I had been thinking bout atavism when I saw a picture of a dolphin. I had seen it before, but suddenly the dorsal fin startled me.

How did this dorsal fin evolve? Is it some atavism from eons ago, a revived feature, or a new development altogether? (Strong evidence of the former would be if the genes ruling dorsal fins in fish and dolphins would be similar.)

I've seen this How and why did mammals go back to the oceans?, but it doesn't ask nor explain about the dorsal fin.


Short answer
I reckon that the dorsal fin of sea-faring mammals are a new development altogether.

The presence of a dorsal fin is one of the defining characteristics of fishes like sharks. Mammals that returned to the sea generally lack a dorsal fin.

However, killer whales and dolphins, while being mammals, indeed do have dorsal fins. In contrast to the dorsal fin in fish, these structures do not have bones (Fig. 1). Instead, the dorsal fins of mammals are made up of dense tissue, somewhat like a ridge of thick, folded skin. Hence, it is not a 'revived structure' that was 'activated' from a latent state in mammals.

Hence, given that mammals returned to sea later in evolution and lacked a dorsal fin on land, I would deduce from these data that dorsal fins are an example of convergent evolution where analogous structures with a similar function (like in fishes, the dorsal fin acts as a stabilizer or a rudder in mammals (source: The National Marine Life Center) develop from unrelated bodily structures. Hence the genes involved may, and likely are, different.

killer whaledorsal fin
Fig. 1. Dorsal fins of a killer whale (left) and in a fish (a redhorse on the right). sources: Sea World and Mosxostoma