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THE EVOLUTION OF THE MAMMALIAN LOWER JAW
EVOLUTION OF THE MAMMALIAN LOWER JAW AND MIDDLE EAR
In mammals, the lower jaw seems fairly boring: it is one solid bone, called the mandible (or dentary). Its evolutionary past is much more exciting, involving not only the jaw but also 2 of the 3 bones of the middle ear (the quadrate of the upper jaw becomes the incus of the middle ear and the articular of the lower jaw becomes the malleus of the middle ear). In reptiles and amphibians, the quadrate and articular bone form the jaw joint: this is where the lower jaw articulates with the upper jaw. In mammals, the jaw joint is between the dentary (mandible) and squamosal (part of the temporal bone). During mammalian evolution, the quadrate and articular were freed from their original functions and formed the middle ear bones that only mammals have (Kemp, 1982; Carroll, 1988; Kardong, 2002).
The following series of illustrations depicts the evolution of the sarcopterygian lower jaw into the mammalian lower jaw. The most important developments include:
1) The blue bone (the dentary) becomes larger and more significant until it is the only bone of the lower jaw and the other bones are lost.
2) The reptilian jaw joint (between the red and yellow bones) is replaced by a joint made by the dentary (blue) and the squamosal bone which is not shown here. Cynodonts are transitional forms which actually have both jaw joints simultaneously: the primitive reptilian joint which is becoming less prominent and the new mammalian jaw joint which is becoming more prominent.
3) When the mammalian jaw joint becomes established, the angular bone (pink) is modified to support the eardrum and the quadrate and articular bones (red and yellow) are incorporated into the middle ear.
Light Green: Surangular
Dark Green: Anterior Coronoid
Yellow: Quadrate (also the Quadrojugal; they may form a complex together)
|The quadrate and the articular became bones of the middle ear which amplify sound. The angular became the part of the temporal bone which supports the eardrum. In marsupial newborns, the quadrate and the articular form in the jaw joint and are incorporated into the middle ear after birth.|
|In human fetuses, the quadrate and articular form in the vicinity of the jaw joint and slowly become encased in a bony part of the temporal bone. In human fetuses, the angular is still visible as a separate bone before it fully fuses with the temporal.|