Heliotrygon, De, Marcelo R. & Lovejoy, Nathan R., 2011

De, Marcelo R. & Lovejoy, Nathan R., 2011, Morphology and phylogenetic relationships of a remarkable new genus and two new species of Neotropical freshwater stingrays from the Amazon basin (Chondrichthyes: Potamotrygonidae), Zootaxa 2776, pp. 13-48 : 17-19

Genus Heliotrygon , n. gen.

Diagnosis. Heliotrygon is distinguished from all other potamotrygonid genera by the following uniquely derived characters: disc very circular, without a median anterior concavity at snout (disc oval in Paratrygon , Plesiotrygon and Potamotrygon [slightly less so in P. brachyura ], and with a clear anterior median concavity in Paratrygon ); a highly reduced and barely noticeable caudal sting, shorter than spiracular length (sting usually at least as great as interorbital space in Paratrygon , Plesiotrygon and Potamotrygon ), and devoid of acute lateral serrations (welldeveloped, sharp lateral serrations present in caudal stings of all three potamotrygonid genera); pelvic girdle very stout, with puboischiadic bar situated very obliquely in relation to median prepelvic process, resembling an upsidedown “V” (puboischiadic process more perpendicular in Plesiotrygon and Potamotrygon , not nearly as stout in all three remaining genera of the family); lateral prepelvic process reduced to a broad lateral angle of the pelvic girdle (processes well-developed in Potamotrygon and Plesiotrygon , but slightly less developed in Paratrygon ); anteriormost segment of propterygium closely contacts anterior margin of nasal capsule for half or most of its length (anterior propterygial segment contacting nasal capsule only at its posteriormost point in Paratrygon , and contacts lateral, not anterior, margins of nasal capsules in Potamotrygon and Plesiotrygon ); anterior propterygial segments almost contact each other medially (these farther apart in all three remaining potamotrygonid genera and widely distant in Potamotrygon and Plesiotrygon ); metapterygium stout and highly arched, reaching laterally to level of propterygium (less stout and less arched in other potamotrygonid genera, not reaching laterally to level of propterygium); basibranchial copula with a very slender, acute anterior projection extending anteriorly to beyond level of ventral pseudohyoid bar (basibranchial anterior projection stouter in all three other potamotrygonid genera, triangular also in Paratrygon but more broadly so, not extending significantly beyond ventral pseudohyoid in Potamotrygon and Plesiotrygon ).

Additional characters that distinguish Heliotrygon from other potamotrygonids, but not exclusive to Heliotrygon or of uncertain polarity within myliobatiforms, are as follows: nostrils circular, not slit-like (slit-like nostrils in Potamotrygon and Plesiotrygon ); snout greatly elongated, preorbital length about one-third of disc width (snout much shorter, with preorbital length smaller than one-fourth disc width in Potamotrygon and Plesiotrygon ); eyes very reduced in size, much smaller than spiracles (eyes about equal or smaller than spiracles in Plesiotrygon and Potamotrygon ); spiracles without posterior knobs (present in Paratrygon ); nostrils circular (elongated, slit-like in Plesiotrygon and Potamotrygon ); branchial basket relatively short, about equal to internasal distance (branchial basket much greater than internasal space in Plesiotrygon and Potamotrygon ); pelvic fins entirely concealed by posterior pectoral disc in dorsal view (pelvics protrude beyond disc in Plesiotrygon and Potamotrygon ); tail width at base much less than one-half interorbital distance (roughly equal in Plesiotrygon and Potamotrygon ); caudal sting situated relatively close to tail base (positioned farther posteriorly in Potamotrygon and Plesiotrygon ); enlarged, acute spines over posterior disc midline and base of tail lacking (present in all three potamotrygonid genera in varying configurations); tail elongated and very slender, whip-like (tail stout and short in Potamotrygon , stout at base but slender distally in Plesiotrygon ); lack of discrete angular cartilages (angulars present in Plesiotrygon and Potamotrygon ); propterygium very wide and highly arched (clearly less wide and less arched in Potamotrygon and Plesiotrygon ); scapulocoracoid very stout, wide and anteroposteriorly extended (these much more slender in Plesiotrygon and Potamotrygon ); iliac processes reduced, not extending beyond level of ischial processes (projecting beyond level of ischial processes in Potamotrygon and Plesiotrygon ); hyomandibulae relatively short, smaller than length of precerebral and frontoparietal fontenellae (greater in Potamotrygon and Plesiotrygon ); nasal capsules separated by a relatively wide internasal septum (nasal capsules very close together with a very slen- der internasal septum in Potamotrygon and Plesiotrygon ).

