Potamotrygonidae, Garman, 1877

Morphological description of the neurocranium in Potamotrygonidae View in CoL

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 View Figure 1 ; Supporting Information, File S1). The same morphology is seen in the species of Potamotrygon and Plesiotrygon ( Fig. 1A, B View Figure 1 ) (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 View Figure 2 ). Heliotrygon and Paratrygon have comparatively shorter nasal capsules anteroposteriorly ( Fig. 3 View Figure 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 View Figure 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 View Figure 3 , 4B View Figure 4 ; 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 View Figure 5 , 6B View Figure 6 ), while on Plesiotrygon and Heliotrygon this condyle is rounded ( Figs 5B View Figure 5 , 6A View Figure 6 ). 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 View Figure 1 , 2 View Figure 2 ). In Heliotrygon the antorbital cartilage has a foramen (acf) that is absent on the other genera ( Fig. 7 View Figure 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 View Figure 5 ), while in Plesiotrygon this angle is less accentuated, close to 170° ( Fig. 5B View Figure 5 ). In Heliotrygon and Paratrygon the inclination is more accentuated and closer to 100° in relation to the basicranium ( Fig. 6 View Figure 6 ). In Styracura , Potamotrygon , Plesiotrygon , and Heliotrygon the condyle for the articulation with the antorbital cartilage is anteroventral to the preorbital process ( Figs 5 View Figure 5 , 6A View Figure 6 ), while in Paratrygon this condyle is situated ventrally and at a level with the preorbital process ( Fig. 6B View Figure 6 ). 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 View Figure 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 View Figure 1 , 3B View Figure 3 ). 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 View Figure 1 , 3A View Figure 3 ). 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 View Figure 1 , 3 View Figure 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 View Figure 3 ), while in the other genera it is lateral and closer to the edge of the preorbital process ( Fig. 1 View Figure 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 View Figure 5 , 6B View Figure 6 ), except in Styracura , where the pfpc is positioned further back beyond the base of the preorbital process ( Fig. 5C View Figure 5 ). In Plesiotrygon both the foramen for anterior and posterior preorbital canal open posteroventrally at the base of the preorbital process ( Fig. 5B View Figure 5 ; 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 View Figure 1 , 3 View Figure 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 View Figure 3 ), while Potamotrygon and Plesiotrygon have the longest epiphysial process, which is longer than the supraorbital process ( Fig. 1A, B View Figure 1 ). Paratrygon shows an intermediate state and its epiphysial process has nearly the same length of the supraorbital process ( Fig. 3B View Figure 3 ).

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 View Figure 4 , 8 View Figure 8 ). The supraorbital process is triangular-shaped and laterally projected, except for Styracura , in which the supraorbital process is anteriorly oriented ( Fig. 1C View Figure 1 ).

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 View Figure 1 ). 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 View Figure 3 ). In relation to its base, the distal margin of the postorbital process is only narrower in Plesiotrygon ( Fig 1B View Figure 1 ). 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 View Figure 1 , 3 View Figure 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 View Figure 1 ).

The frontoparietal fontanelle (fpf) is slender and elongated in all genera ( Figs 1 View Figure 1 , 3 View Figure 3 ). In Styracura , Potamotrygon , and Plesiotrygon the frontoparietal fontanelle is wider anteriorly, narrowing posteriorly, being somewhat triangular in shape ( Fig. 1 View Figure 1 ). In Potamotrygon the frontoparietal fontanelle bears a slight constriction at its midportion, giving it an hourglass-shape ( Fig. 1A View Figure 1 ). In Heliotrygon and Paratrygon the frontoparietal fontanelle has the same width throughout ( Fig. 3 View Figure 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 View Figure 1 ), while in Paratrygon and Heliotrygon it occupies a quarter of the dorsal surface of the neurocranium ( Fig. 3 View Figure 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 View Figure 5 , 6B View Figure 6 ), except for Heliotrygon , which has only one ( Fig. 6A View Figure 6 ). In Styracura the position of the posterior foramen for preorbital canal is completely posterior to the preorbital process ( Fig. 5C View Figure 5 ), 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 View Figure 5 ). 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 View Figure 5 ). 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 View Figure 5 , 6A View Figure 6 ); in Paratrygon it is posterior to the midportion of the optic nerve foramen ( Fig. 6B View Figure 6 ).

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 View Figure 5 , 6B View Figure 6 ), while in Plesiotrygon this distance is equal to the height of the optic nerve foramen ( Fig. 5B View Figure 5 ). In Potamotrygon and Heliotrygon this distance is intermediate ( Figs 5A View Figure 5 , 6A View Figure 6 ). Posterior to the optic nerve foramen is the eye stalk (es), which is rounded, except for Heliotrygon , in which the eye stalk is oval ( Fig. 6A View Figure 6 ). Immediately dorsal to the eye stalk is the oculomotor nerve foramen (III). In Potamotrygon and Heliotrygon the oculomotor nerve foramen is anterodorsal to the eye stalk ( Figs 5A View Figure 5 , 6A View Figure 6 ).

The efferent spiracular artery foramen (esaf) is ventral to the eye stalk, although in Styracura and Potamotrygon it is anteroventral and in-between the eye stalk and the optic nerve foramen ( Fig. 5A, C View Figure 5 ). The internal carotid foramen (icaf) is positioned anterior to the hyomandibular branch of the facial nerve foramen (VII) and in Styracura , Plesiotrygon , and Heliotrygon it is ventrolaterally positioned, occupying the margin that divides the orbital wall and the basicranium ( Figs 5B, C View Figure 5 , 6A View Figure 6 ). In contrast, Potamotrygon and Paratrygon present this foramen positioned ventrally on the basicranium ( Figs 2A View Figure 2 , 3B View Figure 3 ). The orbital fissure (obf) is situated on the posteriormost portion of the orbital region. In Heliotrygon , Paratrygon , and Plesiotrygon the orbital fissure is about the same size as the optic nerve foramen, while in Styracura and Potamotrygon it is about half the size of the optic nerve foramen ( Figs 5 View Figure 5 , 6 View Figure 6 ; Supporting Information, Files S1, S3, S4). In Potamotrygon and Heliotrygon the orbital fissure is covered laterally by the posterodorsal wall of the neurocranium, immediately ventral to the postorbital process ( Figs 5A View Figure 5 , 6A View Figure 6 ), while in the other genera the fissure is completely visible laterally.

