Rhamphichthys Müller & Troschel, 1846

Peixoto, Luiz Antônio Wanderley & de Pinna, Mário, 2022, Patterns of diversification and phylogenetic structure in the dorsolateral head musculature of Neotropical electric eels (Ostariophysi: Gymnotiformes), with a myological synonymy, Neotropical Ichthyology (e 210009) 20 (1), pp. 1-117 : 39-41

Rhamphichthys Müller & Troschel, 1846

Adductor mandibulae. The malaris is fully differentiated from ricto-stegalis and is positioned dorsolaterally to the dorsal portion of the presumably rictalis and the lateroventrally to the stegalis. This muscle arises from the mid-portion of the hyomandibula. The dorsalmost fibers differentiate into an elongated endomaxillary ligament, equal in length to the fibrous portion of the malaris, which inserts on the posteromedial portion of the antorbital and the maxilla ( Fig. 5 ), with some lateral fibers associated with the buccopalatal membrane. The ventral fibers of the malaris converge onto the accessory endomaxillary ligament, which inserts solely on the posteromedial face of the antorbital. There are many ossified tendons in the fibrous portion of the malaris, resulting in several intermuscular bones that are coopted towards its insertion site (Peixoto, Ohara, 2019: fig. 12).

The ricto-stegalis is mesial to the malaris, and originates from parasphenoid, pterosphenoid, metapterygoid, preopercle, quadrate, sympletic and hyomandibula, with its fibers restricted to the anterior margin of the preopercle. In the region near the origin of the ricto-stegalis, both subsections are easily distinguished by the direction of their fibers, with those corresponding to the presumptive stegalis being posteriorlyelongated and displaced dorsomedially; and those of the presumptive rictalis are shorter and located more laterally. At its median portion, the two sections are indistinguishable, becoming slightly differentiated towards the insertion, where fibers of the rictalis insert onto the coronoid process via an elongated ligament, and those of the stegalis converge into the meckelian tendon which, in turn, inserts onto the coronomeckelian bone. The bundles of fibers corresponding to the presumptive stegalis are located laterally relative to the basal region of the endopterygoid and of the adductor arcus palatini, completely overlapping those structures ( Fig. 18 ). The ramus mandibularis trigeminus nerve runs through the ricto-stegalis, located mesial to the presumptive rictalis and malaris; and laterally to the presumptive stegalis.

Levator arcus palatini. The levator arcus palatini has a trapezoidal shape, originating from the ventral margin of the frontal and sphenotics, inserting mainly on the hyomandibula, with posterolateral fibers inserting onto the preopercle. The size of its origin is half that of its insertion, with all fibers located laterally to the malaris at insertion. Only the dorsalmost fibers of the posterior portion of the levator arcus palatini are mesial to the dilatator operculi, but without reaching the median portion of the levator arcus palatini. The composition of the levator arcus palatini is mainly fibrous, with some ossified tendons forming intermuscular bones at its anteriormost portion ( Fig. 8B ).

Figures

FIGURE 5 | Lateral view of adductor mandibulae, pars malaris of Gymnorhamphichthys rosemariae (Rhamphichthyidae), MZUSP 56317. Anatomical abbreviations in Tab. 1. Scale bar = 5 mm.

FIGURE 8 | Rhamphichthys hahni (Rhamphichthyidae), MZUSP 24736, 479.5 mm TL. A. Lateral view of adductor mandibulae, pars ricto-stegalis; B. Mesial view of levator arcus palatini. Anatomical abbreviations in Tab. 1. Scale bars = 10 mm; 2 mm.

FIGURE 18 | Lateral view of dorsolateral musculature of Rhamphichthys hahni (Rhamphichthyidae), MZUSP 24736, 479.5 mm TL. Anatomical abbreviations in Tab. 1. Scale bar = 20 mm.