not a species from Argentina:
Gymnorhamphichthys hypostomus Ellis, 1912

Carvalho et al. assigned the Argentinean records of Gymnorhamphichthys hypostomus to the new described species G. britskii. Besides mere listings in recent papers (López et al. 2003, Liotta 2005) they also assign the first for Argentina (Pozzi 1945) to the new species.
Therefore Gymnorhamphichthys hypostomus should be considered not to be distributed in Argentina until new findings would confirm its presence in this country.

published in:
Carvalho, T.P., C.S. Ramos & J.S. Albert (2011):
A New Species of Gymnorhamphichthys (Gymnotiformes: Rhamphichthyidae) from the Paraná–Paraguay basin.
Copeia 2011 (3): 400-406

abstract (from publication):
Gymnorhamphichthys britskii, new species, is described from the Paraná-Paraguay system. The new species is distinguished from all other congeners except G. hypostomus in having 5–16 (usually 9–13) large dark saddle-shaped pigment bars over the dorsal midline, extending between the nape and a vertical with the end of the anal fin, and sometimes reaching to or slightly below the lateral line. The new species differs from G. hypostomus in having 141–172 vs. 178–211 anal-fin rays; a shorter snout, preorbital distance 48.1–60.3 of head length (mean 54.6 ) vs. 58.3–68.6 (mean 63.9 ); a larger eye, diameter 5.7–9.0 of head length (mean 7.0 ) vs. 3.3–5.9 (mean 4.3 ); a longer distance from tip of snout to posterior nares, 17.1–22.0 of head length (mean 19.2 ) vs. 11.2–17.1 (mean 13.8); 7 vs. 9–13 displaced hemal spines; and 64–69 vs. 71–75 vertebrae to end of anal fin.