Otic region

The lateral commissure (lc) is an arch that bridges over the foramen for the hyomandibular branch of the facial nerve. In Styracura and Plesiotrygon the lateral commissure is septate (bears two openings) and connects medially with the lateral wall of the otic region ( Fig. 5B, C View Figure 5 ). The hyomandibular facet (hmdf), which is posterior to the lateral commissure, corresponds to a nearly elliptical and oblique depression. In Styracura and Plesiotrygon the hyomandibular facet is oval and not with its anterior portion wider than its posterior portion ( Fig. 5B, C View Figure 5 ). Moreover, Heliotrygon is the only genus in which the ventral margin of the hyomandibular facet is somewhat arched medially ( Fig. 6A View Figure 6 ). Plesiotrygon differs from all the other genera by the presence of a chondrified jugal arch (jla) that bridges laterally on the otic region, immediately posterior to the hyomandibular facet ( Fig. 5B View Figure 5 ; Supporting Information, File S2).

The glossopharyngeal nerve foramen (IX) varies in its position, and is somewhere between the otic and occipital regions. In Heliotrygon and Paratrygon this foramen is visible laterally, on the posteroventral portion of the otic region ( Fig. 6 View Figure 6 ). In the other three genera the glossopharyngeal nerve foramen is dorsolateral to the occipital condyles and occupies the ventral lateral portion of the occipital region, being visible in posterior oblique view ( Fig. 5 View Figure 5 ). The sphenopterotic ridge (spr) projects dorsally on the lateral portions of the otic region, with the supraotic ridge (sor) extending laterally from the otic region walls above the hyomandibular facet. In Styracura , Potamotrygon , and Plesiotrygon the spr is as long as the lateral extension of the sor ( Fig. 9 View Figure 9 ). Dorsally, a pair of endolymphatic foramina (elf) is followed posteriorly by a pair of perilymphatic foramina (plf). In Heliotrygon and Potamotrygon the perilymphatic foramina are not immediately posterior, but posterolateral and posteromedial to the endolymphatic foramina, respectively ( Figs 1A View Figure 1 , 3A View Figure 3 ). In Heliotrygon and Plesiotrygon both pairs of foramina are clearly different in size, with the perilymphatic foramina being the largest in Heliotrygon (three times the size of the endolymphatic foramina) ( Fig. 3A View Figure 3 ).

Occipital region

The large and oval foramen magnum (fm) is located in-between a pair of vagus nerve foramina (X). Only in Styracura is the foramen magnum taller than wider, and the lateral limit of the vagus nerve foramen reaches the medial level of the sphenopterotic ridge ( Fig. 9C View Figure 9 ). The foramen magnum is the smallest (eight times the aperture of the vagus nerve foramen) in Heliotrygon and the largest in Styracura and Potamotrygon (12 times the aperture of the vagus nerve foramen) ( Figs 9A, C View Figure 9 , 10A View Figure 10 ). The vagus nerve foramen is positioned dorsal to the midportion of the foramen magnum, closer to its dorsal margin in Styraruca and Potamotrygon ( Fig. 9A, C View Figure 9 ), while in the other genera it is situated level with the midportion of the foramen magnum ( Figs 9B View Figure 9 , 10 View Figure 10 ). The foramen of the lateralis branch of the vagus nerve (flb) is juxtaposed to the vagus nerve foramen in Heliotrygon , Potamotrygon , and Plesiotrygon ( Figs 9A, B View Figure 9 , 10A View Figure 10 ), while in Styracura and Paratrygon this foramen is positioned far from the vagus nerve foramen ( Figs 9C View Figure 9 , 10B View Figure 10 ). The articular surface (as) is located in-between a pair of occipital condyles (oc), which are horizontally arranged and immediately ventral to the foramen magnum. Plesiotrygon presents the narrowest articular surface (one-third of the occipital condyle’s width), while Heliotrygon has the widest in relation to the occipital condyles (two-thirds of the occipital condyle’s width) ( Figs 9B View Figure 9 , 10A View Figure 10 ). In Potamotrygon and Plesiotrygon the ventral margin of the occipital condyles is medially arched ( Fig. 9A, B View Figure 9 ), while in the other three genera the ventral margin is straight. Paratrygon and Heliotrygon have medially and laterally inclined occipital condyles, respectively ( Fig. 10 View Figure 10 ), instead of the usual horizontal orientation seen in Styracura , Potamotrygon , and Plesiotrygon . The occipital condyles extend posteriorly from the basicranium and in Styracura , Potamotrygon , and Plesiotrygon this extension is shorter than half the length of the hyomandibular facet ( Fig. 9 View Figure 9 ), while in Heliotrygon and Paratrygon the posterior extension of the condyles is greater, around two-thirds the length of the hyomandibular facet ( Fig. 10 View Figure 10 ). In Heliotrygon and Paratrygon the occipital condyles reach the lateral limit of the occipital region, resulting in a straighter posterior outline of the neurocranium, while in the other genera they are shorter and the shape of the occipital region is irregular ( Fig. 10 View Figure 10 ).

Character descriptions

The descriptions of 60 newly proposed morphological characters based on the anatomy of the neurocranium, 15 of them consisting of multistate ordered characters, is given below. Characters are organized by regions of the neurocranium.

Ethmoid region

1. Rostral projection: (0) absent; (1) present.

In Styracura (outgroup), Plesiotrygon , and Heliotrygon the neurocranium does not present an anteromedian rostral projection (0). In Potamotrygon the rostral projection is polymorphic, with the majority of the species lacking this structure, while the rostral projection is present in Potamotrygon marquesi , Potamotrygon signata , and Potamotrygon wallacei (0,1). In Paratrygon a small chondrified rostral projection is present in-between the anterior margin of the nasal capsules (1) ( Figs 3B View Figure 3 , 4B View Figure 4 ). This character was previously explored by Rosa (1985).

2. Nasal septum; width: (0) one-sixth the width of the nasal aperture; (1) half the width of the nasal aperture.

In Styracura (outgroup), Potamotrygon , and Plesiotrygon the nasal septum, which divides the two nasal capsules medially, is one-sixth the width of the nasal aperture (0) ( Fig. 2 View Figure 2 ). In Heliotrygon and Paratrygon the nasal septum is wider and about half the width of the nasal aperture (1) ( Fig. 4 View Figure 4 ). This character was previously explored by Carvalho and Lovejoy (2011).

