Autapomorphies of Cynopotamus argenteus:

1. Position of sphenotic spine relative to the orbit (12): (0> 1) distinctly posterior to orbital margin. Paralleled in nodes 193 and 299 and in Acestrorhynchus pantaneiro, Attonitus ephimeros, and Gymnocharacinus bergii .

2. Anterior margin of supraoccipital (51): (0> 1) situated anterior to vertical through posterior orbital margin. Paralleled in node 220 and in Metynnis maculatus .

3. Bony lamellae between second and third basibranchials (184): (0> 1) present.

4. Number of anal pterygiophores anterior to first haemal spine (293): (0> 1) four or more. Paralleled in the Iguanodectinae, in node 214, and in Gymnocorymbus ternetzi .