Autapomorphies of Piaractus mesopotamicus:

1. Branching of laterosensory canals of fourth or fifth infraorbitals (74): (0> 1) present. Paralleled in the Bryconops clade, in nodes 167, 177, 218, 260, and 276, and in Bryconamericus scleroparius and Chalceus macrolepidotus . 2. Canal of lateral line on caudal-fin membrane (92): (0> 1) present. Paralleled in the Characidae .

3. Interdigitations between premaxillae (103): (1> 0) present. Paralleled in the Bryconinae and in Distichodus maculatus .

4. Form of interdigitations between dentaries (112): (0> 1) undulate lamellae. Paralleled in node 183.

5. Anterior extension of interopercle (163): (0> 1) not extending anteriorly beyond terminus of horizontal arm of preopercle. Paralleled in the Heterocharacinae, in nodes 162, 174, and 212, and in Hoplias cf. malabaricus .

6. Number of gill rakers on first hypobranchial and ceratobranchial (195): (1> 0) 16 or more. Paralleled in nodes 177 and 183 and in Astyanax latens, A. cf. rutilus, A. pelegrini Eigenmann, Hoplias cf. malabaricus, Hyphessobrycon socolofi, Moenkhausia dichroura, Parecbasis cyclolepis, and Stichonodon insignis .

7. Teeth on third pharyngobranchial (206): (0> 1) absent. Paralleled in the Gasteropelecidae .

8. Number of branchiostegal rays (213): (0> 1) five. Paralleled in Characidium borellii, Hoplias cf. malabaricus, Rhaphiodon vulpinus, and Thoracocharax stellatus .