3. Condyles on the anterior margin of the nasal capsules: (0) absent; (1) present.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon the nasal capsules do not bear condyles on their anterior margin (0) ( Figs 1 View Figure 1 , 2 View Figure 2 , 3B View Figure 3 , 4B View Figure 4 ). In Heliotrygon the anterior margin of the nasal capsules has two pairs of condyles for the articulation with the distal segment of the propterygium (1) ( Figs 3A View Figure 3 , 4A View Figure 4 , 7 View Figure 7 ).

4. Condyle for the articulation with the antorbital cartilage; position: (0) anteroventral in relation to the preorbital process; (1) ventral in relation to the preorbital process.

In Styracura (outgroup), Plesiotrygon , Potamotrygon , and Heliotrygon the condyle for the articulation with the antorbital cartilage is anteroventral in relation to the preorbital process (0) ( Figs 5 View Figure 5 , 6A View Figure 6 ). In Paratrygon the condyle for the articulation with the antorbital cartilage is ventral and vertically aligned to the preorbital process (1) ( Fig. 6B View Figure 6 ).

5. Condyle for the articulation with the antorbital cartilage; shape: (0) bean-shaped; (1) round.

In Styracura (outgroup) and Paratrygon the condyle for the articulation with the antorbital cartilage is nearly bean-shaped (0) ( Figs 5C View Figure 5 , 6B View Figure 6 ). In Potamotrygon this character is polymorphic, with Potamotrygon brachyura , Potamotrygon histrix , and Potamotrygon orbignyi with the condyle for the articulation with the antorbital cartilage being round in format, while it is bean-shaped on the remaining species (0,1). In Plesiotrygon and Heliotrygon the condyle for the articulation with the antorbital cartilage is rounded (1) ( Figs 5B View Figure 5 , 6A View Figure 6 ).

6. Condyle for the articulation with the antorbital cartilage; depression: (0) anterior; (1) dorsal.

In Styracura (outgroup) the condyle for the articulation with the antorbital cartilage has a depression on its anterior region (0) ( Fig. 5C View Figure 5 ). In Potamotrygon this character is polymorphic (0,1), with Potamotrygon pantanensis bearing a dorsal depression on the condyle for the articulation with the antorbital cartilage. In Paratrygon the depression is dorsal on the condyle for the articulation with the antorbital cartilage (1) ( Fig. 6B View Figure 6 ).

7. Antorbital cartilage; length: (0) half the width of the nasal capsules; (1) one-third the width of the nasal capsules; (2) a quarter the width of the nasal capsules [multistate ordered].

In Styracura (outgroup) and Plesiotrygon the length of the antorbital cartilage is one-third the width of the pair of nasal capsules (0) ( Figs 1 View Figure 1 , 2 View Figure 2 ). In Potamotrygon this character is polymorphic and the antorbital cartilage can be both half or one-third the width of the nasal capsules (0,1). In Heliotrygon and Paratrygon the antorbital length is a quarter the width of the pair of nasal capsules (2) ( Figs 3 View Figure 3 , 4 View Figure 4 ). This character was previously explored by Carvalho and Lovejoy (2011).

8. Antorbital cartilage foramen: (0) absent; (1) present.

In Styracura (outgroup), Potamotrygon , Plesiotrygon ,and Paratrygon the antorbital cartilage foramen is absent (0). In Heliotrygon the antorbital cartilage is pierced by a foramen (1) ( Fig. 7 View Figure 7 ).

9. Nasal capsules in relation to the basicranium; angulation in lateral view: (0) 170°; (1) 130°; (2) 100° [multistate ordered].

In Styracura (outgroup) the nasal capsules are curved ventrally and forming an angle of nearly 130° in relation to the basicranium (1) ( Fig. 5A, C View Figure 5 ). In Potamotrygon this character is polymorphic. In Potamotrygon scobina the nasal capsules form an angle of nearly 170°, while in the remaining species this angle is closer to 130° (0,1). In Plesiotrygon the nasal capsules are slightly curved ventrally and forming an angle of nearly 170° in relation to the basicranium (0) ( Fig. 5B View Figure 5 ). In Paratrygon and Heliotrygon the nasal capsules are considerably curved ventrally and forming an angle of nearly 100° in relation to the basicranium (2) ( Fig. 6 View Figure 6 ).

10. Preorbital process; orientation: (0) posteriorly curved; (1) laterally projected.

In Styracura (outgroup) and Potamotrygon the preorbital process is curved posteriorly (0) ( Figs 1A, C View Figure 1 , 2A, C View Figure 2 ). In Plesiotrygon , Heliotrygon , and Paratrygon the preorbital process extends laterally without curving posteriorly (1) ( Figs 1B View Figure 1 , 2B View Figure 2 , 3 View Figure 3 , 4 View Figure 4 ).

11. Preorbital process; lateral extension in relation to the nasal capsules: (0) preorbital process extends laterally beyond the longitudinal line with the lateral margin of the nasal capsules; (1) preorbital process extends laterally and is longitudinally aligned with the lateral margin of the nasal capsules; (2) preorbital process fails to reach the longitudinal line with lateral margin of the nasal capsules [multistate ordered].

In Styracura (outgroup) and Paratrygon the preorbital process extends laterally beyond the longitudinal line with the lateral margin of the nasal capsules (0) ( Figs 1A, C View Figure 1 , 3B View Figure 3 ). In Potamotrygon this character is polymorphic, with the preorbital process extending laterally beyond or aligned with the longitudinal line with lateral margin of the preorbital process (0,1). In Plesiotrygon the lateral extension of the preorbital process extends laterally and is longitudinally aligned with the lateral margin of the nasal capsules (1) ( Fig. 1B View Figure 1 ). In Heliotrygon the preorbital process fails to reach the longitudinal line with the lateral limit of the nasal capsules (2) ( Fig. 3A View Figure 3 ). This character was previously explored by Carvalho and Lovejoy (2011).

12. Precerebral fontanelle; development in relation to the frontoparietal fontanelle: (0) at least one-third wider than the frontoparietal fontanelle; (1) as wide as the frontoparietal fontanelle.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon , the precerebral component of the fontanelle is at least one-third wider than the frontoparietal component (0) ( Figs 1 View Figure 1 , 3B View Figure 3 ). In Heliotrygon both components of the fontanelle have the same width (1) ( Fig. 3A View Figure 3 ).

