Mammalian Explosion 297The front part is composed of the dentary bone (which contains the teeth) but behind it
were many other bones: the coronoid bone where the jaw muscles pulled up on the jaw, the
articular bone in the jaw hinge, the angular bone in the lower back part (angle) of the jaw,
and several other accessory jawbones. The articular bone of the lower jaw hinged against
the quadrate bone of the amniote skull, and both were abutting against the middle ear and
the “stirrup” bone (the stapes), helping transmit sound from the lower jaw into the ear. But
as we go through the series of more and more advanced synapsids, we see some remark-
able changes in the jaw. All of the nondentary components (shaded bones in right column of
figure 13.5) get smaller and smaller, until by the time we see advanced cynodonts, most of
these nondentary bones are just tiny splints in the inside back of the jaw, and the dentary
bones have expanded to become almost the entire jaw. The probable reason for this trans-
formation is that a single bone (the dentary) is much stronger than a series of bones sutured
together, and as synapsids became more active in chewing, they needed a jaw that could
handle all the stresses. A portion of the dentary, known as the coronoid process, expanded
upward and took over the attachment points of the temporalis muscles and replaced the
FIGURE 13.5. The gradual transformation of the jawbones within the synapsids, as all the nondentary jawbones
(shaded bones: angular, surangular, articular, coronoid, splenials, and so on) are gradually reduced to tiny
splints in the inside back part of the jaw, and the dentary bone (unshaded bone) takes over as the principal
jawbone. Eventually, all the nondentary jaw elements are lost in mammals, except for the articular bone, which
becomes the “hammer” (malleus) bone of the middle ear. (Drawing by Carl Buell)
Morganucodon
Primitive mammalJurassic MammalsTherapsidsPelycosaursTriassicPermianThrinaxodon
Advanced cynodontDimetrodon
Pelycosaur