The Origin and Evolution of Mammals - Moodle

Even at their initial appearance in the fossil
record, therapsids had already diversified into
several distinct groups, although the considerable
number of characters they share indicates that the
Therapsida is a monophyletic group descended from
a single hypothetical pelycosaur-grade ancestor. In
one of the most recent analyses, Sidor and Hopson
(1998) counted as many as 48 possible therapsid
synapomorphies. The most prominent single one
of these is the reflected lamina of the angular
bone of the lower jaw. This is a thin, extensive sheet
of bone lying lateral to and parallel with the main
body of the angular. It is connected anteriorly but
has free dorsal, posterior, and ventral margins.
Therapsids also possess a temporal fenestra that
is much larger than in the pelycosaurs, indicating
an increased mass of jaw closing musculature.
Associated with the latter is the presence of a
single, enlarged canine tooth in both upper and
lower jaws that is sharply distinct from the incisors
and the postcanine teeth. The jaw hinge is more
anteriorly placed, and therefore the occipital plate
is closer to vertical. The whole of the posterior part
of the skull is more robustly built, with massive
supraoccipital and paroccipital processes extending
laterally to brace the squamosal region of the cheek.
Other features probably related to strengthening
the skull include the loss of the supratemporal bone
and the immobility of the basipterygoid articulation
between palate and braincase, which in pelycosaurs
is constructed as a ball and socket joint
between the basisphenoid and the pterygoid bones.
The therapsid postcranial skeleton also has many
new features, related to improved locomotory
ability. The blade of the scapula is narrow and the
shoulder joint is no longer the complex screwshaped
structure that limited the range of humeral
movements of pelycosaurs. Instead, the glenoid
joint is a short, simple notch and the articulating
head of the humerus ball-shaped. An ossified sternum
has evolved behind the interclavicle. In the
hindlimb, the ilium has expanded and the femur
has a slight sigmoid curvature. A trochanter major
has developed behind the femoral head, and the
internal trochanter of the pelycosaur femur has
shifted to the middle of the ventral surface of the
bone. The feet, both front and back, have reduced
certain of the phalanges to discs so that the digits
EVOLUTION OF MAMMAL-LIKE REPTILES 27
are more nearly the same length as each other. In
the vertebral column, the intercentra have disappeared
from the trunk region, although they are
still present in the neck and tail.
That the therapsids are closely related to the
pelycosaurs was established by Broom’s (1910)
classic paper comparing the two, and the affinity
has never been seriously doubted since, although
the relationship is nowadays acknowledged as
a monophyletic Therapsida nesting within a paraphyletic
‘Pelycosauria’. From among the known
pelycosaur groups, the closest relative of
Therapsida is almost universally agreed to be the
family Sphenacodontidae, on the basis of several
shared characters, including the following (Hopson
1991; Laurin 1993).
● Enlarged caniniform tooth, and differentiation of
incisiform teeth in front and postcanine teeth
behind.
● Maxilla enlarged at the expense of the lachrymal,
so that it contacts the nasal bone, an arrangement that
permits accommodation of the upper canine tooth.
● Lower jaw with a high coronoid eminence from
which the posterior part of the jaw curves steeply
down to a jaw hinge well below the level of the
tooth row.
● Sphenacodontids alone among pelycosaurs with
a notch between the angular keel and a downturned
articular region, that is an incipient version
of the reflected lamina of the angular.
● Occiput with well-developed supraoccipital and
paroccipital processes.
● A few characters of the postcranial skeleton,
including a degree of reduction of the trunk intercentra,
and narrowing of the scapular blade.
One final fossil to consider in the context of the
relationships of therapsids is the most mysterious
of all (Fig. 3.7(h)). In 1908, W. D. Matthew
described the crushed, partial skull of a very peculiar
pelycosaur which he named Tetraceratops
(Matthew 1908). It is small, only about 10 cm in
skull length, and has a vaguely sphenacodontid
dentition with an enlarged upper canine. However,
unlike other sphenacodontids, it has an equally
enlarged first upper incisor and is also unique in
possessing a row of extraordinarily large palatal
teeth on the lateral flanges of the pterygoids, and in