Potamotrygonidae Garman 1877

Morphological description of the neurocranium in Potamotrygonidae Ethmoid region The neurocranium in Potamotrygonidae is elongated, with its greatest width at the middle portion of the nasal capsules (ncs). The nasal capsules are located on the anteriormost region, being ventrolaterally expanded and with their openings directed ventrally. In Styracura the nasal capsules are oval and bear a median indentation in-between the convex anterior margins (Fig. 1C; Supporting Information, File S1). The same morphology is seen in the species of Potamotrygon and Plesiotrygon (Fig. 1A, B) (Supporting Information, Files S2, S3, S4). In Styracura, Potamotrygon, and Plesiotrygon the nasal capsules are ventromedially divided by a thin nasal septum that is one-sixth the width of the nasal aperture (Fig. 2). Heliotrygon and Paratrygon have comparatively shorter nasal capsules anteroposteriorly (Fig. 3) (Supporting Information, Files S5, S6), with the anterior margin of the capsules also convex, but less rounded when compared to the general pattern present in the other genera. The nasal septum in Heliotrygon and Paratrygon is wider, with nearly half the width of the nasal aperture (Fig. 4). Paratrygon shows a small rostral projection (rpj) in-between the anterior margin of the two nasal capsules, which can also be seen in some species of Potamotrygon (Figs 3B, 4B; Supporting Information, File S6). The condyles for the articulation with the antorbital cartilage (cac) are situated laterally on the nasal capsules. In Styracura, Paratrygon, and species of Potamotrygon this condyle is bean-shaped (Figs 5A, C, 6B), while on Plesiotrygon and Heliotrygon this condyle is rounded (Figs 5B, 6A). The anteroposteriorly flattened antorbital cartilages (anto) extend posterolaterally from the nasal capsules. In Styracura, Potamotrygon, and Plesiotrygon the antorbital cartilage’s length is one-third of the width of the nasal capsules, whilst in Heliotrygon and Paratrygon they are smaller and not as posteriorly oriented, with its length corresponding to a quarter of the nasal capsule’s width (Figs 1, 2). In Heliotrygon the antorbital cartilage has a foramen (acf) that is absent on the other genera (Fig. 7). The nasal capsules, which are ventrally projected, form a slope in relation to the basicranium. In Styracura and Potamotrygon this ventral projection forms an angle of nearly 130° in relation to the basicranium (Fig. 5A, C), while in Plesiotrygon this angle is less accentuated, close to 170° (Fig. 5B). In Heliotrygon and Paratrygon the inclination is more accentuated and closer to 100° in relation to the basicranium (Fig. 6). In Styracura, Potamotrygon, Plesiotrygon, and Heliotrygon the condyle for the articulation with the antorbital cartilage is anteroventral to the preorbital process (Figs 5, 6A), while in Paratrygon this condyle is situated ventrally and at a level with the preorbital process (Fig. 6B). Furthermore, Heliotrygon can be distinguished from the other potamotrygonins by having two pairs of condyle-like structures (nco) on the anterior face of its nasal capsules (Fig. 7). These condyles articulate with the anterior segment of the propterygium (apro) and are absent in the other genera. The preorbital process (prp) is posterodorsal to the nasal capsules. In Styracura, Potamotrygon, and Paratrygon it extends laterally beyond the longitudinal line with the lateral margin of the nasal capsules (Figs 1A, C, 3B). In Plesiotrygon the preorbital process reaches the longitudinal line with the nasal capsules, while in Heliotrygon the preorbital process is shorter and does not reach that same longitudinal line at the lateral limit of the nasal capsules (Figs 1B, 3A). Furthermore, only in Styracura and Potamotrygon does the preorbital process project posterolaterally, while in Plesiotrygon, Heliotrygon, and Paratrygon the preorbital process projects laterally (Figs 1, 3). The anterior foramen for preorbital canal (afpc) and the posterior foramen for preorbital canal (pfpc) pierce the base of the preorbital process dorsally, and are housed in a fossa that opens anteriorly. In Heliotrygon and Paratrygon this fossa is placed medially (Fig. 3), while in the other genera it is lateral and closer to the edge of the preorbital process (Fig. 1). Differently from the other genera, in Heliotrygon the foramen for preorbital canal is not divided into two openings. In Styracura, Potamotrygon, and Paratrygon the ventral opening of afpc occupies the anterior portion of the base of the preorbital process, while pfpc opens ventrally at the posterior region of the base of the preorbital process (Figs 5A, C, 6B), except in Styracura, where the pfpc is positioned further back beyond the base of the preorbital process (Fig. 5C). In Plesiotrygon both the foramen for anterior and posterior preorbital canal open posteroventrally at the base of the preorbital process (Fig. 5B; Supporting Information, Files S2, S7). The precerebral fontanelle (pcf) is