belong to a female. The specimen in question is NHMUK PV P.2079 and, as Woodward observed, belongs to an older individual. Whilst it is perfectly possible that the specimen does belong to a female, unfortunately the rostrum is missing and the cartilage fragments anterior to the dentition are badly preserved ( Fig. 7B View Fig ); it is possible that if a frontal clasper was present it might have been lost together with the anterior cartilages of the head.
Material and measurements: One complete articulated specimen, preserved in part and counterpart, in dorsal view.
MSNM V 665 – hl: 91 mm; hw: 56 mm; nl: 25 mm; nw: 28 mm (excluding mandibles); pgw: 17 mm (excluding rays); rl: 66.5 mm; sgw: 30 mm (excluding rays); trl: 44 mm; tl: 9.5 mm (as preserved); TL: c. 220 mm.
Description: Head - Broad elongate head (over 1/3 of the total body length), rhomboid in outline, flattened dorsoventrally, widest at the supramaxillary lateral cartilages; dorso-ventrally flattened rostrum elongate and sub-rectangular with smooth lateral margins, relatively wide base and tapering anteriorly; rostral tip unsup- ported by cartilage and slightly tapering; rostrum supported by five sets of cartilage – single median rostral cartilage (mrc), paired lateral rostral cartilages (lrc) anteriorly, paired supramaxillary cartilages (smc) plus inframaxillary (imc) cartilages, and paired premaxillary cartilages (pmc) laterally; mrc lance-like, slightly flattened, gradually tapering distally, with wide proximal base overlying nasal capsules and distal tip bifurcating with a deep, concave distal margin; dorsal surface of mrc differentiated from tessellated cartilage into evenly distributed coarse granulations ( Fig. 9A View Fig ) which do not contact each other; lrc each extending half way down full length of rostrum, curved and roughly triangular in shape, flaring anterolaterally, distally pointed with base fused to preorbital region of neurocranium; proximal posterior margin of lrc incomplete, exposing underlying symphysis and mandibular dentition; smc project laterally at roughly 40 o to midline and are slightly curved, with slight distal tapering; short, medially-directed transverse cartilage strut (?pre-labial cartilage) seems to connect the anterior mid-point of the smc to the lrc; imc short, distally pointed, posteriorly-directed extends from tip of each smc; pmc each scimitar-like, distally pointed and posteriorly-directed with base obscured by overlying smc ( Fig. 9C View Fig ).
Neurocranium - Neurocranium compressed dorsoventrally, partially crushed, with poorly preserved brain- case and orbital margins; wide sub-pentagonal neurocranium with rounded lateral margins; neurocranium laterally constricted, slightly elevated in the orbital region, lacking evidence of preorbital lateral projections; neurocranium extended anteriorly by fusion with the elongate rostral cartilages; orbits wide, sub-ovoid and dorsolater- ally positioned; occipital margin wide, slightly convex medially and poorly preserved ( Fig. 9B View Fig ).
Dentition - Lower dentition exposed in occlusal view; strong mandibular arc gently convex, converging mesi- ally to the symphyseal cartilage and extending to form a short protruding and bluntly rounded point; each mandible bears a single elongate tooth plate; the occlusal surface is crossed by bands of hypermineralised tissue which are roughly parallel to the labial margin of the tooth plate; the hypermineralised tissue is exposed by antemortem wear in transverse strips angled away from the mesial angle in symphyseal view, revealing laminated development of the tritural tissue ( Fig. 9D View Fig ).
Postcranial skeleton - Vertebral column reinforced by closely-set thick calcified rings, slightly tapering posteriorly to the narrow elongate tail; calcifications in the sheath of the notochord continue in the occipital region of the neurocranium where the notochord was enclosed; synarcual made of calcified cartilage present just behind occiput enclosing anterior distal section of notochord; no dorsal fin spine or dorsal fin present ( Fig. 10A View Fig ).