13. Precerebral fontanelle; shape: (0) rounded; (1) elliptical.

In Styracura (outgroup), Plesiotrygon , and Paratrygon the precerebral fontanelle is as wide as it is long, being rounded in shape (0) ( Figs 1 View Figure 1 , 3B View Figure 3 ). In Potamotrygon , only Potamotrygon schroederi has an elliptical precerebral fontanelle (0,1). In Heliotrygon the precerebral fontanelle is anteroposteriorly elongated, with its width about half its length (1) ( Fig. 3A View Figure 3 ). This character was previously explored by Carvalho and Lovejoy (2011).

14. Epiphysial process: (0) absent; (1) present.

In Styracura (outgroup) the epiphysial process is absent (0) ( Fig. 1C View Figure 1 ). In all members of Potmotrygoninae, namely Potamotrygon , Plesiotrygon , Paratrygon , and Heliotrygon the chondrified epiphysial process is present (1) ( Figs 1A, B View Figure 1 , 3 View Figure 3 ).

15. Epiphysial process; development: (0) with one-third the length of the supraorbital process; (1) with the same length of the supraorbital process; (2) one-third longer than the length of the supraorbital process [multistate ordered].

In Heliotrygon the epiphysial bar is short, with about one-third the length of the supraorbital process (0) ( Fig. 3A View Figure 3 ). In Paratrygon the epiphysial bar extends medially having the same length of the supraorbital process (1) ( Fig. 3B View Figure 3 ). In Plesiotrygon the epiphyseal bar is one-third longer than the length of the supraorbital process (2) ( Fig. 1B View Figure 1 ). In Potamotrygon this character is polymorphic, with all three character states occurring among the species (0,1,2) (Supporting Information, Files S3, S4). This character is inapplicable for Styracura ( Fig. 1C View Figure 1 ).

16. Epiphysial process; orientation: (0) medially projected; (1) anteromedially projected.

In Heliotrygon , Plesiotrygon , and Paratrygon the epiphysial bar is oriented medially (0) ( Fig. 1B View Figure 1 , 3 View Figure 3 ). In Potamotrygon the epiphysial bar can be anteromedially or medially oriented (0,1) ( Fig. 1A View Figure 1 ; Supporting Information, Files S3, S4). This character is inapplicable for Styracura .

Dorsal optic region

17. Frontoparietal fontanelle; width in relation to the neurocranium: (0) one-third the width of the neurocranium on its anteriormost portion; (1) a quarter the width of the neurocranium on its anteriormost portion.

In Styracura (outgroup) and Plesiotrygon the anterior portion of the frontoparietal fontanelle is one-third the width of the neurocranium (0) ( Fig. 1 View Figure 1 ). In Paratrygon and Heliotrygon the width of the frontoparietal fontanelle is a quarter the width of the neurocranium (1) ( Fig. 3B View Figure 3 ). In Potamotrygon this character is polymorphic and the width of the frontoparietal fontanelle can be one-third or a quarter the width of the neurocranium (0,1). This character was previously explored by Carvalho and Lovejoy (2011).

18. Frontoparietal fontanelle; ratio between anterior and posterior regions: (0) anterior region 1.5 times wider than its posterior portion; (1) anterior region as wide as its posterior portion.

In Styracura (outgroup) and Plesiotrygon the anterior region of the frontoparietal fontanelle is around 1.5 times wider than its posterior region (0) ( Fig. 1B, C View Figure 1 ). In Heliotrygon and Paratrygon the anterior and posterior portions of the frontoparietal fontanelle have the same width (1) ( Fig. 3 View Figure 3 ). In Potamotrygon the anterior portion of the frontoparietal fontanelle can be 1.5 times wider or may have the same width as its posterior portion (0,1). This character was previously explored by Carvalho and Lovejoy (2011).

19. Frontoparietal fontanelle; midportion constriction: (0) absent; (1) present.

In Styracura (outgroup) and Plesiotrygon the frontoparietal fontanelle has no constriction on its midlength (0) ( Figs 1B, C View Figure 1 ). In some Potamotrygon species the frontoparietal fontanelle has a midlength extension of the neurocranium ceiling forming a constriction, resulting in an hourglass-like outline of the fontanelle, while other species lack such constriction, making this character polymorphic (0,1) ( Fig. 1A View Figure 1 ). This character is coded as inapplicable for Heliotrygon and Paratrygon .

20. Supraorbital crest; development: (0) with one-fifth or less the width of the supraorbital process; (1) as wide as the supraorbital process.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon the lateral extension of the supraorbital crest is about one-fifth or less the width of the supraorbital process (0) ( Figs 2 View Figure 2 , 4B View Figure 4 , 8 View Figure 8 ) (Supporting Information, Files S1–S4, S6). In Heliotrygon the supraorbital crest is clearly developed, being as wide as the supraorbital process (1) ( Figs 3A View Figure 3 , 4A View Figure 4 , 8 View Figure 8 ; Supporting Information, File S5).

21. Postorbital process; distal margin: (0) straight; (1) concave.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon the distal margin of the postorbital process has a straight outline (0) ( Figs 1 View Figure 1 , 3B View Figure 3 ). In Heliotrygon the distal margin of the postorbital process is concave (1) ( Fig. 3A View Figure 3 ).

22. Postorbital process; width of its distal margin in relation to its proximal region: (0) its distal portion a quarter narrower in relation to its proximal region; (1) its distal portion a quarter wider than its proximal region; (2) its distal portion with twice the width of its proximal region [multistate ordered].

In Styracura (outgroup) and Paratrygon the distal margin of the postorbital process is nearly a quarter wider than its proximal region (1) ( Figs 1C View Figure 1 , 3B View Figure 3 ). In Heliotrygon the distal margin of the postorbital process has twice the width of its proximal region (2) ( Fig. 3A View Figure 3 ). In Plesiotrygon the distal portion of the postorbital process is a quarter narrower in relation to its proximal region (0) ( Fig. 1B View Figure 1 ). This character is polymorphic in Potamotrygon and the distal margin of the postorbital process can be both a quarter narrower or a quarter wider than its proximal region (0,1).

23. Postorbital process; anterolateral extension: (0) surpassing anteriorly the lateral tip of the supraorbital process; (1) aligned with the lateral tip of the supraorbital process.

