![]() Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Living mammal species can be identified by the presence in females of mammary glands which produce milk. A typical mammalian middle ear: sound makes the tympanum (eardrum) vibrate 3 small bones, the malleus, incus and stapes, transmit the vibrations to the labyrinth (inner ear), which transforms the vibrations into nerve signals. Other implicated genes include the Dlx genes, Prx genes, and Wnt genes. Bapx1, also known as Nkx3.2 (a member of the NK2 class of homeobox genes), is implicated in the change from the jaw bones of non-mammals to the ossicles of mammals. Recent genetic studies are able to relate the development of the ossicles from the embryonic arch to hypothesized evolutionary history. ĭuring embryonic development, the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla, and are served by mandibular and maxillary division of the trigeminal nerve. The transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the fossil record until the 1950s with the elaboration of such fossils as the now-famous Morganucodon. Work on extinct theromorphs by Owen (1845), and continued by Seeley, Broom, and Watson, was pivotal in discovering the intermediate steps to this change. The discovery of the link in homology between the reptilian jaw joint and mammalian malleus and incus is considered an important milestone in the history of comparative anatomy. These ideas were advanced by Ernst Gaupp, and are now known as the Reichert–Gaupp theory. The shortened columella connected to these bones within the middle ear to form a chain of three bones, the ossicles, which serve to effectively transmit air-based vibrations and facilitate more acute hearing.įollowing on the ideas of Étienne Geoffroy Saint-Hilaire (1818), and studies by Johann Friedrich Meckel the Younger (1820), Carl Gustav Carus (1818), Martin Rathke (1825), and Karl Ernst von Baer (1828), the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Karl Bogislaus Reichert (in 1837, before the publication of On the Origin of Species in 1859). ![]() Over the course of the evolution of mammals, one bone from the lower and one from the upper jaw (the articular and quadrate bones) lost their function in the jaw joint and migrated to the middle ear. ![]() In reptiles, the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in mammals. The reptilian quadrate bone, articular bone, and columella evolved into the mammalian incus, malleus, and stapes (anvil, hammer, and stirrup), respectively. The ossicles evolved from skull bones present in most tetrapods, including the reptilian lineage. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution. These bones, or ossicles, are a defining characteristic of all mammals. The evolution of mammalian auditory ossicles was an evolutionary process that resulted in the formation of the bones of the mammalian middle ear.
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