Sometimes the pectoral fin
phylogenetically splits in two, creating a new structure anterior to the main body of the pectoral fin.
(genus: Prionotus, Fig. 1) have finger-like appendages anterior to the pectoral fins.
(genus: Manta, Fig. 2) extend a subset of the pectoral fin forward to create flexible external mouth scoops.
like Polydactylus (Fig. 3), also split the pectoral fin to form sea robin like feelers. Once nested with flatfish, the large reptile tree (LRT, 1625 taxa) nested Polydactylus with the sea robin, Prionotus (above).
Figure 4. Spotted eagle ray (Aetobatus) skull shows the anterior portions of the pectoral fins joined medially to create a digging snout.Eagle rays
like Aetobatus (Fig. 4), splits then merges the anterior portion of the pectoral fins, creating a snout used for digging in soft sand. Outside it looks like a nose. Inside the pectoral fin structure is apparent.
This is the closest tetrapods have come to becoming six-legged, not quite like those seen in the sci-fi movie, Avatar (2009, which considered six appendages inappropriate for the tall blue humanoids, a morphology first imagined by Edgar Rice Burroughs for Barsoom (Mars).
A Google search reveals ‘split pectoral fin’ is also a malady
that affects certain Koi goldfish. Details here.