The transition from a four-boned lower jaw to the mammalian jaw with a single element. Skull illustrations from Kardong (2002), as reproduced by Theobald (2004). The jawbones on the left illustrate the the inside of the mouth; the illustrations on the right show the outside of the jaw. The quadrate (the incus or anvil of the mammalian ear) is in turquoise, the articular (malleus or hammer in the mammalian ear) is in yellow, and the angular (tympanic annulus in the mammalian ear) is in pink. Teeth are not shown, and skulls are scaled to constant size for clarity. Q = quadrate, Ar = articular, An = angular, I = incus (anvil), Ma = malleus (hammer), Ty = tympanic annulus, D = dentary.
Skull figures reproduced from Kardong, K. V. (2002) Vertebrates: Comparative Anatomy, Function, Evolution. 3 ed. New York: McGraw Hill, fig. 1.4.3. The bubble plot of mammalian evolution is based on figure 17.1 in Carroll (1988), and shows the diversification of major groups and the separation of distinct lineages through time.