In Styracura (outgroup) and Paratrygon the anterolateral portion of the postorbital process extends anteriorly surpassing the lateral tip of the supraorbital process (0) ( Figs 1C View Figure 1 , 3B View Figure 3 ). In Plesiotrygon and Heliotrygon the anterolateral portion of the postorbital process is horizontally aligned with the lateral tip of the supraorbital process (1) ( Figs 1B View Figure 1 , 3A View Figure 3 ). In Potamotrygon this character is polymorphic. In some species the anterolateral portion of the postorbital process surpasses the lateral tip of the supraorbital process, while in others it is horizontally aligned with it (0,1) ( Fig. 1A View Figure 1 ).

24. Supraorbital process; length: (0) with one-fifth the length of the postorbital process; (1) with one-third the length of the postorbital process; (2) with half the length of the postorbital process [multistate ordered].

In Styracura (outgroup) and Plesiotrygon the supraorbital process corresponds to one-third the length of the postorbital process (1) ( Fig. 1B, C View Figure 1 ). In Paratrygon the supraorbital process is shorter, corresponding to one-fifth the length of the postorbital process (0) ( Fig. 3B View Figure 3 ). In Heliotrygon the lateral projection of the supraorbital process is half the length of its postorbital process (2) ( Fig. 3A View Figure 3 ). In Potamotrygon the three character states can be found (0,1,2) ( Fig. 1A View Figure 1 ).

25. Supraorbital process; orientation: (0) anterolaterally oriented; (1) laterally oriented.

In Styracura (outgroup) the supraorbital process is anterolaterally oriented (0) ( Fig. 1C View Figure 1 ). In Potamotrygon , Plesiotrygon , Paratrygon , and Heliotrygon the supraorbital process is laterally oriented (1) ( Figs 1A, B View Figure 1 , 3 View Figure 3 ).

26. Postorbital process; length: (0) its anterolateral portion failing to reach the longitudinal level of the lateral tip of the preorbital process; (1) its anterolateral portion longitudinally aligned with the lateral tip of the preorbital process; (2) its anterolateral portion extending laterally beyond the lateral tip of the preorbital process [multistate ordered].

In Styracura (outgroup) and Heliotrygon the anterolateral edge of the postorbital process is longitudinally aligned with the lateral tip of the preorbital process (1) ( Figs 1C View Figure 1 , 3A View Figure 3 ). In Potamotrygon the anterolateral edge of the postorbital process may be longitudinally aligned with the lateral tip of the preorbital process or may fail to reach its lateral extension (0,1) ( Fig. 1A View Figure 1 ). In Plesiotrygon and Paratrygon the postorbital process extends laterally beyond the lateral edge of the preorbital process (2) ( Fig. 1B View Figure 1 , 3B View Figure 3 ).

27. Posterior margin of postorbital process; shape: (0) slightly convex; (1) straight.

In Styracura (outgroup) the posterior margin of the postorbital process is slightly convex near its distal portion (0) ( Fig. 1C View Figure 1 ). In Plesiotrygon , Paratrygon , and Heliotrygon the posterior margin of the postorbital process is straight (1) ( Figs 1B View Figure 1 , 3 View Figure 3 ). In Potamotrygon this character is polymorphic and can be slightly convex or, as in most of the species here observed, straight (0,1) ( Fig. 1A View Figure 1 ).

28. Postorbital process; angle in relation to the otic wall: (0) 120°; (1) 90°.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon the angle of the postorbital process in relation to the otic wall is nearly 120° (0) ( Figs 1 View Figure 1 , 3B View Figure 3 ). In Heliotrygon the angle of the postorbital process in relation to the otic wall is close to 90° (1) ( Fig. 3A View Figure 3 ).

Orbital region

29. Foramen for preorbital canal; number: (0) two; (1) one.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Paratrygon there are two foramina for the preorbital canal (0) ( Figs 5 View Figure 5 , 6B View Figure 6 ). In Heliotrygon there is a single foramen for the preorbital canal(1) ( Fig. 6A View Figure 6 ; Supporting Information, File S6).

30. Anterior foramen for preorbital canal; position: (0) anteroventral to the preorbital process; (1) posteroventral to the preorbital process.

In Styracura (outgroup), Potamotrygon , and Paratrygon the anterior foramen for preorbital canal opens ventrally, anterior to the base of the preorbital process (0) ( Figs 5A, C View Figure 5 , 6B View Figure 6 ). In Plesiotrygon the anterior foramen for preorbital canal opens ventrally, posterior to the base of the preorbital process (1) ( Fig 5B View Figure 5 ; Supporting Information, File S3). Heliotrygon has a single branch of the foramen for preorbital canal and is coded as inapplicable.

31. Posterior foramen for preorbital canal; position: (0) posterior to the preorbital process; (1) posteroventral to the preorbital process.

In Styracura (outgroup) the posterior foramen for preorbital canal is posterior to the preorbital process (0) ( Fig. 5C View Figure 5 ). In Potamotrygon , Plesiotrygon , and Paratrygon the posterior foramen for preorbital canal is posteroventral to the preorbital process (1) ( Figs 5A, B View Figure 5 , 6B View Figure 6 ). Heliotrygon lacks this foramen and is coded as inapplicable ( Fig. 6A View Figure 6 ).

32. Anterior cerebral vein foramen; position: (0) anterior, longitudinally aligned with the anterodorsal margin of the optic nerve foramen; (1) anterodorsal to the optic nerve foramen.

In Styracura (outgroup), Heliotrygon , Paratrygon , and Potamotrygon , the anterior cerebral vein foramen is anterior and at level with the dorsal margin of the optic nerve foramen (0) ( Figs 5A, C View Figure 5 , 6 View Figure 6 ). In Plesiotrygon the anterior cerebral vein foramen is anterodorsal in relation to the optic nerve foramen (1) ( Fig. 5B View Figure 5 ).

33. Trochlear nerve foramen; position: (0) dorsal to the midportion of the optic nerve foramen; (1) dorsal to the posterior margin of the optic nerve foramen.

In Styracura (outgroup), Potamotrygon , Plesiotrygon , and Heliotrygon the trochlear nerve foramen is dorsal to midportion of the optic nerve foramen (0) ( Figs 5 View Figure 5 , 6A View Figure 6 ). In Paratrygon the oculomotor nerve foramen is dorsal and closer to the posterior margin of the optic nerve foramen (1) ( Fig. 6B View Figure 6 ).

