4x the pectoral fin phylogenetically splits in two

Sometimes the pectoral fin
phylogenetically splits in two, creating a new structure anterior to the main body of the pectoral fin.

Figure 2. Prionotus, the sea robin, has more of a barracuda-like face. Uniquely, medial spines from the pectoral fin evolve to act like fingers for walking on the sea floor.

Figure 2. Prionotus, the sea robin, has more of a barracuda-like face. Uniquely, medial spines from the pectoral fin evolve to act like fingers for walking on the sea floor.

Sea robins
(genus: Prionotus, Fig. 1) have finger-like appendages anterior to the pectoral fins.

Figure 6. Three views of the skeleton of Manta, colors added. Green represents the maxilla. Note the terminal mouth, distinct from other rays, skates and guitarfish. The pectoral fins do not reach the orbit. The cephalic fins are highly modified maxillae, still gathering food. Note the attachment to the quadrate. The premaxilla extends across the mouth.

Figure 6. Three views of the skeleton of Manta, colors added. The cephalic fins are highly modified anterior portions of the pectoral fins.

Manta ray
(genus: Manta, Fig. 2) extend a subset of the pectoral fin forward to create flexible external mouth scoops.

Figure 3. The threadfin, Polydactylus, also splits the pectoral fin to form threadlike feelers.

Figure 3. The threadfin, Polydactylus, also splits the pectoral fin to form threadlike feelers.

Threadfins
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 1. Spotted eagle ray skull shows the anterior portions of the pectoral fins jointed medially to create a digging snout.

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.

Hexapod tetrapods
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).

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

A Google search reveals ‘split pectoral fin’ is also a malady
that affects certain Koi goldfish. Details here.


 

 

 

 

 

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