If a vertebrate had a paired fin (specifically a sarcopterygian-like fin) in the correct place, would it be plausible for the fin to adapt into a structure analogous to the middle and outer ears of tetrapods? The vertebrate has no hearing parts besides the inner ear, and would be transitioning to land as this structure evolves. It also has many fins along its body
$\begingroup$ Why do you want to have a fin evolve into an ear? I think it's implausible that it would happen, but there may be another way to get to your end goal. $\endgroup$– Rob WattsAug 13, 2021 at 15:36
$\begingroup$ Unrelated, but it is very common for mermaid and fish-like humanoids to have "earfins". $\endgroup$– MermakerAug 13, 2021 at 15:37
$\begingroup$ Do you mean lobe fin? Seems unlikely with an inner ear already in place. It would be like evolving true eyes on your shoulders when you already have eye spots on your head. $\endgroup$– rekAug 13, 2021 at 15:49
1$\begingroup$ I understand the mechanism you're proposing, I just don't think it's more likely than the jaw-to-ear model tetrapods followed. $\endgroup$– rekAug 13, 2021 at 17:26
1$\begingroup$ its more likely that it would evolve an ear that looks like a fin, keep in mind external ears are just made of skin and cartilage and did not evolve from anything else. $\endgroup$– JohnAug 13, 2021 at 19:18
Likely not, at least with hearing as we think of it.
Darwin's mechanism for evolution and natural selection requires gradual changes, where each one is slightly better than the last. Any change that does not produce immediate results should be filtered out by Natural Selection.
A fin would start with no apparatus analogous to an ear, and I can't think of any sequence of small changes (each improving on the previous body design) that would give a fin an entirely new function.
That said, we do have limited ability to hear through our bones. I can't find it upon googling, but my band director once showed me a silent tuning fork, where it would vibrate without making noise, but if you placed the end on your elbow and touched your eardrum, you could hear the tone. Another example would be placing the vibrating tines of a fork between your teeth and "hearing" the sound transmitted through your bones.
You could probably develop some similar structure to this where the creature "hears" through vibrations conducted up through its fin (especially if that fin is used to walk on later in the evolutionary sequence). This would be much easier to create via a Darwinian evolutionary path.
Now that I think about it, that is a very similar function to how snakes can "hear" vibrations in the ground. You might look into that as well for inspiration.
I hope that helps you!
$\begingroup$ But couldn't this bone conduction be used to transmit vibrations from an eardrum, like the ossicles in real tetrapods? $\endgroup$ Aug 13, 2021 at 16:30
$\begingroup$ @IchthysKing the reason we have said ossicles to begin with, as far as we know, is because our ancestors would use their jaw bones to hear vibrations in the ground. Over time, some of the bones in the jaws came to become part of the ear, making hearing a lot better. You'd probably need a process in which, for whatever reason, this creatures started to use its fins to hear instead. I'd say it might be easier to explain actual ears becoming similar to fish fins through sexual selection than it'd be to have limbs develop into a hearing apparatus (unless you want to take a look at grasshoppers). $\endgroup$ Aug 13, 2021 at 17:38