The Origin and Evolution of Mammals - Moodle

48 THE ORIGIN AND EVOLUTION OF MAMMALS
edge of the scapula. In the hindlimb, the ilium
extends forwards and the pubis is reduced and
rotated backwards more than in any other dicynodont.
King (1985) interpreted these modifications
as indicating a forelimb that relied primarily on
long-axis rotation of a laterally directed humerus to
produce the forelimb stride. The hind limb anatomy
indicates that the knee was turned forwards and the
foot lay under the body, in mammalian fashion.
Quite what use Kingoria made of its modified
anatomy is not at all clear; she tentatively suggested
that the forelimb had a digging function, and the
hindlimb was adapted to generate a greater thrust.
Whatever its mode of life, Kingoria is uncommon
in the fossil record, possibly because it lived in an
environment less conducive to fossilisation. The
only other known kingorioid is the Lower Triassic
Kombuisia (Hotton 1974), a small dicynodont representing
one of the only three lineages that survived
the end-Permian and through to the Cynognathus
Assemblage Zone.
Diictodontoidea. The diictodontoids share with
Kingoria a reduction of the palatine bone, and it has
been proposed on the basis of this extremely limited
evidence that they may be related (King 1988).
Diictodontoids are, however, distinguished in their
own right by the presence of an embayment, or
notch in the margin of the maxilla, immediately in
front of the caniniform process, whether a tusk is present
or not. The shape of the notch indicates a sharp,
horny blade against which a dentary blade would
have acted, capable of cutting slender but tough
stems, roots, and rhizomes. These would then have
been triturated between opposing plates borne by the
palate and the front part of the dentaries. Primitive
members of the group include Robertia which retained
a few small postcanine teeth, and Emydops which lost
them, both from the Tapinocephalus Assemblage Zone
of South Africa.
Diictodon itself is more advanced, having completely
lost the teeth and developed a narrow
intertemporal region of the skull to permit further
elaboration of the adductor musculature. King
(1990) described evidence for a complex form
of horny beak. Longitudinal ridges on the bony
ventral surface of the premaxillae correspond to the
medial edges and to troughs on the dorsal surface
of the symphyseal part of the dentaries, indicating
a mechanism for shredding up the food that had
been collected by cutting between the very prominent
caniniform notch and edge of the dentary.
Hotton (1986) thought the most likely diet was subterranean
roots and tubers, dug up by the strong,
clawed hand, and indirect evidence supporting this
view is the remarkable discovery by Smith (1987)
of Diictodon specimens curled up at the bottom
of 50 cm deep helical-shaped burrows. The skull
shows none of the adaptations typical of powerful,
obligatory burrowing animals, but there are signs
in the postcranial skeleton of a fossorial ability
enhanced beyond that of typical dicynodonts. Ray
and Chinsamy (2003) interpreted the cylindricalshaped
body, very short tail, relatively small, broad
limbs, and wide manus as those of an adept
digging animal using its forelimbs to dig and its
hindlimb to push away the soil. Possibly Diictodon
only made shallow burrows, or adopted existing
ones made by other taxa. Whatever its mode of life,
Diictodon was one of the most successful and widespread
of dicynodont genera. It has been found in
the northern hemisphere and China, as well as in
southern Africa, and in the latter it lasted from the
Tapinocephalus Assemblage Zone right through to
the Dicynodon Assemblage Zone at the very end of
the Permian.
The idea of burrowing was taken much further in
Cistecephalus (Cluver 1978) and the closely related
Cistecephaloides and Kawingasaurus (Cox 1972).
These are small animals, with a very short, broad
skull. The snout is pointed, the intertemporal roof
extremely broad and strongly built, and the occiput
wide to accommodate powerful neck muscles. The
forelimb is mole-like, with a short, powerfully built
humerus, large olecranon process on the ulna, and
a broad forefoot with enlarged digits. Surprisingly
perhaps, the hindlimb is much less modified, indicating
that it played a lesser role in digging and,
presumably, burrowing (King 1990).
Pristerodontoidea. This is both the most diverse and
the longest lived of the four major dicynodont taxa,
for in addition to some of the commonest Permian
forms it also includes almost all of the Triassic
dicynodonts. Pristerodontoids are distinguished by
a large, leaf-shaped palatine bone on the palate and