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 mammalian middle ear contains three tiny bones known as the ossicles: malleus, incus, and stapes. In mammals, the quadrate and articular bones have evolved into the incus and malleus bones in the middle ear. 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). 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. Courtesy of the Peabody Museum of Natural History Division of Vertebrate Zoology Yale University.Įarly 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). Upper and lower portions of a python skull, displaying multiple bony components of the upper and lower jaws. 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. 8 Fossil evidence for mammal-like jaws and earsįollowing 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).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. 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 purpose 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 event that resulted in the formation of the bones of the mammalian middle ear.

Auditory ossicles function