Evolution__3rd_Edition

544 PART 5 / Macroevolution
Figure 18.11
Articulation of the jaw in
mammal-like reptiles. (a) In
the early form Biarmosuchus,
the jaw muscle is at the basal
articulation. In the evolution
of mammals, the muscles
move forward, and by
Probainognathus (c), an
advanced mammal-like reptile,
the masseter has split into two.
The superficial masseter joins
a characteristic region of the
upper jaw, and the presence
of the advanced jaw condition
can be recognized from the
jaw alone (see Figure 18.10,
where these three forms are
again illustrated but without
the muscles drawn in).
Thrinaxodon (b) was probably
like Probainognathus. The
positions of muscles in fossils
can be inferred from the bone
shapes, which imply the
attachment sites for muscles.
Note also the increasing
complexity and serial
differentiation of the teeth. For
the different gaits of reptiles
and mammals, see Figure 15.2
(p. 427). From Carroll (1988).
© 1988 WH Freeman &
Company, with permission.
Stages in mammal evolution
include the pelycosaurs ...
. . . therapsids ...
Adductor
mandibulae
externus
(a) Biarmosuchus (b) Thrinaxodon
(c) Probainognathus
Temporalis
Masseter
Temporalis
Deep
masseter
Superficial
masseter
by then the dinosaurs had proliferated. No other terrestrial tetrapods thrived until after
the dinosaurs went extinct at the end of the Cretaceous.
The characters that can most clearly be reconstructed in fossils are those concerned
with locomotion and feeding, because these are simply related to the form of preserved
bones and teeth. The reptilian jaw contrasts in many respects with the mammalian
(Figure 18.11). Mammalian teeth have a complex, multicusped structure and are differentiated
down the jaw into canines, molars, and so on, whereas reptilian teeth form a
relatively undifferentiated row and have a simpler structure. The top and bottom of the
reptilian jaw articulates (that is, hinges) at the back, where it has muscles that simply
snap it shut. The mammalian jaw has cheek muscles which surround the cheek teeth
and enable the jaw to close more powerfully and accurately than the reptile’s. As the
point of jaw articulation moved forward in mammalian evolution, the bones at the rear
of the jaw were evolutionarily liberated, and went on to evolve into the ear bones a but
we shall not follow that fascinating story. Here we concentrate on how the jaw, and gait,
changed in the evolution of the mammal-like reptiles.
We can distinguish three main phases in mammal-like reptilian evolution. The first
phase corresponds to one of the two major divisions of the group, the pelycosaurs.
Pelycosaur fossils are preserved from the Pennsylvanian and Permian, particularly
from southwest USA where rocks of this age are located (see Plate 10, between pp. 68
and 69). Archaeothyris lived there about 300 million years ago, and is an early pelycosaur
(Figure 18.10). It was a lizard-like animal, about 20 in (50 cm) long. Its distinctive
difference from other reptile groups is an opening in the bones behind the eye. The
opening is called a temporal fenestra, and in the living animal a muscle passed through
it. The muscle acted to close the jaw, and the opening up of the temporal fenestrae is the
first sign of the more powerful jaw mechanism of the mammals. (The temporal fenestra,
by the way, is the defining character of the Synapsida.) A better known pelycosaur
was Dimetrodon, with its enigmatic back-sails. Pelycosaurs showed little or no tooth
differentiation, and had the reptilian sprawling gait (Figure 15.2, p. 427). They evolved
into three main groups during their 50 million-year history, and most of them went
extinct quite suddenly about 260 million years ago.
A few of the sphenacodontids survived and it was from an unknown line within the
sphenacodontids that the second main group of mammal-like reptiles evolved. This
group was the therapsids. The evolution of the therapsids makes up the second main
phase of the mammal-like reptiles, in the Permian and Triassic. Therapsid fossils are
found in many regions of the world, with a site in South Africa having the best deposits.
The therapsids underwent a remarkably similar pattern of evolution to the pelycosaurs
..