34. Trochlear nerve foramen; distance from the optic nerve foramen: (0) less than half the height of the optic nerve foramen; (1) half the height of the optic nerve foramen; (2) the same height as the optic nerve foramen [multistate ordered].

In Styracura (outgroup) and Paratrygon the distance of the oculomotor nerve foramen in relation to the optic nerve foramen is less than half the height of the optic nerve foramen (0) ( Figs 5C View Figure 5 , 6B View Figure 6 ). In Heliotrygon the distance of the oculomotor nerve foramen in relation to the optic nerve foramen is half the height of the optic nerve foramen (1) ( Fig. 6A View Figure 6 ). In Potamotrygon the distance of the oculomotor nerve foramen in relation to the optic nerve foramen can be less than or half the height of the optic nerve foramen (0,1) ( Fig. 5A View Figure 5 ). In Plesiotrygon the distance between the oculomotor nerve foramen and the optic nerve foramen is equal to the height of the optic nerve foramen (2) ( Fig. 5B View Figure 5 ).

35. Efferent spiracular artery foramen; position: (0) anteroventral to the eye stalk; (1) ventral to the eye stalk.

In Styracura (outgroup) and Potamotrygon the efferent spiracular artery foramen is positioned anteroventrally to the eye stalk, in the space between it and the optic nerve foramen (0) ( Figs 5A, C View Figure 5 ). In Plesiotrygon , Paratrygon , and Heliotrygon the efferent spiracular artery foramen is located ventral to the eye stalk (1) ( Figs 5B View Figure 5 , 6 View Figure 6 ).

36. Eye stalk; morphology: (0) rounded; (1) oval.

In Styracura (outgroup), Plesiotrygon , and Paratrygon the eye stalk is rounded (0) ( Figs 5B, C View Figure 5 , 6A View Figure 6 ). In Heliotrygon the eye stalk is oval (1) ( Fig. 6A View Figure 6 ). In Potamotrygon both states are present, with species bearing a rounded or oval eye stalk (0,1) ( Fig. 5A View Figure 5 ).

37. Internal carotid artery foramen; position: (0) ventrolateral; (1) ventral.

In Styracura (outgroup), Plesiotrygon , and Heliotrygon the internal carotid foramina are positioned ventrolaterally on the neurocranium at the limit between the ventral face and the lateral face of the neurocranium (0) ( Figs 5 View Figure 5 , 6A View Figure 6 ). In Paratrygon the internal carotid foramina are ventrally positioned, occupying only the basicranium (1) ( Fig. 4B View Figure 4 ). This character is polymorphic for Potamotrygon , with species presenting the internal carotid foramina positioned ventrally or ventrolaterally (0,1) ( Fig. 5A View Figure 5 ).

38. Oculomotor nerve foramen; position: (0) dorsal to the eye stalk; (1) anterodorsal to the eye stalk.

In Styracura (outgroup), Plesiorygon, and Paratrygon the oculomotor nerve foramen is immediately dorsal to the eye stalk (0) ( Figs 5B, C View Figure 5 , 6B View Figure 6 ). In Heliotrygon the oculomotor nerve foramen is anterodorsal in relation to the eye stalk (1) ( Fig. 6A View Figure 6 ). In Potamotrygon this character is polymorphic and the oculomotor nerve foramen can be anterodorsal or dorsal in relation to the eye stalk (0,1) ( Fig. 5A View Figure 5 ).

39. Postorbital wall; anterolateral projection: (0) not anterolaterally extended; (1) extends anterolaterally, partially covering the orbital fissure; (2) extends anterolaterally over the orbital fissure, covering it entirely [multistate ordered].

In Styracura (outgroup), Paratrygon , and Plesiotrygon , the postorbital wall of the neurocranium is not anterolaterally expanded, leaving the orbital fissure uncovered and completely visible laterally (0) ( Figs 5B, C View Figure 5 , 6B View Figure 6 ). In Heliotrygon , the postorbital wall extends anterolaterally covering the orbital fissure completely (2) ( Fig. 6A View Figure 6 ). In Potamotrygon this character is polymorphic. The majority of the species have states 1 or 2, and Potamotrygon schroederi is the only species in which the orbital fissure is completely exposed, and coded as 0 (0,1,2) ( Fig. 5A View Figure 5 ).

40. Orbital fissure; proportion: (0) one-third the diameter of the optic nerve foramen; (1) half the diameter of the optic nerve foramen; (2) the same size of the optic nerve foramen [multistate ordered].

In Styracura (outgroup) the orbital fissure has half the size of the optic nerve foramen (1) ( Fig. 5C View Figure 5 ). In Heliotrygon , Paratrygon , and Plesiotrygon the orbital fissure has the same size of the optic nerve foramen (2) ( Figs 5B View Figure 5 , 6 View Figure 6 ). In Potamotrygon this character is polymorphic and the orbital fissure may have one-third, half, or the same size of the optic nerve foramen (0,1,2) ( Fig. 5A View Figure 5 ).

Otic region

41. Otic region; length: (0) shorter than the proximal portion of the postorbital process; (1) 1.5 times longer than the proximal portion of the postorbital process; (2) 3 times the proximal portion of the postorbital process [multistate ordered].

In Styracura (outgroup) the length of the otic region is shorter than the proximal portion of the postorbital process (0) ( Fig. 1C View Figure 1 ). In Potamotrygon , Plesiotrygon , and Paratrygon the length of the otic region is 1.5 times the proximal portion of the postorbital process (1) ( Figs 1A, B View Figure 1 , 3B View Figure 3 ). In Heliotrygon the length of the optic region has 3 times the proximal portion of the postorbital process (2) ( Fig. 3A View Figure 3 ).

42. Perilymphatic foramina; position in relation to the endolymphatic foramina: (0) posterior; (1) posterolateral; (2) posteromedial.

In Styracura (outgroup), Plesitorygon, and Paratrygon the perilymphatic foramina are posterior and longitudinally aligned with the endolymphatic foramina (0) ( Figs 1B, C View Figure 1 , 3B View Figure 3 ). In Heliotrygon the perilymphatic foramina are posterolateral in relation to the endolymphatic foramina (1) ( Fig. 3A View Figure 3 ). In the majority of Potamotrygon species the perilymphatic foramina are posteromedial in relation to the endolymphatic foramina, and only in some of them the perilymphatic foramina are posterior (0,2) ( Fig. 1A View Figure 1 ).

