Chordates - Chordates: Trait History 1. Third ear bone =...

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Chordates: Trait History 1. Third ear bone = Mammalian hearing adaptation is made possible by a sophisticated middle ear of three tiny bones, known as the hammer (malleus), the anvil (incus), and the stirrup (stapes), plus a bony ring for the eardrum (tympanic membrane). These mammal middle ear bones evolved from the bones of the jaw hinge in their reptilian relatives. The third ear bone came about when the jawbones evolved and separated from the jaw, moving into the middle ear of modern mammals. The evolution of the third ear bone can be interpreted as having a "secondarily reversal to the ancestral condition," meaning that the adaptation is the caused by changes in development. It has been suggested that the third middle ear is actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution. 2. Notochord = The notochord is a flexible, rod-shaped body found in embryos of all chordates . It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo . Notochords were the first "backbones" serving as support structures in chordates such as Haikouichthys . Notochords were advantageous to primitive fish-ancestors because
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Chordates - Chordates: Trait History 1. Third ear bone =...

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