Autapomorphies of Hyphessobrycon elachys:

1. Nasal (33): (0> 1) absent. Paralleled in Coptobrycon bilineatus .

2. Length of laterosensory canal of dentary (79): (0> 1) reduced or absent. Paralleled in node 279 and in Aphyocharax nattereri, Bryconamericus rubropictus, Hyphessobrycon luetkenii, Nantis cf. indefessus, and Thayeria boehlkei.

3. Rows of gill rakers on first ceratobranchial (192): (0> 1) two. Reversal of synapomorphy 3 of node 179. Paralleled in the Iguanodectinae, in node 280, and in Brycon orbignyanus, Bryconaethiops macrops, Carlana eigenmanni, Cheirodon interruptus, Hoplocharax goethei, Odontostilbe microcephala, Parecbasis cyclolepis, Prodontocharax melanotus, and Rhaphiodon vulpinus . Some trees: Paralleled in node 249 and in Attonitus ephimeros .

4. Rows of gill rakers on second ceratobranchial (193): (0> 1) two. Paralleled in nodes 210, 225, 276, and 297 and in Brycon orbignyanus, Hoplocharax goethei, and Rhaphiodon vulpinus .

5. Anterior rays of dorsal fin of adult males (268): (0> 1) elongate and reaching posteriorly to position close to adipose fin. Paralleled in Pseudocorynopoma doriae .

Some trees:

6. Relative size of frontal and parietal fontanels (23): (1> 0) length of frontal fontanel up to 2/3 length of parietal fontanel. (k9-11). Paralleled in Hyphessobrycon socolofi .

7. Maxillary teeth (134): (1> 0) absent. (k9-11). Paralleled in Aulixidens eugeniae, Coptobrycon bilineatus, Iguanodectes geisleri, Parecbasis cyclolepis, and Stichonodon insignis . Some trees: Paralleled in Psellogrammus kennedyi .

8. Denticles on gill rakers (201): (0> 1) absent. (k9-11). Paralleled in the Gymnocharacinae, in nodes 245 and 253, and in Axelrodia lindeae and Pseudochalceus kyburzi . Some trees: Paralleled in Hyphessobrycon herbertaxelrodi .

9. Number of branched pelvic-fin rays (258): (1> 0) six or less. (k9-11). Paralleled in the Aphyocharacinae, in nodes 220, 236, 280, and 302, and in Axelrodia lindeae, Cheirodon interruptus, Cyanocharax alburnus, Hollandichthys multifasciatus, Hoplocharax goethei, and Hyphessobrycon luetkenii . Some trees: Paralleled in Hasemania nana .

10. Dorsal-fin rays articulating with first dorsal pterygiophore (266): (1> 0) two. (k12-14). Reversal of synapomorphy 4 of node 203. Paralleled in Bario steindachneri and Thayeria obliqua. Some trees: Paralleled in Paracheirodon axelrodi . 11. Bony hooks on fin rays (307): (1> 0) absent. (k9-11). Paralleled in the Gymnocharacinae, in nodes 207, 283, 297, and 301, and in Astyanax paris, Bryconamericus mennii, Exodon paradoxus, Inpaichthys kerri, Pseudochalceus kyburzi, and Rhoadsia altipinna . Some trees: Paralleled in Hasemania nana .

12. Contact between dorsal margin of adductor mandibulae and ventral margin of dilator operculi (335): (1> 0) absent. (k12-14). Paralleled in Creagrutus anary, Inpaichthys kerri, Piabucus melanostomus, Prionobrama paraguayensis, Pristella maxillaris, Prodontocharax melanotus, and Pyrrhulina australis .

13. Insertion of pterotic aponeurosis (340): (0> 2) on pterotic or sphenotic, distinctly dorsal to horizontal semicircular canal. (k12-14). Some trees: Paralleled in Paracheirodon axelrodi .