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.