43. Perilymphatic foramen; size: (0) same diameter as the endolymphatic foramen. (1) 2 times the size of the endolymphatic foramen; (2) 3 times the size of the endolymphatic foramen [multistate ordered].

In Styracura (outgroup) and Paratrygon the perilymphatic foramen has the same diameter as the endolymphatic foramen (0) ( Figs 1C View Figure 1 , 3B View Figure 3 ). In Plesiotrygon the perilymphatic foramen’s diameter is twice the diameter of the endolymphatic foramen (1) ( Fig. 1B View Figure 1 ). In Potamotrygon this character is polymorphic and the perilymphatic foramen may have the same size or be twice the size of the endolymphatic foramen (0,1) ( Fig. 1A View Figure 1 ). In Heliotrygon the perylimphatic foramen’s diameter is three times that of the endolymphatic foramen (2) ( Fig. 3A View Figure 3 ).

44. Lateral commissure; shape: (0) medially septate on its anterior margin; (1) not septate medially.

In Styracura (outgroup) and Plesiotrygon the lateral commissure bears a chondrified median septum and two openings anteriorly (0) ( Fig. 5B, C View Figure 5 ). In Potamotrygon , Paratrygon , and Heliotrygon the lateral commissure is not septate, forming a simple arch over the foramen for the hyomandibular branch of the facial nerve (1) ( Figs 5A View Figure 5 , 6 View Figure 6 ).

45. Hyomandibular facet; shape: (0) its anterior region wider than its posterior region; (1) same width throughout its extension.

In Styracura (outgroup) and Plesiotrygon the hyomandibular facet is wider at its anterior region, resulting in a nearly ovalshaped structure (0) ( Figs 5B, C View Figure 5 ). In Paratrygon and Heliotrygon the hyomandibular facet has the same width anteroposteriorly (1) ( Fig. 6 View Figure 6 ). Both states can be found in species of Potamotrygon (0,1) ( Fig. 5A View Figure 5 ).

46. Hyomandibular facet; ventral margin: (0) convex; (1) straight; (2) concave.

In Styracura (outgroup) and Plesiotrygon the ventral margin of the hyomandibular facet is convex (0) ( Fig. 5B, C View Figure 5 ). In Paratrygon the ventral margin of the hyomandibular facet is straight (1) ( Fig. 6B View Figure 6 ). In Heliotrygon the ventral margin of the hyomandibular facet is medially concave (2) ( Fig. 6A View Figure 6 ). In Potamotrygon this character is polymorphic and the ventral margin of the hyomandibular facet can be convex or straight (0,1) ( Fig. 5A View Figure 5 ).

47. Jugal arch: (0) absent; (1) present.

In Styracura (outgroup), Potamotrygon , Paratrygon , and Heliotrygon the neurocranium lacks a jugal arch (0) ( Figs 5A, C View Figure 5 , 6 View Figure 6 ). In Plesiotrygon the jugal arch is present and posterior to the hyomandibular facet (1) ( Fig. 5B View Figure 5 ). However, it’s important to highlight that it is unclear if the jugal arch may be present in the other genera of Potamotrygoninae as an unchondrified arch considering that this fragile structure may be damaged during dissection.

Occipital region

48. Sphenopterotic ridge; development: (0) as long as the lateral extension of the supraotic ridge; (1) shorter than the lateral extension of the supraotic ridge.

In Styracura (outgroup) and Plesiotrygon the dorsal projection of the sphenopterotic ridge is as long as the lateral extension of the supraotic ridge (0) ( Fig. 9B, C View Figure 9 ). In Heliotrygon and Paratrygon the dorsal projection of the sphenopterotic ridge is shorter than the lateral extension of the supraotic ridge (1) ( Fig. 10 View Figure 10 ). In Potamotrygon the sphenopterotic ridge may be as long as or shorter than the lateral extension of the supraotic ridge (0,1) ( Fig. 9A View Figure 9 ).

49. Vagus nerve foramen; diameter: (0) one-third the width of the foramen magnum; (1) one-fifth the width of the foramen magnum; (2) one-fourteenth the width of the foramen magnum [multistate ordered].

In Styracura (outgroup) the vagus nerve foramen diameter is one-third the width of the foramen magnum (0) ( Fig. 9C View Figure 9 ). In Paratrygon and Plesiotrygon the vagus nerve foramen diameter is one-fifth the width of the foramen magnum (1) ( Figs 9B View Figure 9 , 10B View Figure 10 ). In Potamotrygon the vagus nerve foramen may have a diameter equal to one-third or one-fifth the width of the foramen magnum (0,1) ( Fig. 9A View Figure 9 ). In Heliotrygon the diameter of the vagus nerve foramen is one-fourteenth of the width of the foramen magnum (2) ( Fig. 10A View Figure 10 ).

50. Vagus nerve foramen; dorsal margin: (0) closer to the dorsal margin of the foramen magnum; (1) at the midlength of the foramen magnum.

In Styracura (outgroup) the dorsal margin of the vagus nerve foramen is closer to the dorsal margin of the foramen magnum (0) ( Fig. 9C View Figure 9 ). In Paratrygon , Plesiotrygon , and Heliotrygon the dorsal margin of the vagus nerve foramen does not reach the same level as the dorsal margin of the foramen magnum (1) ( Figs 9B View Figure 9 , 10 View Figure 10 ). In Potamotrygon this character is polymorphic (0,1) ( Fig. 9A View Figure 9 ).

51. Foramen of the lateralis branch of the vagus nerve; position: (0) dorsolateral to the vagus nerve foramen; (1) dorsal to the vagus nerve foramen.

In Styracura (outgroup) and Paratrygon the foramen of the lateralis branch of the vagus nerve is dorsolateral to the vagus nerve foramen (0) ( Figs 9C View Figure 9 , 10B View Figure 10 ). In Heliotrygon and Plesiotrygon the foramen of the lateralis branch of the vagus nerve is immediately dorsal to the vagus nerve foramen (1) ( Figs 9B View Figure 9 , 10A View Figure 10 ). In Potamotrygon most species have the foramen of the lateralis branch of the vagus nerve dorsal to the vagus nerve foramen and few species have it positioned dorsolateral and farther from the vagus nerve foramen (0,1) ( Fig. 9A View Figure 9 ).

