298 Evolution? The Fossils Say YES!
primitive amniote coronoid bone. Likewise, the primitive articular jaw joint with the quad-
rate bone of the skull also became highly reduced. Meanwhile, a portion of the dentary bone
expanded upward, meeting the skull in the squamosal bone region and initiating a new jaw
joint. Eventually, the dentary/squamosal jaw joint will take over completely, and the primi-
tive articular/quadrate jaw joint will be superseded.
There is even one remarkable fossil, a trithelodont known as Diarthrognathus (fig. 13.6),
which shows how this transition took place. Its name means “two jaw joint,” and indeed
that’s what it has: both the old amniote articular/quadrate jaw joint still attached on both
sides of the skull alongside the new dentary/squamosal jaw joint. We could not have asked
for a more perfect transitional fossil, which has been caught in the act of making the transi-
tion from one set of jaw joints to the other. Eventually, the articular/quadrate jaw joint gets
smaller and smaller, and no longer serves as a jaw joint, as the dentary and squamosal take
over completely.
So what happened to the quadrate and articular? They could have vanished completely,
as did most of the nondentary bones of the lower jaw. But remember the point we discussed
earlier? Primitive amniotes hear with their lower jaws, and the sound is transmitted from
FIGURE 13.6. Diarthrognathus actually has the old articular/quadrate jaw joint of primitive synapsids in
operation side by side with the mammalian dentary/squamosal jaw joint on both sides of the skull. (Drawing
by McLoughlin 1980; Viking, New York; used with permission)
dentary (shaded)lower jawarticularquadratestapessquamosal
(shaded)