Neoplecostomus jaguari is distinguished from all congeners by having a conspicuous sexual dimorphism: females with 30-38 teeth in the dentary and 30-39 in the premaxilla, and males with 13-18 and 14- 20 teeth respectively (vs. number of teeth in the premaxilla and dentary equivalent in all other species, except for N. langeanii in the premaxilla); females with larger mandibulary width, 18.5- 21.6% HL, males 11.8-15.7% HL (vs. females and males with mandibulary width /head length equivalent in the other species); females with teeth longer and thin, males with teeth shorter and robust (vs. teeth similar in males and females in all other species, except for N. langeanii in the premaxilla). It further differs from all other species from the upper rio Paraná basin by having a lateronasal plate either forming the lateral border of the nostril or posteriorly displaced (vs. lateronasal plate absent). In addition, it can be diagnosed from other congeners by having four to six plates between the frontal and prefrontal, here called posteronasal plates (vs. one to three in all other species) (Ref. 97853).

Description: Dorsal-fin rays ii,7; anal-fin rays i,5; pectoral-fin rays i,6; pelvic-fin rays i,5 (Ref. 97853).