Yes, I know owls have feathery "ear tufts", but these are less suited for hearing and more for display.

And I find it hard to believe that animals like dinosaurs or other cursorial archosaurs would not rely on sound as much as a mammal would. It's just too useful of a sense.

One potential clue is the fact that most non-mammal tetrapods appear to use electrical tuning to discern different sounds, whereas mammals use the hair cells of their longer cochlea to mechanically discriminate sound. I feel like this is related but not completely connected to why a fleshy outer ear structure that can collect and focus in on the direction of a sound.

Why did the pinnae evolve in mammals and why are they the only ones with such a structure?

  • $\begingroup$ If an organism does not have outer ear, it does not mean it does not rely on sounds. Birds heavily rely on sounds for so many aspect of their life. Owl is a great example, $\endgroup$ – Remi.b Jun 28 at 19:55
  • $\begingroup$ Which isn't the question. The whole idea of the pinnae is that they aid in the collection and focusing of sound, yet other animals seem to be able to do this relatively well so there must be some reason they don't have pinnae. $\endgroup$ – Thesaurus Rex Jun 28 at 23:15
  • $\begingroup$ I understood (maybe misunderstood) the sentence I find it hard to believe that animals like dinosaurs or other cursorial archosaurs would not rely on sound as much as a mammal would as assuming that absence of pinnea means poor use of the sense of hearing. $\endgroup$ – Remi.b Jun 29 at 1:37
  • $\begingroup$ Indeed. It was more of a rhetorical doubt. $\endgroup$ – Thesaurus Rex Jul 4 at 22:21
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    $\begingroup$ @David The question is literally "why did this evolve in only one group of animals", and I just got such an objective answer. It's always puzzled me why StackExchange users try to belittle the OP into thinking their question is stupid/too broad/impossible to answer while it clearly isn't that insurmountable. $\endgroup$ – Thesaurus Rex Jul 22 at 8:38

Short answer Mammalian hearing is unique and amazing.

The mammalian ear is unique and highly sensitive with a built in amplification system that means even minute changes in sound can be detected. The bony amplification is also the reason the mammalian (or at least therian) cochlea is huge, every incremental increase yielded markedly more information, it has to get quite large before you hit the point of diminshing returns. This means mammals can get a lot out of even small investments into improving hearing. The vast majority of other groups just would not get any benefit from an external ear. their ears are not sensitive enough to gain anything the faint echoes external ears collect. The one group that would get some benefit (owls), has something functionally similar to an external ear.
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The mammalian ear evolved from the repurposing of vestigial jaw bones no other group had (or more correctly they did not become vestigial in other groups). Mammals are the only group with a single bone in the lower jaw. it is a fascinating story and well worth a look. It kinda makes up for how awful mammalian eyes are.


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