Evolution of the mammalian ear
By Lindsey Hughes
Significance
• Typically, animals are unable to generate
entirely new anatomical structures in evolution
• Mammals = 3 ossicles (malleus, incus, stapes)
• Nonmammals = 1 ossicle (stapes)
• The fossil record suggests the answer to this
issue lies in the reptilian masticatory system
Human Middle Ear
There are three bones in the human ear that aid in
hearing: the malleus, the incus, and the
stapes.
The pinna is the external part of the ear
Nonmammal Middle Ear and Masticatory System
The middle ear of nonmammals consists of a
single bone, the hyomandibula.
The articular and the quadrate are two bones
in the jaw of the shark and aid in mastication.
On the Visceralbogen of vertebrates in general
and their metamorphosis in the Säugethieren and
birds by Karl Reichert (1837)
Method: Followed gill arches of various species to
understand where they ended up in the different
skulls
Results: Two bones in the mammalian ear
correspond to pieces of the reptilian jaw
Conclusions: The same gill arch that formed part
of the jaw in reptiles also formed the ear bones
in mammals. They are the same thing.
Critique of recent work on the morphology of the
vertebrate skull, especially in relation to the
origin of mammals by W.H. Gregory (1913)
• Method: Analysis and comparison of previously
studied embryos and South African fossils
• Results: The most reptilian of the mammal-like
reptiles had a single bone in the middle ear and
many bones that make up the jaw. The bones at the
back of the reptilian jaw decreased over time until
they ultimately lie in the mammalian middle ear
• Conclusion: The malleus and the incus did evolve
from the jaw bones of reptiles
Evolution of the Mammalian Masticatory
Apparatus by A. W. Crompton (1978)
• Prediction: The small bones of the reptilian jaw
joint were eventually incorporated into the
middle ear as the malleus and the incus.
• Method: Comparison of tooth-replacement
patterns, jaw muscles, jaw bones, and middle ear
bones in mammals and reptiles.
Evolution of the Mammalian Masticatory
Apparatus by A. W. Crompton (1978)
• Conclusions: The fossil record makes it possible
to understand how selective forces improved
both the masticatory and auditory systems,
allowing them to evolve from the unit in the
reptilian skull into two separate morphological
parts in the mammalian skull.
Your Inner Fish by Neil Shubin (2008)
• Method: Trace the hyomandibula from sharks to
amphibians and note how it changes
• Results: The hyomandibula decreases in size
over time until it no longer supports the upper
jaw and, instead, aids in hearing
• Conclusions: The hyomandibula in the jaws of
reptiles and amphibians is homologous to the
stapes in the middle ear of mammals
Possible directions for future research
• Unsolved mystery: how did the tympanic
membrane form in the lower jaw domain in the
ancient ancestor of mammals?
• Addressing the issue: Use comparative studies
conducted at a molecular genetic level to focus
on evolutionary changes in pharyngeal arch
development.
Procedure for addressing the issue
1. Identify comparable developmental stages in mammals and
nonmammals.
2. Describe the critical events that lead to mammalian tympanic
membrane development in the lower jaw in both
developmental and molecular terms.
3. Determine which developing tissues shift along the
anteroposterior, dorsoventral, or mediolateral axes in
mammals and nonmammals.
4. Consider developmental signals involved in skeletal shape to
explain the shift that occurred in mammalian evolution.
References
Crompton, A. W., and P. Parker. 1978. Evolution of the mammalian masticatory apparatus.
American
Scientist 66:193 – 201.
Lankester, R. , 1909, Amia calva head, Treatise on Zoology Vol 9,
http://commons.wikimedia.org/wiki/File:AmiaCalvaHeadLankester. pg (November 8,
2010)
NaboHome, 2009, Comparative Osteology,
http://www.nabohome.org/products/manuals/fishbone/fish/fish.html (November 9,
2010)
Shubin, N., 2009, Your Inner Fish, Random House, Inc., New York, 158 – 164 p.
Takechi, M. and Kuratani, S. (2010), History of studies on mammalian middle ear evolution: A
comparative morphological and developmental biology perspective. Journal of
Experimental Zoology Part B: Molecular and Developmental Evolution, 314B: 417–433.