The Origin and Evolution of Mammals - Moodle

62 THE ORIGIN AND EVOLUTION OF MAMMALS
have a more complex form. There is a single large,
main cusp, at the inner base of which is a row of
small, but very distinct cuspules. These teeth
represent the first step towards the complex multicusped,
occluding molar teeth that were eventually
to play such a fundamental role in the radiation of
mammals. At this stage, however, the upper and
lower teeth did not even occlude directly, but
appear merely to have increased the general tearing
effect as they dealt with food such as insect cuticle
(Fig. 3.19(c)). Several features of the skull and lower
jaw indicate that there had been further elaboration
of the jaw musculature beyond the stage found in
the therocephalians, although still to a degree well
short of advanced cynodonts and mammals. The
temporal fenestra had expanded, both medially to
the extent that there is now a deeper, vertical-faced
sagittal crest, and laterally to create an outwardly
bowed zygomatic arch. The latter is nevertheless
still a relatively delicate structure. The lower jaw is
comparably modified. The coronoid process has
become broadened, and bears a depression on its
lateral surface that indicates the first stage of the
invasion of the lateral surface of the jaw by adductor
musculature. The dentary being relatively
enlarged in this way, the postdentary bone complex
is relatively reduced in size, a trend to be continued
right up to the mammals. Other changes in the
skull of Procynosuchus include the development of a
secondary palate in the roof of the mouth, although
at this stage the paired extensions of the maxillae
and palatals do not meet in the midline so the
osseous secondary palate is incomplete; presumably
it was completed by soft tissue.
Despite these considerable modifications to the
skull, the postcranial skeleton of Procynosuchus has
progressed little beyond a basic therapsid condition
(Fig. 3.19(d)). There is certainly very clear differentiation
of the vertebral column into thoracic and
lumbar regions, but the limb girdles illustrate the
cynodont characteristics to only a small degree. In
fact, the Zambian specimen (Kemp 1980c) shows
unexpected specialisations indicating a semiaquatic,
perhaps otter-like mode of life. The zygapophyses
of the lumbar region of the vertebral column are
extraordinarily broad and horizontally oriented.
Uniquely among therapsids, they must have permitted
very extensive lateral undulation of the hind
region of the vertebral column. The tail is relatively
long, with uniquely expanded haemal arches that
suggest a laterally operated swimming organ. The
limbs themselves are relatively short and the individual
bones stout. The hind foot is not preserved,
but the forefoot is broad and the individual bones
flattened and poorly ossified. The only other postcranial
skeleton attributed to Procynosuchus was
described rather superficially by Broom (1948, as
the genus Leavachia). This specimen lacks these
highly distinctive specialisations of Procynosuchus
and may actually belong to a different genus, one
that was adapted for a fully terrestrial existence.
Dvinia
The only other cynodont of the same primitive grade
as Procynosuchus is Dvinia (Fig. 3.19(b)) from the
Russian Late Permian (Tatarinov 1968), for it also has
a relatively small coronoid process and adductor
fossa of the dentary, and relatively large postdentary
bones. Indeed, it is even more primitive than
Procynosuchus in certain details of its braincase such
as a very limited contact between the prootic and the
epipterygoid bones and a large post-temporal fossa
(Hopson and Barghusen 1986). At the same time,
however, Dvinia has much more elaborate molariform
teeth. In both upper and lower jaws, there is a
series of small, simple premolariform teeth, followed
by the molariform teeth, each of which is
expanded across the jaw so that the crown surface is
roughly circular, and bears a complete ring of cuspules.
Dvinia was similar in size to Procynosuchus
and its diet can only be speculated upon. Perhaps it
was an early cynodont herbivore, although certainly
unrelated to the advanced herbivorous members of
the group.
Basal epicynodontia—middle-grade cynodonts
All other cynodonts have progressed beyond the
procynosuchian grade by enlargement of the coronoid
process, and development of a more robust,
and dorsally bowed zygomatic arch. There is a further
reduction in the size of the quadrate, and the
number of incisor teeth is reduced to four uppers
and three lowers. The most distinctive feature of the
postcranial skeleton is the presence of overlapping
costal plates on the dorsal ribs.