9/20/2023 0 Comments Lend me your ears drawingThere is no evidence that these bones were specialized for transmitting airborne sound in basal synapsids. The basalmost synapsids had a typical tetrapod skull with the quadrate, articular, and angular forming the parts of the jaw they are supposed to. In modern mammals, only the anteriormost bone is left in the lower jaw, the dentary (which holds the teeth), and the jaw hinge is instead formed by articulating it with the squamosal of the upper jaw. The quadrate and articular form the jaw hinge in non-mammalian bony vertebrates. The ectotympanic is a modified angular, another lower jaw bone. The malleus is a modified articular and prearticular, bones in the lower jaw of non-mammalian bony vertebrates. Where did they come from? The incus is actually modified from the quadrate, a bone in the upper jaw of non-mammalian bony vertebrates. The incus and malleus, however, are unique to mammals among modern animals. The stapes is present in most tetrapods, having been inherited from part of the hyoid arch in fish. They also have an ectotympanic ring that helps support the eardrum. Modern mammals have three ossicles (small bones) in their middle ear: the stapes, incus, and malleus. Important to note off the bat: modern mammals, reptiles, and frogs all generally have a tympanic (eardrum-bearing) ear for hearing airborne sounds, but the tympanic ear is not homologous between these groups! Instead, it evolved at least three different times (and probably more) in vertebrate history. After seeing one-too-many Dimetrodon incorrectly depicted with reptile-like ears, I decided to step out of my comfort zone and take a shot at explaining the whole deal myself. Yet there are very few popular treatises on the subject available, much less explanation on what it might mean for paleoartists. The evolution of the mammalian ear is one of the best documented evolutionary transitions in the fossil record.
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