The Origin and Evolution of Mammals - Moodle

Cynognathidae
Cynognathus (Fig. 3.21(a)) occurs in the Lower to
Middle Triassic of South Africa and is sufficiently
common to have given its name to the Cynognathus
Assemblage Zone. Specimens of the genus have also
been found in beds of the same age in Argentina
and Antarctica (Hammer 1995). It is a large form
with a robustly built 30 cm long skull and strongly
carnivorous dentition. The canines are large and the
postcanines blade-like with a recurved main cusp.
The latter lack cingulum cuspules around the base
but there are sharp accessory cusps in line with the
main cusp, increasing in prominence on teeth from
the front to the back of the postcanine tooth row.
The way in which the postcanine teeth wear indicates
that there was no direct, precise occlusion
between uppers and lowers, but a more general
opposition between the two rows of teeth. While
the lower jaw has the fully expressed eucynodont
structure, the temporal fenestra is quite short. The
hind wall of the fenestra, forming the occiput, does
not have the deep embayment between itself and
the root of the zygomatic arch that is found in other
eucynodonts, possibly an adaptation for extreme
carnivory where the major jaw-closing muscle
would be a postero-dorsally oriented temporalis.
Diademodontoidea
The diademodontoid eucynodonts include a range
of genera characterised by expanded postcanine
teeth that engaged in precise tooth-to-tooth occlusion,
a design taken to indicate a herbivorous diet
even though well-developed canine teeth are also
retained. Diademodon (Fig. 3.22(a)) was a contemporary
of Cynognathus in the Lower to Middle Triassic.
It has simple incisors, sizeable canines, but a
remarkably specialised postcanine dentition. There
are three morphologically distinct kinds of postcanine
teeth (Fig. 4.12(a)). The anterior three or four
consist of simple, conical crowns. Following these
there are up to nine, depending on age, transversely
widened, multicusped teeth described as gomphodont
(Fig. 3.22(b)). The crowns of the uppers are
broadly expanded across the line of the jaw. The
single main cusp occupies the centre of the outer
edge of the tooth and sharp crenulated ridges run
backwards and forwards from its apex. The other
three margins of the occlusal surface are marked by
EVOLUTION OF MAMMAL-LIKE REPTILES 67
a series of small cuspules. The lower gomphodont
teeth are less expanded, but morphologically similar
to the uppers. These detailed features are only
seen in freshly erupted, unworn teeth. Tooth wear
was rapid, removing the thin layer of enamel and
reducing the teeth to more or less featureless pegs;
each smaller lower tooth bit within the basin
formed between two adjacent larger upper teeth, in
a mortar-and-pestle fashion. Behind the gomphodont
teeth there are between two and five sectorial
teeth not unlike those of Cynognathus. The
recurved main cusp is transversely flattened and
bears an anterior and a smaller posterior accessory
cusp. Comparing a range of Diademodon skulls of
different sizes and therefore presumably different
ages reveals a remarkable pattern of tooth replacement.
Teeth are successively lost from the front of
the tooth row, and new ones added at the back. In
order to retain the same numbers of the three kinds
of postcanine teeth, the middle, gomphodont type
teeth are shed from the front backwards and
replaced by anterior-types. Meanwhile, the posterior,
sectorial types are replaced by middle types. The
teeth added at the hind end are posterior types.
A number of other members of the family
Diademodontidae occur, notably Trirachodon which
differs in lacking the simple type of anterior postcanines,
and in the cusp pattern of the gomphodont
teeth. Both the uppers and the lowers have a transverse
row of three cusps across the middle of the
occlusal surface, of which the central one is the
largest. A row of cuspules occupies both the front
and the hind edges of the crown. Middle Triassic
trirachontines occurred widely, having been discovered
in Africa, China, Russia, India, and possibly
North America.
The second diademodontoid family Traversodontidae
(Fig. 3.22(c)–(f)) are related to diademodontids
but have a simpler dentition insofar as both the simple
anterior type teeth and the sectorial posterior
type teeth are lacking. However, the remaining
middle type of gomphodont postcanines are more
elaborate, bearing an enlarged transverse crest
across the occlusal surface. In the case of an upper
tooth it lies well towards the posterior end of the
tooth, and in a lower, close to the anterior edge.
Crompton (1972a) analysed the mode of action of
traversodontid teeth in detail, showing how a