page_title: Evolution of mammalian auditory ossicles
section_title: Defining characteristic of mammals
hierarchical_section_title: Evolution of mammalian auditory ossicles / Defining characteristic of mammals
caption_attribution_description: English: Skull, including lower jaw, of a python (YPM HERR 010752): photo by Taylor, N. Courtesy of the Peabody Museum of Natural History, Division of Vertebrate Zoology, Yale University; peabody.yale.edu.
context_page_description: The evolution of mammalian auditory ossicles was an evolutionary event that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.
  The ossicles evolved from skull bones present in most tetrapods, including the reptilian lineage. The reptilian quadrate bone, articular bone, and columella evolved into the mammalian incus, malleus, and stapes, respectively.
  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. Over the course of the evolution of mammals, one bone from the lower and one from the upper jaw lost their purpose in the jaw joint and migrated to the middle ear. 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.
context_section_description: Living mammal species can be identified by the presence in females of mammary glands which produce milk. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Paleontologists therefore use the ossicles as distinguishing bony features shared by all living mammals (including monotremes), but is not present in any of the early Triassic therapsids ("mammal-like reptiles"). 
  Early amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including lizards, crocodilians, dinosaurs (and their descendants the birds) and therapsids; so the only ossicle in their middle ears is the stapes. The mammalian jaw joint is composed of different skull bones, including the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). In mammals, the quadrate and articular bones have evolved into the incus and malleus bones in the middle ear.
  The mammalian middle ear contains three tiny bones known as the ossicles: malleus, incus, and stapes. The ossicles are a complex system of levers whose functions include: reducing the amplitude of the vibrations; increasing the mechanical force of vibrations; and thus improving the efficient transmission of sound energy from the eardrum to the inner ear structures. The ossicles act as the mechanical analog of an electrical transformer, matching the mechanical impedance of vibrations in air to vibrations in the liquid of the cochlea. The net effect of this impedance matching is to greatly increase the overall sensitivity and upper frequency limits of mammalian hearing, as compared to reptilian hearing. The details of these structures and their effects vary noticeably between different mammal species, even when the species are as closely related as humans and chimpanzees.
caption: Upper and lower portions of a python skull, displaying multiple bony components of the upper and lower jaws. Courtesy of the Peabody Museum of Natural History; Division of Vertebrate Zoology; Yale University.
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