518 CHAPTER 20and the Eutheria (placental mammals). The crown Mammalia also includes many
extinct groups. Some of the critical characteristics of the Mammalia are these:- The lower jaw in reptiles consists of the articular and several other bones, but
in mammals it is only a single bone (the dentary). - The primary (and in all except the earliest mammals, the exclusive) jaw ar-
ticulation is between the dentary and the squamosal skull bone, rather than
between the articular and the quadrate bone, as in other tetrapods. - Early amniotes have a single sound-transmitting bone in the middle ear, the
stirrup (or stapes). Mammals have three bones: not only the stirrup, but also
the hammer (malleus) and anvil (incus). - Mammals’ teeth are differentiated into incisors, canines, and multicusped
(multipointed) cheek teeth (premolars and molars), whereas most other tet-
rapods have uniform, single-cusped teeth.
Other features that distinguish most mammals from other amniotes include an
enlarged braincase, a large space (temporal fenestra) behind the eye socket, and
a secondary palate that separates the breathing passage from the mouth cavity.
The early synapsids had a temporal fenestra that provided space for jaw muscles
to expand into when contracted (FIGURE 20.1B). The temporal fenestra became
progressively enlarged in later synapsids (FIGURE 20.1C–E). Permian synapsids
in the order Therapsida (see Figure 20.1C) had large canine teeth, and the center
of the palate was recessed, suggesting that the breathing passage was partially
separated from the mouth cavity. The hind legs were held rather vertically, more
like a mammal than a reptile.
Among the Therapsida, the cynodonts, which lived from the late Permian to the
late Triassic, represent several steps in the approach toward mammals. The rear of
the skull was compressed, giving it a doglike appearance (see Figure 20.1D,E); the
dentary was enlarged relative to the other bones of the lower jaw; the cheek teeth
had a row of several cusps rather than only one; and a bony shelf formed a sec-
ondary palate that was incomplete in some cynodonts and complete in others. The
quadrate was smaller and looser than in previous forms and occupied a socket in the
squamosal (FIGURE 20.2A).
In the advanced cynodonts of the middle and late Triassic (FIGURE 20.1F), the
cheek teeth had not only a linear row of cusps, but also a cusp on the inner side of the
tooth. This seemingly trivial, but actually profoundly important, innovation begins a
history of complex cheek teeth of mammals, which are modified in different lineages
for chewing different kinds of food. In some late Triassic and Jurassic cynodonts, the
lower jaw had not only the old articular/quadrate articulation with the skull, but also
an articulation between the dentary and the squamosal, marking a critical transition
between the ancestral condition and the mammalian state (FIGURE 20.2B). All these
features—molars, a strong lower jaw composed mostly of a single bone, an enlarged
fenestra to accommodate large jaw muscles, and a secondary palate that enabled the
animal to breathe while consuming and chewing large prey—imply increasingly
active, efficient predators, probably with a heightened metabolism.
Morganucodon, of the late Triassic and very early Jurassic (FIGURE 20.1G), is con-
sidered an early mammal. Morganucodon had two jaw-skull joints: a weak articular/
quadrate hinge and a fully developed mammalian articulation between the dentary
and the squamosal. The articular and quadrate bones were sunk into the ear region,
similar to the condition in modern mammals (FIGURE 20.2C,D). Hadrocodium, from
the early Jurassic, was very similar to Morganucodon, but the lower jaw consisted
entirely of the dentary, and the articular and quadrate bones were fully separated
from the jaw joint and fully lodged in the middle ear, where they are now called the
hammer and anvil, and transmit sound, together with the stirrup. Morganucodon,20_EVOL4E_CH20.indd 518 3/22/17 1:44 PM