The evolution of mammalian auditory ossicles is a well-documented and important evolutionary event, demonstrating an excellent example of exaptation, the re-purposing of existing structures during evolution.

In reptiles, the eardrum is connected to the inner ear via a single bone, while the upper and lower jaws contain several bones not found in mammals. Over the course of the evolution of mammals, one lower and one upper jaw bone (the articular and quadrate) lost their purpose in the jaw joint and were put to new use in the middle ear, connecting to the existing stapes bone and forming a chain of three bones (collectively called the ossicles) which transmit sounds more efficiently. In mammals, these three bones are known as the malleus, incus, and stapes (hammer, anvil, and stirrup respectively).

They are homologous to the reptilian articular and quadrate, supporting the transition from reptiles to mammals. Living mammals can be identified by the presence in females of mammary glands, but when classifying fossils, mammary glands and other soft-tissue features are not visible. Paleontologists therefore use features shared by all living mammals but is not present in therapsids ("mammal-like reptiles"). Mammals have a different jaw joint, composed only of 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 become the incus and malleus bones in the middle ear.

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