Pectoral girdle and fins - Pectoral girdle close to the posterior occipital margin, arc-like in form with large scapular process and moderately broad coracoid; wide metapterygium with 26 distinct radials segmented distally produced in a broad wing-like fin ( Fig. 10B View Fig ).
Pelvic girdle and fins - The two halves of the pelvic girdle are not fused; curved halves arcuate posteriorly, lacking an anterior process; single metapterygium with 18 articulated segmented radials; pelvic fins smaller than pectoral fins at only half their width ( Fig. 10C View Fig ).
Dorsal and caudal fins - Despite the exceptional detailed preservation of the specimen, there is no direct evidence or trace of any structure suggesting the presence of dorsal median and caudal fins; a flattened epichordal lobe could be present in the terminal half of the narrow tail, but evidence of supporting radials is lacking.
Body scales - Squamation comprises placoid scales distributed over the whole body except the rostrum; scales small and scattered around the orbits; dorsal and lateral trunk scales wider and coarser with different shape; scales between pectoral and pelvic girdles and in various loca- tions on the proximal part of the tail have rounded crown margins produced into elongate, radially directed unequal rays; hooked-shape scales ranged in a pair of parallel rows dorsally unite to form a single row towards the distal part of the tail; hooked-shape scales situated along the inferior margin of each side of the tail, decreasing in size towards the tail tip ( Fig. 10D View Fig ).
Mechanosensory canals [the nomenclature for the different elements of the lateral line sensory system used herein follows that used in Patterson (1965) and Didier (2004)] ( Fig. 11 View Fig ). – The mechanoreceptive lateral line system forms a pattern of grooves or tubes on the snout and head regions; they are exceptionally well preserved; the lateral line canals are supported by c-shaped cartilaginous rings which are open to the external surface; the trajecto- ries of the different components of the lateral line sensory canal system can be traced through the survival of mini- mally disarticulated lengths of such cartilaginous rings on the specimen; the lateral line sensory canal (LSC) of the trunk passes down the length of the body on the flanks of either side; both left and right trunk canals can be clearly seen extending from a point just posterior to the base of the pelvic fin to the tip of the tail.
Preservation of the specimen in dorsal view means that it is sometimes difficult to discern more subtle changes in the three-dimensional trajectory of individual canals; nevertheless, a slight kink in the pathway of the trunk canals approximately 15 mm from the tip of the tail marks a change in position from the upper flanks to the lower flanks in that region, a characteristic feature of all extant chimaeroids and all Mesozoic holocephalians in which the tail region is known.
In the skull region, the sensory canal system is best preserved around the anterior part of the neurocranium and the rostrum. A supratemporal canal (SC) ascends the postero-dorsal surface of the head, joining to form a commissural structure crossing the dorsal surface of the neurocranium (this canal is visible in the counterpart of MSNM V665); the trunk canal (LSC) is connected to the base of the supratemporal canal by a short occipital canal (OC), again visible in the counterpart; the base of the left supraorbital canal (SOC) is just discernible at the OC/SC junction in the counterpart, and some indistinct lengths of c-shaped rings indicate the presence of the supraorbital canal extending to a point just anterior to the orbit. An otic canal (OTC) branches from the SC/OC junction and swings down to a position ventral to the orbit; at an antero-ventral point to the orbit it branches at a complex junction; the most posterior branch is what is interpreted herein as the hyomandibular or preopercular canal (HC), but note that this canal is in a much more anterior position than in extant chimaeroids, with the exception of Callorhincus (Didier, 1995: figs. 8, 10). A second, shorter branch descends to the tissues of the lower jaw and is the oral canal (ORC); this branch is succeeded anteriorly by a bifurcation; the lower of these two branches is the angular canal (AC) which extends forwards to a point approximately half way along the rostrum; here, it loops beneath the lateral rostral cartilage; the upper branch is the infraorbital canal (IOC) which runs forward over the rostrum in front of the orbit to a bifurcation point in front of the anteriormost extent of the AC; from here, an anterior branch, the dorsal rostral canal ( DRC) runs over the tip of the rostrum and then loops back to a point just less than half-way back to the mouth close to the midline; a posterior branch, the central rostral canal (CRC) flanks the medial border of the lateral rostral cartilage extending posteriorly to meet the anterior ventral rostral cartilage (AVRC), which itself runs anteriorly until ending blindly about two thirds of the way along the underside of the rostrum; thus, the distal parts of the DRC and the AVRC have a reasonably extensive overlap on the underside of the distal part of the rostrum; a second branch, the posterior ventral rostral canal (PVRC) runs posteriorly toward the mouth; terminating near the rostrum base, it seems to have at least one and possibly two branches, the proximal rostral canals leading to the underside of the proximal part of the rostrum.
