The Origin and Evolution of Mammals - Moodle

consist of a large labial cusp and a row of smaller
lingual cusps. They intermesh in such a way that
the anterior edge of an upper tooth shears against
the posterior edge of the corresponding lower tooth
to generate a cutting action (Gow 1978). The diet of
Bauria is assumed to have included tough, fibrous
material, although, for all the sophistication of its
dental battery, it shows no sign of a particular
increase in the size of its adductor musculature. The
postorbital bar is in fact absent, and the zygomatic
arch not significantly expanded beyond that of earlier,
primitive baurioids. Bauria is the last occurring
South African therocephalian, being found in the
Lower-Middle Triassic Cynognathus Assemblage
Zone. The Chinese Traversodontoides is closely
related (Sun 1991), as possibly are certain Russian
specimens (Battail and Surkov 2000).
Kemp (1972a, 1982) analysed the therocephalian
jaw mechanism. The more primitive forms retained
well-developed incisors and canines but reduced
postcanines, and probably had similar jaw mechanics
to the gorgonopsians, with the adductor jaw
musculature imparting kinetic energy to the jaws
that was dissipated by the canines entering the
prey. There is no evidence for interdigitation of the
incisors as found in gorgonopsians, and therefore
dismembering of prey may have been a cruder
affair consisting of tearing rather than cutting the
flesh. The jaw musculature (Fig. 4.3(b)) was largely
restricted to the medial and posterior surfaces of
the temporal fenestra, and a connective tissue sheet
that may have covered it. Invasion by adductor
musculature of the lateral surface of the lower jaw
seems to have been at most very limited. On the
other hand, the structure of the reflected lamina of
the angular of therocephalians is more complex than
that of any other therapsid. It consists of a relatively
huge, thin sheet of bone, free along the dorsal posterior
and ventral margins, and strengthened by three
or four broad ridges or corrugations radiating from
the front. These ridges on the lamina are difficult to
interpret since no comparable structure exists in
modern tetrapods. However, they do suggest that
there was a complex arrangement of posteriorly and
ventrally directed musculature extending from the
reflected lamina behind and below the lower jaw.
It would presumably have included musculature
associated with jaw opening, and with the operation
EVOLUTION OF MAMMAL-LIKE REPTILES 59
of hyoid apparatus in the floor of the mouth, and
therefore complex tongue movements may have
occurred. It may be that the considerable range of
adaptive types found among the therocephalians
compared to the otherwise rather similar gorgonopsians
was made possible by greater adaptability of
the oral manipulating mechanism in the former.
Large and small carnivores, insectivores, scavengers,
venom-producers, and herbivores all manifested
versions of the therocephalian jaw structure.
The locomotory function of therocephalians has
been considered by Kemp (1986). The ratio of limb
length to body size is comparable to modern noncursorial
mammals, and like the latter there is a distinct
lumbar region with short, stout, immoveable,
horizontal ribs behind the functional ribcage
(Fig. 3.17(d)). The tail is very reduced and the ilium
expanded forwards, presumably to accommodate a
gluteal-like muscle for femoral retraction. The mode
of action of the limbs is discussed later, since the
therocephalians best illustrate an important hypothetical
stage in the origin of mammalian locomotion
(page 105ff).
Of all the therapsid groups other than the cynodonts
with their particular relationship to the mammals,
it is the progressive members of the Baurioidea
that most give the impression of an advanced level of
temperature physiology. As in the cynodonts, they
evolved a secondary palate, with the implication that
feeding and food manipulation in the mouth needed
to occur simultaneously, and which is correlated with
an elevated metabolic rate. The triturating teeth of
Bauria may have increased the rate of food assimilation,
and further evidence is the disassociation of the
lumbar region from the thoracic, implying the
possibility of a diaphragm and thus elevated levels of
gas exchange. The fact that several lineages of baurioids
of very small body size survived the Permo–
Triassic boundary, which is to say survived the
greatest mass extinction event in the Earth’s history
could also be correlated with the relatively high level
of independence of environmental stresses such as
cold temperature that is associated with endothermy.
Nevertheless, despite such intriguing possibilities the
therocephalians actually had an extremely modest
presence in the Triassic, and it is to their relatives the
cynodonts that this history must now turn to see
the next great advances in the synapsid story.