The Origin and Evolution of Mammals - Moodle

40 THE ORIGIN AND EVOLUTION OF MAMMALS
specimens classified as Dromasauria from the South
African Karoo (e.g. Kemp 1982; King 1988). The picture
has since improved significantly on the basis of
the discovery of South African specimens along with
new and newly described Russian material.
Anomocephalus
Anomocephalus (Fig. 3.11(a)) is the most basal
anomodont so far known, although not the oldest,
having been discovered recently in the Tapinocephalus
Assemblage Zone of the South African Karoo
(Modesto et al. 1999). Unfortunately, as it is based so
far only on a single, incomplete skull, the full potential
of this genus for helping understand anomodont
evolution has yet to be realised.
It is quite large for an early anomodont, with a
skull length of around 20 cm. Numerous primitive
characters have been retained, such as a preorbital
length which, while short compared to other
therapsids, is still relatively long at about 45% of
the total skull length. Of particular interest, the part
of the squamosal bone forming the zygomatic arch
is still short and rod-like rather than horizontally
expanded for the origin of extra adductor jaw musculature,
which indicates that there had been rather
little development towards the highly characteristic
dicynodont arrangement of the jaw musculature.
Nevertheless, the bowed zygomatic arch and the
coronoid eminence of the lower jaw do indicate a
modest degree of enlargement of the lateral part of
the adductor mandibulae musculature. The dentition
of Anomocephalus consists of a row of about
eight teeth on either side, upper and lower. The
individual teeth are large, peg-like and decrease in
size regularly from front to back of the row. Each
individual tooth increases in diameter towards its
tip, and has a flat, sloping wear facet on its crown
indicating a masticating function. The dentition as
a whole appears to be adapted for a relatively soft
herbivorous diet.
Patronomodon
In 1990, Rubidge and Hopson reported a small
anomodont, Patronomodon (Fig. 3.11(b)), from the
South African Eodicynodon Assemblage Zone,
which makes it slightly older than Anomocephalus,
notwithstanding its more derived structure. It is
represented by a well-preserved, 6 cm long skull,
with lower jaw and partial postcranial skeleton
(Rubidge and Hopson 1990, 1996). Primitive features
include the failure of the premaxilla to meet
the palatine in the palate, and the absence of a fossa
indicating extension of the adductor mandibuli
musculature onto the lateral surface of the
squamosal. Perhaps most significantly, the jaw
articulation did not permit the movement of the
lower jaw backwards and forwards, the property of
propaliny that is so distinctive a part of the jaw
apparatus of other anomodonts. The dentition of
Patronomodon consists of much smaller teeth than in
Anomocephalus. In the upper jaw there are probably
at least three premaxillary and up to seven maxillary
teeth, all about the same size and forming a
continuous row. The dentary has about six similar
teeth. In both upper and lower jaws, the teeth lie
medial to the jaw margin, with a greater medial
than lateral exposure and yet there is no indication
on the bony surfaces of the development of horny
tooth plates alongside the tooth rows. King (1994)
on the basis of the dentition, the absence of
propaliny, and the primitive, relatively undivided
nature of the external adductor mandibulae musculature
concluded that Patronomodon was not herbivorous,
but subsisted on a generalised carnivorous
diet. Given the skull size of only about 6 cm,
this would imply a diet of insects and other small
invertebrates.
Venyukovioidea
There are several genera of primitive anomodonts
from late Kazanian or early Tatarian deposits of
Russia, the taxonomy of which has until recently
been confusing (Ivakhnenko 1996). Venyukovia prima
was described by Amalitzky in 1922 on the basis
of a left dentary and an isolated jaw symphysis,
found in an unknown locality within the Copper
Sandstones. Subsequently, Efremov (1938) attributed
an incomplete skull and jaw fragments from the
Early Tatarian Isheevo locality to the same genus, as
Venyukovia invisa, and Tchudinov (1983), referring to
new material from Isheevo, synonomised the two
species. Ivakhnenko (1996) reviewed the material
and concluded that all the Isheevo specimens are
sufficiently distinct from Amalitzky’s original specimens
for them to require a new generic attribution,
Ulemica invisa (Fig. 3.11(e)), leaving Amalitsky’s