Discussion: According to Didier (2004), the mature males in extant chimaeroids have distinctive secondary sexual dimorphic characters, such as an elongate frontal clasper which opposes the rostrum, and paired pre-pelvic tenaculae and claspers, characters shared in all the best-preserved specimens of Squaloraja polyspondyla from Lyme Regis ( UK). However, these characters are clearly absent in the specimen described here, allowing us to exclude that it represents a mature male. Possible female specimens of S. polyspondyla include NHMUK PV P.3184 from the Enniskillen collection in NHMUK, and NHMUK PV P.2079 (see discussion above), both reported by Woodward (1886: 527) ( Figs. 7A, B View Fig ). Whilst the Lyme Regis specimens are both only partial, the Italian specimen is represented by a complete and articulated body in both part and counterpart and in a state of exceptional preservation. The diagnostic characters revealed in the specimen, such as the absence of the frontal clasper, pre-pelvic tenaculae and pelvic clasper cartilages indicates that the specimen belongs to a female Squaloraja , and possibly a juvenile or immature individual. The latter conclusion is supported by the following considerations: 1) the poor degree of calcifica- tion in juvenile tissues might mean that the true outline of the rostral tip (bifurcating in the Italian specimen vs. rounded in S. polyspondyla from the UK) is missing or not developed yet; 2) the granular surface of the rostral cartilage might represent a condition in which juvenile tesserae have not achieved their full geometric shape and size, as in all known specimens of S. polyspondyla ; 3) the lack of dorsal and epichordal fin radials (partially observed in S. polyspondyla ) might also indicate that these elements might not yet be calcified (= a juvenile condition). Finally, the shorter body length compared to all specimens of S. polyspondyla , although not considered to be a valid distinctive character, could be an additional datum, suggesting juvenile/immature status for MSNM V665.
Despite the excellent state of preservation, we prefer to leave the studied specimen in open nomenclature for the following reasons: 1) as previously pointed out, the diagnosis and description of S. polyspondyla are based on several male specimens (plus a single, poorly preserved, and incomplete female); as result all the main distinctive characters of the type species are based on the description of masculine features, precluding any comparison with the studied specimen; 2) there are too few morphological characters to allow adequate comparison with the type species or to establish a confident diagnosis of a new taxon within the genus Squaloraja ; 3) it is impossible to judge if some morphological characters in the studied specimen, such as the different number of radials in both fins compared to those of the type species could be related to sexual dimorphism or could be considered a specific character, since specimens of the type species lack preserved fin radials in the supposed female examples; this question is still un- resolved due to the lack of appropriate data from extant and fossil holocephalians.
The palaeogeographic position of the Osteno basin during the Sinemurian (Lower Jurassic), located at the northern boundary of the Tethyan faunas, just at the li- mit of the deep water sediments of the Western Tethys Sea and surrounded by shallow waters and emergent land masses, outlines an environmental scenario unfavourable to a possible migration/distribution within the two faunal Provinces for a species considered to have a benthic deep water to bathyal style of life.
Based on these considerations, we cannot exclude a priori that the studied specimen might represent a new taxon within Squaloraja , a hypothesis that can only be ve- rified and supported by the discovery of other specimens, especially adult males.
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Royal British Columbia Museum - Herbarium |
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