One of the great transformations in vertebrate evolution is that of the jaws and ears. Several key differences between the skulls of Mammals and other vetebrates hinges on the Quadrate Bone. The bone forms the cranial side of the jaw joint in fishes, amphibians and reptiles, with the mandibular side made of the Articular Bone (or Malleus in mammals). As part of the jaw joint, the Quadrate can be large and sutured to the skull like in Alligators and Tyrannosaurus, or stout, strutlike and wiggly in many birds, lizards and snakes. During mammal evolution the jaw joint changed and the Quadrate was repurposed into a bone used in sound transmission in the Middle Ear. This earned the bone a new name, the Incus, or anvil. So, we still have a tiny ancient jaw joint in our ear, with muscles, ligaments, and articular cartilage. As Neil Shubin said, inside our ears, is ‘our inner reptile’. Thanks to Alec Wilken for assembling this model.
6 comments