Mesozoic Earth and Life History - The Changing Earth: Exploring Geology and Evolution - page 623

TABLE 22.2 Summary Chart Showing Some Characteristics and How They Changed during the Transition from Reptiles to Mammals
Features

Typical Reptile Cynodont Mammal

Lower Jaw Dentary and several other

bones

Dentary enlarged, other bones
reduced

Dentary bone only, except in
earliest mammals

Jaw-Skull Joint Articular-quadrate Articular-quadrate; some advanced

cynodonts had both the reptile
jaw-skull joint and the mammal
jaw-skull joint

Dentary-squamosal

Middle-Ear Bones Stapes Stapes Stapes, incus, malleus

Secondary Palate Absent Partly developed Well developed

Teeth No differentiation; chewing

teeth single rooted

Some differentiation; chewing teeth
partly double rooted

Fully differentiated into
incisors, canines, and
chewing teeth; chewing teeth
double rooted

Tooth Replacement Teeth replaced continuously Only two sets of teeth in some

advanced cynodonts

Two sets of teeth

Occipital Condyle Single Partly divided Double

Occlusion (chewing teeth
meet surface to surface to
allow grinding)

No occlusion Occlusion in some advanced

cynodonts

Occlusion

Endothermic vs. Ectothermic Ectothermic Probably endothermic Endothermic

Body Covering Scales One fossil shows that it had skin

similar to that of mammals

Skin with hair or fur

Origin and Evolution of Mammals
Recall from Chapter 21 that mammal-like reptiles called the-
rapsids diversifi ed into many species of herbivores and carni-
vores during the Permian Period. Among the therapsids, one
group, known as cynodonts, was the most mammal-like of all,
and by Late Triassic time, true mammals evolved from them.
Cynodonts and the Origin of Mammals We can easily recog-
nize living mammals as warm-blooded animals that have hair
or fur and mammary glands and, except for the platypus and
spiny anteater, give birth to live young. However, these criteria
are not suffi cient for recognizing fossil mammals; for them,
we must rely on skeletal structure only. Several skeletal modi-
fi cations took place during the transition from mammal-like
reptiles to mammals, but distinctions between the two groups
are based mostly on details of the middle ear, the lower jaw,
and the teeth (Table 22.2). Fortunately, the evolution of mam-
mals from cynodonts is well documented by fossils.
Reptiles have one small bone in the middle ear (the sta-
pes), whereas mammals have three: the incus, the malleus,
and the stapes. Also, the lower jaw of a mammal is com-
posed of a single bone called the dentary, but a reptile's jaw
is composed of several bones (

of the dentary until it became the only element in the mam-
malian jaw. Likewise, a progressive change from the reptile to
mammal jaw joint is documented by fossil evidence. In fact,
some of the most advanced cynodonts were truly transitional,
because they had a compound jaw joint consisting of (1) the
articular and quadrate bones typical of reptiles and (2) the
dentary and squamosal bones as in mammals (Table 22.2).
In Chapter 18, we noted that the study of embryos pro-
vides some of the evidence for evolution. Opossum embryos
show that the middle-ear bones of mammals were originally
part of the jaw. In fact, when opossums are born, the middle
ear elements are still attached to the dentary, but as they de-
velop further, these elements migrate to the middle ear and a
typical mammal jaw joint develops.
Several other aspects of cynodonts also indicate they
were ancestors of mammals. Their teeth were becom-
ing double rooted as they are in mammals, and they were
somewhat differentiated into distinct types that performed
specifi c functions. In mammals, the teeth are fully differen-
tiated into incisors, canines, and chewing teeth (premolars
and molars) but typical reptiles do not have differentiated
teeth ( Figure 22.30). In addition, mammals have only two

sets of teeth during their lifetimes—a set of baby teeth and
the permanent adult teeth. Reptiles have teeth replaced con-
tinuously throughout their lives, the notable exception being
some cynodonts who, in mammal fashion, had only two sets
of teeth. Another important feature of mammal teeth is oc-
clusion; that is, the chewing teeth meet surface to surface to
allow grinding. Thus mammals chew their food, but reptiles,
amphibians, and fish do not. However, tooth occlusion is
known in some advanced cynodonts (Table 22.2).

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Figure 22.29). In addition,
a reptile's jaw is hinged to the skull at a contact between the
articular and quadrate bones, whereas in mammals the den-
tary contacts the squamosal bone of the skull (Figure 22.29).
During the transition from cynodonts to mammals, the
quadrate and articular bones that had formed the joint be-
tween the jaw and skull in reptiles were modifi ed into the in-
cus and malleus of the mammalian middle ear (Figure 22.29,
Table 22.2). Fossils document the progressive enlargement

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