Max Bernt: There is a key in Tiago Carvalho’s dissertation, but I don’t have it handy. However, the working concept I have of the 5 Gymnorhamphichtys spp is as follows:

G. hypostomus – Long snout and dark, thick bands along the back/sides.
G. rosamariae – Moderate snout length, usually no pigment on the body, pale yellow to transparent. Amazon Basin.
G. bogardusi – Very similar to G. rosamariae with no pigment on the back or sides, but with a dark tail. Limited to the Orinoco Basin.
G. britskii – Shorter snout with dark, thick bands along the back/sides. Limited to the La Plata Basin.
G. rondoni – Shorter snout. Color variable, sometimes transparent, bands normally not extending onto the fishes’ sides. Widespread.

Encyclopedia Aquatica