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What Makes a Mammal? Special Earbones

Did you know there's a jawbone in your ear?

That's right. The small bones that enable mammals to hear better than fish, birds, or any other animals were located in the jawbones of the animals that came before us. How did they end up in our ears?

Varanosaurus (early Permian synapsid, 270 MYA)

Mammals evolved from reptile-like animals that had more than one bone in their lower jaw (dentary bone, quadrate bone, articular bone and angular bone)...

Thinaxodon (early Triassic cynodont, 210 MYA)

...Very slowly, over tens of millions of years, two of these bones moved to the middle...

Opossum (modern mammal)

...In the ear, the bones became part of sensitive sound-ampligying system found only in mammals. The modern mammal middle ear is comprised of the malleus (hammer), incus (anvil), stapes (stirrup), tympanum (ear drum) and tympanic bone.

© Smithsonian Insitution