Psilorhynchus sucatio

Conway, Kevin W., 2011, Osteology of the South Asian Genus Psilorhynchus McClelland, 1839 (Teleostei: Ostariophysi: Psilorhynchidae), with investigation of its phylogenetic relationships within the order Cypriniformes, Zoological Journal of the Linnean Society 163 (5), pp. 50-154 : 61-63

A. Psilorhynchus sucatio B. Psilorhynchus balitora situated medially to the ventralmost part of the opercle. The preopercle is a short well-ossified bone. It sits lateral to the symplectic, and bridges the gap between the posterior process of the quadrate and the anterior edge of the opercle. The preopercle lacks the dorsally curved portion at its posterior end that typically rims the anterior face of the opercle in other teleosts (e.g. Harrington, 1955; Weitzman, 1962; Forey, 1973; Grande, 1985; Arratia, 1997; Hilton, 2002). Anteriorly, the preopercle exhibits a small, peg-like process on its medial face ( Figs 7A, 8A ). This process is heavily ossified and exhibits a meniscuslike covering. It articulates with a similar process on the lateral face of the interopercle (see below). The preopercular sensory canal runs along the lateral face of the preopercle before connecting with the ·

infraorbital sensory canal at a point between infraorbitals 3 and 4. It does not connect with the otic sensory canal. There are four or five preopercular canal openings. The first sits on the anteroventral edge of the preopercle, directly below the anteriormost point of overlap between the posterior arm of the quadrate and the preopercle. The second opening is situated on the ventral edge of the preopercle, directly below the posteriormost tip of the posterior arm of the quadrate. The third and fourth openings (when present) are situated on the ventral edge of the preopercle, posterior to the second opening, and the posteriormost opening is situated on the posterior edge of the preopercle, which marks the termination of the preopercular sensory canal. The interopercle is an irregularly shaped bone, similar in length to the subopercle. Its anteriormost point serves as the point of origin for the interopercular–retroarticular ligament, which inserts on the posterior tip of the small retroarticular. Dorsal to the origin of the interopercle– retroarticular ligament the interopercle exhibits a small, peg-like process on its lateral face, similar in size and shape to the process on the medial face of the preopercle with which it articulates ( Figs 7A, 8A ). Such an articulation between the interopercle and the preopercle has, to the best of my knowledge, not yet been described in any teleost fishes. The name interoperculo–preopercular articulation is suggested.

The premaxilla is short and curves towards the midline to meet its antimere, completely excluding the maxilla from the gape, as is common in other cypriniforms ( Howes, 1991). It exhibits a needle-like ascending process on its anterodorsal surface that extends dorsally towards the kinethmoid, to which it is attached via a short ligament (Fig. 5A). A large process is present on the posteroventral surface of the premaxilla, which articulates in a deep notch on the lateral face of the dentary. The maxilla is a complicated structure and is much larger than the premaxilla, which it completely surrounds anteriorly and dorsally. It exhibits a sharp ridge along its anteroventral edge that extends posteriorly and provides support for the fleshy upper lip. Its anterodorsal surface bears a short palatine process that extends posterodorsally towards the maxillary process of the autopalatine and a large ascending process that extends towards the dorsal midline. The head of the ascending process is bifurcate, resembling a hammer, and exhibits a dorsal process with a large circular tip, and a much smaller ventral process that is triangular in shape and extends towards the ascending process of the premaxilla. A large, square pad of dense connective tissue is present between the ascending process of the maxilla and the preethmoid ( Fig. 4D ). This meniscus-like structure was visible in cleared and stained specimens only with the aid of transmitted light. A similar, although smaller, structure is also present between the palatine process of the maxilla and the maxillary process of the autopalatine. The posterior end of the maxilla exhibits a short process on its dorsal edge that curves towards the short coronoid process midway along the dentary on its dorsal surface.

The lower jaw comprises Meckel’s cartilage, dentary, anguloarticular, retroarticular, and the coronomeckelian. The anterior portion of the dentary is similar in shape to the premaxilla to which it opposes. Along its midlength it exhibits a short coronoid process that extends dorsally and is closely associated with the curved process on the posterodorsal edge of the maxilla. A large foramen for the passage of the internal mandibular branches of the trigeminal (V) nerve is present directly below the coronoid process, close to the point of articulation with the premaxilla. The posterior region of the dentary exhibits a shallow groove along its medial edge that accommodates the anterior portion of the anguloarticular. The anguloarticular articulates posteriorly with the quadrate, via a broad articulatory facet ( Fig. 8A ). Its posteroventralmost tip is drawn out into a fine point that extends past the articular facet. A small cartilaginous strip, a remnant of the Meckel’s cartilage, runs along the anterodorsal edge of the medial face of the anguloarticular. A relatively large ovoid-shaped coronomeckelian lies on the posterodorsal surface of Meckel’s cartilage. The small retroarticular articulates with the posteroventral surface of the anguloarticular. It is conical in shape, and its ventralmost tip is curved posteriorly to accept the interopercular–retroarticular ligament that originates on the anterior tip of

Figures

Figure 4. Serial sections through the head of Psilorhynchus. A, transverse section through the masticatory plate of Psilorhynchus sucatio, KU 40642, male, 48.6-mm standard length (SL). B, transverse section through the masticatory plate of Psilorhynchus balitora, KU 40375, 36.1-mm SL. C, sagital section through the lateral ethmoid of Psilorhynchus sucatio, KU 40642, 49.8-mm SL, inset image shows close-up of connective tissue between transverse shelf on medial face of infraorbital 2 and the ventral surface of the lateral ethmoid of a more medial section. D, sagital section through the upper jaw and ethmoid region of P. sucatio, KU 40642, 49.8-mm SL, inset image shows close-up of boxed area. E, sagital section through the upper jaw and ethmoid region of P. balitora, KU 40375, 29.9-mm SL, inset image shows close-up of boxed area. Abbreviations: Boc, basioccipital; CT, connective tissue; DA, dorsal aorta; E, esophagus; Ep, epidermis; IE, inner ear; IO2, inner ear; LE, lateral ethmoid; M, meniscus; MaPL, masticatory plate; Mx, maxilla; N, notochord; Pc, perichordal ossification of the basioccipital; PE, preethmoid; PT, pharyngeal teeth of ceratobranchial 5.

Figure 8. Hyopalatine arch and opercular series of Psilorhynchus, right side in lateral (i) and medial (ii) view. Anterior to left (images reversed). A, Psilorhynchus sucatio, CAS 50289, 39.6-mm SL. B, Psilorhynchus balitora, UMMZ 244782, 31.2-mm SL. C, Psilorhynchus breviminor, ZRC 43452, paratype, 32.0-mm SL. D, Psilorhynchus gracilis, UMMZ 205345, paratype, 39.1-mm SL. E, Psilorhynchus robustus, CMK 17941, paratype, 58.1-mm SL. F, Psilorhynchus pseudecheneis, KU 29517, 41.5-mm SL. Cartilage highlighted in grey. Abbreviations: Ana, anguloarticular; Apa, autopalatine; Cm, coronomeckelian; De, dentary; Ecpt, ectopterygoid; Enpt, endopterygoid; Hy, hyomandibular; Iop, interopercle; MC, Meckel’s cartilage; Mpt, metapterygoid; Mx, maxilla; Op, opercle; Pmx, premaxilla; Pop, preopercle; Q, quadrate; Ra, retroarticular; Sop, subopercle; Sy, symplectic.