A correspondent recently asked me about the evolution of the mammalian middle ear in relation to the fossil record. Based on data gathered from embryology, it is widely thought that the bones of the mammalian middle ear (the region just inside the eardrum) evolved from bones of the reptilian lower jaw joint. Besides the paleontological data, this hypothesis is based on the fact that, in mammals, Meckel’s cartilage plays a role in forming the middle ear bones and mandible before subsequently disappearing. In reptiles, it ossifies to become part of the jaw.
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