Plesiotrygon, Rosa, Castello & Thorson, 1987

De Carvalho, Marcelo R. & Ragno, Maíra P., 2011, An Unusual, Dwarf New Species Of Neotropical Freshwater Stingray, Plesiotrygon Nana Sp. Nov., From The Upper And Mid Amazon Basin: The Second Species Of Plesiotrygon (Chondrichthyes: Potamotrygonidae), Papéis Avulsos de Zoologia 51 (7), pp. 101-138 : 101-138

Quo vadis Plesiotrygon ?

Recent molecular phylogenetic studies have placed Plesiotrygon iwamae nested with species of Potamotrygon in approaches using neighbor joining, likelihood, and Bayesian methods ( Toffoli et al., 2008). The results are highly variable, as Plesiotrygon changes position frequently among the analyses presented ( Toffoli et al., 2008). What is more or less consistent, however, is that when Plesiotrygon is nested among species of Potamotrygon , it usually does so with P. schroederi from the Negro and Orinoco basins. This is similar to conclusions achieved through a more thorough molecular parsimony analysis using POY (Marques et al., in prep; F. Marques, pers. comm.), and not incompatible with the earlier molecular phylogeny of Lovejoy et al. (1998), in which Plesiotrygon iwamae is grouped with Potamotrygon orbignyi , but again from the Río Orinoco. These conclusions, if corroborated, have implications for the acceptance of Plesiotrygon as a valid potamotrygonid genus.

In the present paper, further characters supporting the monophyly of Plesiotrygon , found in both species, were uncovered. These include their anteroposteriorly elongated puboischiadic bar, greatly concave posterior aspect of puboischiadic bar, single and uniquely stout and short angular cartilage, and greater posterior position of caudal stings on dorsal tail (this character still needs scrutiny, as some specimens of Potamotrygon scobina and P. falkneri may rival Plesiotrygon in this regard). Additionally, we can cite the posterior placement of the pelvic fins, the extremely long caudal filament, and the well developed ventral tail fold as unique among potamotrygonids. But substantiating the monophyly of P. nana and P. iwamae together as a clade (of which there can be little doubt) will not affect conclusions as to the validity of the nominal genus Plesiotrygon (i.e. both species can be sister groups and nested within Potamotrygon , corroborating the molecular phylogenetic scenario). For Plesiotrygon to be considered valid, both species must either be basal to all Potamotrygon species, which themselves need to be monophyletic without the inclusion of P. nana and P. iwamae , or Potamotrygon would require subdivision into multiple genera to allow for separate monophyletic subgroups (of which only those species nested with the type-species, Potamotrygon histrix , would remain in Potamotrygon ).

The real question, therefore, concerns not the monophyly of Plesiotrygon per se, but the monophyly of Potamotrygon without Plesiotrygon . Along these lines, we have alluded to one character above, regarding the greater undulation of the infraorbital and supraorbital lateral-line canals in species of Potamotrygon . There are other features of the lateralis system that also figure in support of Potamotrygon monophyly, but these will not be advanced here. Skeletal features of the gill arches also potentially corroborate Potamotrygon as monophyletic, along with other morphological characters. More obvious features unique for Potamotrygon within the family include its much thicker, taller head and disc ( Plesiotrygon , Paratrygon and Heliotrygon have a very low head and very thin, flat disc), and bulging, much larger eyes proportionally, but these characters may be primitive when all myliobatiform genera are taken into account (e.g. Carvalho et al., 2004). In any case, this issue can only be resolved with a species-level phylogeny of all potamotrygonid taxa; in the meantime, we see merit in recognizing Plesiotrygon as valid.