The Origin and Evolution of Mammals - Moodle

mandibulae musculature. This, as in the dicynodonts,
would have provided the necessary posteriorly
directed force on the mandible during occlusion
(Rybczynski 2001). The form and evident action of
the teeth indicate a committed herbivorous animal.
Dromasauroidea
There are three genera of small, primitive anomodonts
from the Late Permian of South Africa generally
included in a taxon Dromasauria. They are
represented by a total of four specimens, each one of
which is preserved as a natural sandstone cast of the
skull and postcranial elements (Brinkman 1981).
Galeops (Fig. 3.11(d)) shows features otherwise
restricted to the dicynodonts, leading to the view
that among the dromasaurs it is the dicynodont sistergroup
(Modesto and Rybczynski 2000; Rybczynski
2000). The characters in question include further
shortening of the preorbital region of the skull,
absence of anterior teeth on either premaxilla or dentary,
and the remaining posterior dentition reduced to
a short row of small, peg-like teeth. The arrangement
implies that dicynodont-like horny tooth plates had
evolved at the front of the jaws. Other features of
dicynodonts that are found are fusion of the paired
dentaries at the symphysis, and a quadrate condyle
divided by a longitudinal groove into lateral and
medial condyles, corresponding to elongated articular
condyles. According to King (1994), the structure
of the jaw articulation would have permitted propalinal
movement of the lower jaw.
The preserved specimens of neither of the other
two genera of dromasaurs, Galepus and Galechirus
(Fig. 3.11(c)) are as revealing. They are much the same
size as Galeops, and look superficially very similar.
However, there are differences in the dentition which
justifies at least the generic distinction between
the three (Brinkman 1981). Galechirus possesses premaxillary
teeth, which are decidedly procumbent,
but the situation of the lower teeth is unclear. Galepus
has at least six small lower teeth, but in this case the
nature of the upper dentition is unclear.
Dicynodontia
The great majority of anomodonts belong to the very
well-defined group Dicynodontia, which makes its
appearance in the Eodicynodon Assemblage Zone
EVOLUTION OF MAMMAL-LIKE REPTILES 43
contemporary with the earliest of the primitive
African anomodonts described above. Indeed, it is
the eponymous Eodicynodon itself that first represents
the group (Rubidge 1984, 1990a).
Eodicynodon
The trends in cranial evolution seen in the primitive
anomodonts up to the stage represented by Galeops
reach their culmination in the fully dicynodont form
of Eodicynodon (Fig. 3.12(a)). All the incisor teeth are
absent, there is an enlarged caniniform upper tusk,
and its postcanine dentition is reduced to a small
number of relatively minute teeth. (Rubidge, 1990b,
described a specimen of Eodicynodon, which lacks the
caniniform tooth; he attributed it to a different
species). The triturating surface of the jaws consisted
virtually exclusively of a horny beak. Cox (1998)
concluded from the structure of the bony surfaces
that there were three parts to the beak. A medial
anterior dentary beak bit within the front margins
of the palate, presumably with a cropping function.
Further back, a pad on the dentary table acted against
a palatal pad medial to the caniniform process. Third,
a posterior horny blade on the dentary bit against a
palatine pad behind the caniniform process. In some
specimens the lower blade also carries a row of small
teeth. In order to operate these biting structures, the
external adductor musculature had a much enlarged
lateral component, originating from the horizontally
flattened and dorsally bowed zygomatic arch and
inserting at least partially on a lateral shelf on
the dentary. The temporal fenestra as a whole is
elongated, while the lateral pterygoid processes are
reduced. These point to the increased importance of
a posteriorly directed power stroke during the closing
phase of the jaw action, accompanied by a reduction
in the importance of the anterior pterygoideus
musculature with its anteriorly directed component
(Fig. 3.12(c)). The jaw articulation (Fig. 3.12(d)) has
the uniquely dicynodont structure of more or less
equal lateral and medial convex condyles on the
quadrate and corresponding but elongated articular
condyles on the lower jaw. King et al. (1989) believed
that the feeding mechanism of Eodicynodon had
achieved the essential dicynodont arrangement but
that the degree of propaliny was still less than
occurred in later dicynodonts, a view reiterated by
King (1994) and Cox (1998).