The Origin and Evolution of Mammals - Moodle

the presence of a deep, longitudinal groove along
the dorsal edge of the dentary, called the dentary
sulcus. The posterior part of the feeding apparatus
appears to have been more important than in other
dicynodonts, with a slicing, knife-like blade on the
lower jaw shearing against a horny palatal pad. In
most members of the group, the premaxillary margin
is also sharp and fits quite closely against the
outer edge of the dentary, indicating a cutting
edge perhaps suitable for cropping palate-sized
bites of leafy vegetation, with the different species
specialised for handling the foliage of different
forms of glossopterid trees, shrubs, etc. (Cox 1998).
Pristerodon is a primitive member of the group. It is
small in size with a broad intertemporal roof, and possesses
small postcanine teeth in both upper and lower
jaws (King and Rubidge 1993). Common and widespread
in Africa, it existed from the Tapinocephalus
to the Dicynodon Assemblage Zones, a longevity
matched otherwise only by Diictodon (Angielczyk
2001), and it also occurs in India. The adaptive
radiation which occurred from a hypothetical
EVOLUTION OF MAMMAL-LIKE REPTILES 49
Pristerodon-like ancestor includes such familiar
types as Oudenodon which lost its tusks but retained
the wide intertemporal region and narrow snout.
According to Hotton (1986), Oudenodon held its
snout forwards when feeding, indicating that its
source of food was vegetation well above ground
level. This group also includes the largest of the
Permian dicynodonts, with skull lengths up to 50 cm.
Aulacocephalodon is a fairly large, wide-snouted
pristerodontoid in which the front edge of the
premaxilla formed a transverse cutting blade.
In contrast, the even larger Dinanomodon possessed
a narrow, pointed snout which might perhaps have
adapted it to browsing on higher-level foliage.
These two inevitably have been compared to white
and black rhinos, respectively.
The familiar and once much overused genus
Dicynodon retained a pair of tusks, evolved a
narrow intertemporal region, and tended to
shorten the length of the palate. Haughton and
Brink (1954) famously listed 111 species of
Dicynodon, a number whittled down by King (1988)
(a) (b) (c)
Dicynodon Kwazulusaurus Lystrosaurus
Figure 3.14 Triassic dicynodonts. (a) Dicynodon (King 1990). (b) Kwazulusaurus shakai (Maisch 2002). (c) Lystrosaurus declivis (King 1988).
(d) Skeleton of Kannemeyeria (after Pearson). (e) The shansiodontine Tetragonius njalilus (King 1988, from Cruickshank). (f) Stahleckeria
(King 1990 after Camp). (g) Ischigualasto jenseni (King 1988, after Cox). Continued overleaf