The Origin and Evolution of Mammals - Moodle

94 THE ORIGIN AND EVOLUTION OF MAMMALS
Biarmosuchian grade
The next stage is the primitive therapsid, or biarmosuchian
grade (Fig. 3.8(a) and 4.3(a)). Relatively little
innovation had occurred, but rather an
exaggeration of the novelties that had evolved by the
sphenacodontine-grade condition. The principle of
well-developed incisors, large canines, but lessprominent
postcanines was retained, but with even
greater distinction between them. The canines were
relatively enormous and must have been used in a
kinetic fashion in which the lower jaw was accelerated
from a widely open position and the kinetic
energy so generated was dissipated by the teeth
sinking into the prey. The reason for thinking this,
apart from the size of the canines, is that the jaw
adductor musculature still acted at the posterior end
of the jaw, so no great static force could have been
generated between teeth situated towards the front
of the jaw. The incisors, in addition to being well
developed, were probably capable of interdigitating,
lowers between uppers, which increased the effectiveness
of their biting action.
The overall mass of the adductor musculature
had certainly increased considerably, as indicated
by the increased size of the temporal fenestra. It
extends further dorsally, ventrally, and posteriorly
than in the sphenacodontine stage, thereby providing
a larger area of connective sheet over it, and bony
edging around it for the attachment of muscle. There
is still no development of a discrete coronoid
process of the dentary rising above the dorsal margin
of the jaw, although the coronoid eminence is
prominent. The most striking change in the lower
jaw was the evolution of a full-sized reflected lamina
of the angular. Instead of being merely the keel of
that bone separated by a notch from the depressed
articular region of the jaw as in the sphenacodontines,
it has become a very large, thin sheet enclosing
laterally a deep, narrow space between itself and
the main body of the jaw. The exact function of this
prominent structure in basal therapsids has never
been fully agreed upon. Despite a number of earlier
suggestions that it housed a gland, or an air-filled
diverticulum associated with hearing, there is little
doubt now that its primary function was for muscle
insertion. In Biarmosuchus (Fig. 3.8(a)), Ivakhnenko’s
(1999) reconstruction indicates a pattern of ridges
on the external surface of the lamina indicative of
strengthening against muscle-generated forces. The
most plausible general explanation for the form of
the reflected lamina is that it served as the insertion
site for a complex set of muscles running in a variety
of different directions. At the biarmosuchian grade,
this may have included tongue and hyoid musculature
inserted on the lower part of the lamina, and
jaw-opening musculature running from the posterior
region backwards towards the shoulder girdle
region. In the more advanced carnivorous groups,
gorgonopsians, and therocephalians, the strengthening
ridges are more strongly developed, although
in quite different patterns respectively. Certainly in
gorgonopsians and possibly in therocephalians,
there is evidence for an invasion of part of the
reflected lamina by adductor mandibuli musculature.
However, this could not have been true of
biarmosuchians because their zygomatic arch was
not expanded laterally to create room for the attachment
of such a muscle.
The jaw articulation of Biarmosuchus is a simple
roller and groove system, restricting the jaw to
strictly orthal movements. Thus, the biting mechanism
at this stage was little more sophisticated,
although more powerful than in pelycosaurs, with a
disabling bite delivered by the canines, a tearing
action using the incisors, and a finer tearing, or food
retention by the modest-sized postcanines.
Therocephalian grade
The next definable stage is the therocephalian
grade (Fig. 4.3(b)), in which the principal innovation
is enlargement of the temporal fenestra by
extreme medial extension. This has occurred to
such an extent that the intertemporal region of the
skull roof is reduced to a narrow girder, the sagittal
crest. The midline of the crest tends to be sharp and
the sides face dorso-laterally, providing an area for
the origin of the innermost part of the adductor
mandibuli musculature. This area of muscle attachment
continues smoothly round on to the front face
of the occiput, while the almost horizontal temporal
fenestra, covered by its connective tissue sheet,
gives further origin for the muscle. By this stage,
a large part of the adductor musculature is distinguishable
as a medial component, namely a true
temporalis muscle. A coronoid process of the dentary
had evolved, as a postero-dorsal extension of