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.