Seems it would have enhanced their hearing and hence their survival to have such big ears. Why did it only show up with mammals?
Possibly divergent evolution may be an answer. External ears are not the only means to perceive vibrations or sound waves - though they seem to be efficient enough at detecting some discrete bands of frequencies among the electro-magnetic spectrum. I believe that external ears in mammals are more of a means to aid in localizing (directionally) the originating source of the perceived sound as opposed to just the sensing of all sounds. For example Elephants have large ears, but their sheer mass enables them to also very efficiently sense vibrations (especially the very low frequency sub-sonic) through their feet. If you have ever been near an efficient sub-woofer, a passing train, large vehicle or even the movement of a large animal - you have probably experienced or sensed this phenomena.
I would think it is hard to say why in any group some feature did NOT evolve. Presumably there was not sufficient selective pressure for it, for whatever reasons. I did however come across the passage below, which may shed some light on the issue from the reverse direction, i.e. why mammals DID evolve an outer ear. Seems to do with small, nocturnal creatures needing to hear higher frequencies in stereo for direction-finding:- "The earliest mammals, coexisting with dinosaurs, probably occupied and exploited nocturnal niches. This places great emphasis on good olfaction and the ability to identify the source of a sound accurately with no visual cues. Animals with small heads also need to detect sounds at higher frequencies for only then will the acoustic shadow of the head produce a detectable interaural difference in sound intensity. Directional clues can also be enhanced by skin folds around the ear canal and these must have become the outer ear (pinna)." The whole article is here: http://www.answers.com/topic/evolution-of-the-ear
They seemed to survive just fine without them, so from an evolutionary viewpoint it is not clear what advantage external ears may have given them? They could survive without them well enough.... As to why mammals evolved them... Possibly they were beneficial when mammals were mostly small, and preyed upon, in that they may provide better locational information, to be able to avoid prey etc. just a guess, though. Please Register or Log in to view the hidden image!
Enhanced directional hearing wouldn't have been an advantage for dinosaur predators and prey? That's ridiculous. Ofcourse it would. The history of outer ear evolution in mammals completely refutes that.
good question. we don't know for certain, afaik. i suspect it has to do with the fact that mammals have reptilian jaw-bones that translocated to the ear, greatly enhancing the ability to hear. with that, directional location became possible, and ears developed to aid that. but this is just a guess, and I've seen no studies on it. again, good question.
That may be a key factor right there. The acoustical shadow was too small to provide sufficient differentiation of direction and amplification of high frequency rustlings of foilage. Big ears in mice provided the perfect answer. So the gene for this trait got passed on even as mammals grew larger. Apparently there is more required of a trait than that it just be advantageous. There has to be a genome to support it as well. Dinosaur ancestors were never small enough for this to have been a problem for them. Just guessing..
No one in this Thread has stated that dinosaurs/reptiles did NOT have "Enhanced directional hearing" because of NO outer ears! No one, except yourself, Magical Realist, are presuming or assuming the aural sensing abilities of dinosaurs/reptiles! So far most Posters are trying to point out what it would appear or seem and could therefore be the reason that THE SMALLER MAMMALS evolved outer ears.
Whatever method of hearing the dinosaurs had was sufficient, and they evolved hearing that was sufficient... that was fit for purpose. If you're asking why they didn't evolve something better it was because there was no evolutionary need to. Evolution doesn't result in the best... just something that is fit for purpose. So the simple answer to your question is that there was no evolutionary need to: their own method was fit for purpose.
Captain Obvious here. The required mutation didn't happen. I mean, it's not like Mother Nature sits around and says, "How can I make this better?" It's more like, "What happens if I do this?"
I think sense of smell and sight were sufficient for hunting and survival among the dinosaurs, because these are brain core wired and therefore would have a more direct connection to the arousal centers within the brain stem. Hearing appears to be wired in later and added as an addendum to these core senses. Please Register or Log in to view the hidden image! Please Register or Log in to view the hidden image!
We have evidence, from bones and birds, that dinosaurs did not hear all that well compared with mammals. The advantage of an external accumulator is greatly amplified by the sophistication and extraordinary frequency response of the three-bone inner mammalian ear, and more easily covers the cost - in most other animals, the liability and cost of an external structure like that is a far bigger comparative factor. The cost benefit comparison is light in benefits, compared with the hyper-sensitive mammalian ear. On the other hand, even though as mostly daylight active animals (apparently, from the eyesockets etc) and without the advanced ear structure, they would have in general derived less benefit from ears that stick out, we don't actually know whether all dinos lacked them. Some birds have them, after all - certain owls, etc - and some birds have collective feather surfaces on their faces that are a kind of whole-head external "ear", these features are not always easily visible in skeletal remains and might not be visible in fossils (compare the skull with the "radar dish" face of a barn owl, for example: http://en.wikipedia.org/wiki/Barn_Owl )
First, how do we know that none did? Second, for many mammals, especially the big ones, it is a heat shedding mechanism rather that just an auditory enhancer. Dinosaurs didn't have that need.
Some dinosaurs probably had serious heat shedding problems, and many of the odd features of their skeletons have been explained as heat shedding features. But the point is taken - we don't know much about the shapes of the ears on many dinosaurs. The absence of muscle attachment points on the skulls, the lack of ears in any of the fine-grained preservations (that show skin texture, etc), tend to indicate what we see now in most birds - but some birds have external ears.
