Vertebrates in general have conserved body plans. New appendages, whether fins or limbs, evolve rarely. ~ American evolutionary biologist Michael Coates
An adipose fin is a small, soft, fleshy fin found on the back of a fish, behind the dorsal fin and just in front of the caudal fin. The adipose fin is a sensory organ, though its exact employment is not known. Over 6,000 species of fish have adipose fins, including salmon, trout, catfishes, and characids.
The adipose fin did not descend from a common ancestor. Instead, it repeatedly arose. Moreover, the adipose fin convergently evolved with different structures, indicating selfsame functionality obtained via various physical means.