52. Articular surface; width: (0) one-third the width of the occipital condyle; (1) half the width of the occipital condyle; (2) two-thirds the width of the occipital condyle [multistate ordered].

In Styracura (outgroup) and Paratrygon the width of the articular surface is half the width of the occipital condyle (1) ( Figs 9C View Figure 9 , 10B View Figure 10 ). In Plesiotrygon the articular surface has one-third the width of the occipital condyle (0) ( Fig. 9B View Figure 9 ). In Potamotrygon the articular surface may have half or one-third the width of the occipital condyle (0,1) ( Fig. 9A View Figure 9 ). In Heliotrygon the articular surface is wider, with two-thirds the width of the occipital condyle (2) ( Fig. 10A View Figure 10 ).

53. Foramen magnum; shape: (0) taller than wider; (1) wider than taller.

In Styracura (outgroup) the opening of the foramen magnum is oval and taller than wider (0) ( Fig. 9C View Figure 9 ). In Plesiotrygon , Heliotrygon , and Paratrygon the foramen magnum is also ovoid but is wider than taller (1) ( Figs 9B View Figure 9 , 10 View Figure 10 ). In Potamotrygon this character is polymorphic, but in most of the species the foramen magnum is wider than taller (0,1) ( Fig. 9A View Figure 9 ).

54. Occipital condyle; ventral margin, shape: (0) straight; (1) concave.

In Styracura (outgroup), Heliotrygon , and Paratrygon the occipital condyles are straight and devoid of a concavity on their ventral margin (0) ( Figs 9C View Figure 9 , 10 View Figure 10 ). In Plesiotrygon the occipital condyles are arched on their mid portion and have a clear concavity on their ventral margin (1) ( Fig. 9B View Figure 9 ). In Potamotrygon the occipital condyles may be straight or arched (0,1) ( Fig. 10A View Figure 10 ).

55. Occipital condyles; orientation: (0) horizontal; (1) medially inclined, with its lateral portion dorsal to its medial portion; (2) laterally inclined, with its medial portion dorsal to its lateral portion.

In Styracura (outgroup) and Potamotrygon the occipital condyles are horizontally oriented (0) ( Fig. 9A, C View Figure 9 ). In Paratrygon the occipital condyles are medially oblique, with their lateral portions dorsal to their medial portions (1) ( Fig. 10B View Figure 10 ). In Heliotrygon and Plesiotrygon the condyles are laterally obliquely, with their medial portions dorsal in relation to their lateral portions (2) ( Fig. 10A View Figure 10 ).

56. Occipital condyle; posterior extension in relation to the basicranium: (0) one-third the length of the hyomandibular facet; (1) two-thirds the length of the hyomandibular facet.

In Styracura (outgroup), Potamotrygon , and Plesiotrygon the posterior extension of the occipital condyle corresponds to one-third the length of hyomandibular facet (0) ( Fig. 5 View Figure 5 ). In Paratrygon and Heliotrygon the posterior extension of the occipital condyle corresponds to two-thirds the length of the hyomandibular facet (1) ( Fig. 6 View Figure 6 ).

57. Occipital condyles; lateral extension: (0) medial to the lateral surface of otic capsules; (1) at level with the lateral surface of otic capsules.

In Styracura (outgroup), Potamotrygon , and Plesiotrygon the lateral extension of the occipital condyles does not reach the lateral surface of the otic capsules (0) ( Figs 2 View Figure 2 , 9 View Figure 9 ). In Heliotrygon and Paratrygon the lateral extension of the occipital condyles is aligned with the lateral surface of the otic capsules (1) ( Figs 4 View Figure 4 , 10 View Figure 10 ).

58. Glossopharyngeal nerve foramen; position on the occipital region: (0) ventrolateral; (1) posteroventral.

In Styracura (outgroup) and Plesiotrygon the glossopharyngeal nerve foramen is situated ventrolaterally on the occipital region, and visible on a posterior oblique view (0) ( Fig. 5 View Figure 5 , 9 View Figure 9 ). In Heliotrygon and Paratrygon the glossopharyngeal nerve foramen is placed posteroventrally on the otic region and is only visible in lateral view, being immediately posterior to the hyomandibular facet (1) ( Fig. 6 View Figure 6 ). In species of Potamotrygon the glossopharyngeal nerve foramen may be seen on posterior oblique view or only laterally (0, 1) ( Fig. 9A View Figure 9 ).

59. Otic region; lateral wall: (0) concave; (1) convex.

In Styracura (outgroup), Plesiotrygon , and Paratrygon the lateral wall of the otic region is concave (0) ( Fig. 9B, C View Figure 9 , 10B View Figure 10 ). In Heliotrygon the lateral wall of the otic region is convex (1) ( Fig. 10A View Figure 10 ). In Potamotrygon the lateral wall of the otic region is concave in most of the species, except for Potamotrygon limai (0, 1) ( Fig. 9A View Figure 9 ).

60. Supraotic ridge; lateral projection: (0) surpassing the lateral limit of the hyomandibular facet; (1) reaching the lateral limit of the hyomandibular facet; (2) failing to reach the lateral limit of the hyomandibular facet [multistate ordered].

In Styracura (outgroup) and Plesiotrygon the lateral extension of the supraotic ridge is similar to the lateral extension of the hyomandibular facet (1) ( Fig. 9B, C View Figure 9 ). In Heliotrygon and Paratrygon the lateral extension of the supraotic ridge is shorter than that of the hyomandibular facet (2) ( Fig. 10 View Figure 10 ). In Potamotrygon this character is polymorphic and the lateral projection of the supraotic ridge may surpass, reach or fail to reach the lateral limit of the hyomandibular facet (0, 1, 2) ( Fig. 9A View Figure 9 ).

Phylogenetic analyses

A matrix with five terminal taxa and 60 characters from the neurocranium yielded a single most parsimonious tree with 79 steps, consistency index (CI) of 0.899, and retention index (RI) of 0.636 ( Figs 11 View Figure 11 and 12 View Figure 12 ). The characters above correspond to those from the matrix (Supporting Information, File S8). The resulting tree supports the monophyly of the subfamily Potamotrygoninae , and indicates that Potamotrygon and Plesiotrygon are successive sister-groups to a clade formed by Heliotrygon and Paratrygon ( Figs 11 View Figure 11 , 12 View Figure 12 ).