Re: Ears

["Matthew Bonnan" <mbonnan@hotmail.com>]

Jerzy Dyczkowski asks:
"How do
we know what shape were "visible" or "outside" ears of extinct animals and
especially, that mammals had them and dinosaurs had not?"

Good question and fascinating story.  The evolution of a tympanic ear (i.e., 
an ear that uses a thin, vibrating membrane to pick up high-frequency air 
borne sounds) has occurred three times -- once in frogs, once in sauropsids 
("reptiles" and birds), and once in mammals.  In sauropsids, which include 
the dinosaurs, the typmanic membrane is located in a recessed area on the 
skull behind (posterior to) the jaws (usually referred to as the otic 
region).  We can also describe the location of the tympanic membrane in 
sauropsids as "postquadrate" because it lies just posterior to the quadrate 
bone.  (The bottom portion of the quadrate bone articulates with the lower 
jaw FYI.)  Usually, the tympanic membrane of sauropsids lies close to the 
surface of the ear canal, and the quadrate bone usually partially encloses 
the tympanic membrane and a portion of the ear canal.  Because dinosaurs 
have a quadrate bone and an ear canal in a similar region as found in other 
sauropsids, this suggests dinosaurs had a similar sort of ear.

In mammals, numerous changes in the jaws occurred that affected the ear as 
well.  I will not detail them here, but suffice it to say that the tympanic 
membrane of mammals is located further inside the ear canal, mammals are the 
only vertebrates with three inner ear ossicles (instead of the single 
ossicle (stapes) in sauropsids -- yes, there are Webberian ossicles in some 
fishes but these are modified from verts and do not actually sit in the 
inner ear), and, because mammals expanded hyoid muscles onto their faces 
which became the facial muscles, they developed soft tissue structures on 
their skulls (i.e., external ears or pinnae) that could be directed these 
muscles.

Because the living relatives of dinosaurs (crocodylians and birds) do not 
have a recessed inner eardrum (typmanic membrane), three inner ear ossicles, 
or facial muscles with external ear structures, it is considered doubtful 
that dinosaurs would be much different.  This does not mean dinosaurs or 
other living archosaurs hear "worse" than mammals, they just do it a 
different way.  Many predatory birds (such as owls) have exceptional hearing 
abilities.

The evolution of the mammalian jaw and inner ear is a fascinating story -- 
many books and articles have been published on it.  The recent textbook by 
Liem, Bemis, Walker, and Grande (Functional Anatomy of the Vertebrates, 3rd 
Ed., 2001) would be a great place to start to get more info.

Hope this helps,

Matt

Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155
mbonnan@hotmail.com
MF-Bonnan@wiu.edu

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