Type-species. Heliotrygon gomesi , n. sp., by original designation.

Included species. Currently the new genus is composed of two species, Heliotrygon gomesi , n. sp., and Heliotrygon rosai , n. sp., both described below.

Etymology. From the Greek helios, meaning “sun”, in reference to its distinctively arranged pectoral disc radials that appear to “radiate” outward; and trygon , Greek for stingray. Gender feminine.

Remarks. The generic diagnosis above is based on characters that are common to both new species described below; monophyly of Heliotrygon is well supported. Even though the diagnosis is restricted to comparisons with other potamotrygonid genera, these same characters, either in isolation or in combination, serve to distinguish Heliotrygon from all other myliobatiform genera as well. Urogymnus Müller & Henle 1837 , of uncertain validity (but cf. Last & Compagno, 1999; Compagno, 2005; Last & Stevens, 2009), also has a somewhat rounded disc, but much more oval and not nearly as circular as in Heliotrygon . The snout region in Urogymnus is also significantly shorter than in Heliotrygon . Furthermore, no other myliobatiform genus has a caudal sting as reduced as in Heliotrygon ; although a few myliobatiform species lack caudal stings, these belong to genera in which the caudal stings are present as well developed structures in congeneric species. No nominal genus available for a potamotrygonid can be applied to our new species either. Elipesurus Schomburgk, 1843 , following Rosa (1985a, b) and Carvalho et al. (2003) cannot be unequivocally applied and is best considered doubtful, whereas Disceus Garman, 1877 is a junior synonym of Paratrygon Duméril, 1865 (for further comments, see Rosa 1990; Carvalho et al., 2003). We note that the first authors to report on the distinctiveness of the new form were Ishihara & Taniuchi (1995), based on a juvenile aquarium specimen reportedly from the Orinoco basin (locality unconfirmed).

Most specimens of both new species do not have caudal stings, but these may be damaged upon being captured, and appear not to be as firmly attached as in other potamotrygonids or myliobatiforms. Under magnification, however, a small groove on the dorsal tail base is clearly visible where the caudal sting was originally located, even in smaller specimens. The lack of caudal stings in some specimens therefore does not invalidate this feature (i.e. reduced caudal sting) as a generic-level diagnostic character, one considered (see below) to be derived in Heliotrygon .

Both species of Heliotrygon have greatly reduced eyes and are sometimes captured in relatively deeper channels of usually murky rivers in the Amazon basin. In this regard, species of this genus have a life-style and geographic distribution that is notably similar to Plesiotrygon iwamae , and we expect that specimens of Heliotrygon spp. will be captured where Plesiotrygon iwamae is known to occur. Parallels with other batoid genera are present in the morphology of Heliotrygon as well. Species of Heliotrygon and Gymnura (and also Paratrygon ) are relatively flat and wide batoids that have expanded scapulocoracoids, pectoral basals and pectoral radials, morphological designs that provide enhanced skeletal support for the disc region. However, these similarities are clearly independently derived for Gymnura and Heliotrygon + Paratrygon (see Phylogenetic relationships of Heliotrygon , below).

At present, only two new species are described in Heliotrygon , but more material from a greater distribution is needed in order to shed light on morphological variation within and among species. Individuals of both species present variation in coloration that requires further study.