rounded in Styracura, Potamotrygon, Plesiotrygon, and Paratrygon, while in Heliotrygon the precerebral fontanelle is elliptical and narrow (Figs 1, 3). Posterior to the precerebral fontanelle there is an epiphysial process (epp), which delimits the two fontanelles of the neurocranium, except for Styracura, where this structure is absent. In Heliotrygon the epiphysial process is the shortest among all genera analysed (with one-third the width of the supraorbital process) (Fig. 3A), while Potamotrygon and Plesiotrygon have the longest epiphysial process, which is longer than the supraorbital process (Fig. 1A, B). Paratrygon shows an intermediate state and its epiphysial process has nearly the same length of the supraorbital process (Fig. 3B). Dorsal optic region Thesupraorbitalcrest (soc) ispiercedbyaseriesofsmallforamina for the superficial ophthalmic nerve (sup). They appear posterior to the preorbital process and extend posteriorly throughout the orbital region, reaching the small (half the postorbital process length at most) and triangular supraorbital process (sp). In all genera the lateral extension of the supraorbital crest in relation to the basicranium is short, except for Heliotrygon, in which the crest is a well-developed, shelf-like structure (Figs 4A, 8). The supraorbital process is triangular-shaped and laterally projected, except for Styracura, in which the supraorbital process is anteriorly oriented (Fig. 1C). The postorbital process (pop) is posterior to the supraorbital process, being a rectangular and shelf-like plate that extends anterolaterally from the anterodorsal portion of the otic region of the neurocranium. In Styracura the postorbital process is larger than the other four genera, with its base wider than the length of the otic region (Fig. 1C). In Heliotrygon the distal margin of the postorbital process is concave, distinct from the other four genera, in which this margin is straight (Fig. 3A). In relation to its base, the distal margin of the postorbital process is only narrower in Plesiotrygon (Fig 1B). In Styracura, Potamotrygon, and Paratrygon the anterolateral portion of the postorbital process surpasses anteriorly the lateral tip of the supraorbital processes, while in Heliotrygon and Plesiotrygon it is horizontally aligned with the lateral tip of the supraorbital process (Figs 1, 3). Furthermore, the posterior margin of the postorbital process is straight in all genera, except for Styracura, which has a slight convex projection near its distal tip (Fig. 1C). The frontoparietal fontanelle (fpf) is slender and elongated in all genera (Figs 1, 3). In Styracura, Potamotrygon, and Plesiotrygon the frontoparietal fontanelle is wider anteriorly, narrowing posteriorly, being somewhat triangular in shape (Fig. 1). In Potamotrygon the frontoparietal fontanelle bears a slight constriction at its midportion, giving it an hourglass-shape (Fig. 1A). In Heliotrygon and Paratrygon the frontoparietal fontanelle has the same width throughout (Fig. 3). In Styracura, Potamotrygon, and Plesiotrygon the width of the anterior portion of the frontoparietal fontanelle occupies approximately one-third of the dorsal surface of the neurocranium (Fig. 1), while in Paratrygon and Heliotrygon it occupies a quarter of the dorsal surface of the neurocranium (Fig. 3). Orbital region The neurocranium is pierced laterally by several foramina. Ventrally at the base of the preorbital process all genera bear two foramina for the preorbital canal (Figs 5, 6B), except for Heliotrygon, which has only one (Fig. 6A). In Styracura the position of the posterior foramen for preorbital canal is completely posterior to the preorbital process (Fig. 5C), while in the other genera this posterior opening is located on the posterior margin of the preorbital process. Only in Plesiotrygon, both foramina for the preorbital canal are on the posterior margin of the preorbital process (Fig. 5B). The anteriormost foramen on the orbital wall is the anterior cerebral vein foramen (acvf). In all genera its position is anterior to, and close to, the dorsal margin of the optic nerve foramen (II). Only Plesiotrygon presents a different placement of this foramen, which is anterodorsal and not at the same level of the dorsal margin of the optic nerve foramen (Fig. 5B). The wide (at least the same size as the orbital fissure) and oval optic nerve foramen (II) is situated centrally on the orbital wall. The trochlear nerve foramen (IV) is located dorsally to the optic nerve foramen, and may vary in its position amongst genera of Potamotrygonidae. In Styracura, Potamotrygon, Plesiotrygon, and Heliotrygon it is dorsal to the midportion of the optic nerve foramen (Figs 5, 6A); in Paratrygon it is posterior to the midportion of the optic nerve foramen (Fig. 6B). In Styracura and Paratrygon the distance between the trochlear nerve foramen and the optic nerve foramen is less than half the height of the optic nerve foramen (Figs 5C, 6B), while in Plesiotrygon this