The Evolution and Development of Middle Ears in Land Vertebrates
New fossil evidence and supporting data from embryological studies have helped to consolidate interpretations of the structures that assemble the middle ear apparatus of different lineages of land vertebrates. The middle ears of modern land vertebrate groups evolved independently of one another during the Triassic era of the Mesozoic. Thus, two dogmata have fallen: (1) The tympanic middle ear is not a monophyletic development, i.e., the eardrum-bearing middle ears of modern land vertebrates are not descended from one common ancestral type. (2) The mammalian middle ear did not emerge by the addition of two more ossicles to an existing, one-ossicle middle ear because mammalian ancestors, like all other vertebrate lineages of late Permian–early Triassic times, lacked a tympanic middle ear. Whereas most lineages evolved a single-ossicle system, mammals developed a three-ossicle system. This difference is due to mammals simultaneously evolving a secondary jaw joint, a process that freed up small bones at the rear of the jaw that became incorporated into the middle ear. Functionally, there are only small differences between the resulting two types of middle ear. Because all middle ear systems were constructed from preexisting components of the skull that subserved other functions, there are striking similarities in the embryological origins and the developmental pathways of all land vertebrate middle ears and homologous, ancestral jaw components.
KeywordsEmbryology of middle ear Middle ear development Middle ear evolution Three-ossicle Tympanic
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