The Origin and Evolution of Mammals - Moodle

To date the postcranial skeleton of Eodicynodon
is incompletely known (Rubidge et al. 1994).
However, it has been described in some detail for
the small, relatively unspecialised dicynodont
Robertia (King 1981b), which occurs in the
Tapinocephalus Assemblage Zone, immediately
overlying the Eodicynodon Assemblage Zone
(Fig. 3.12(b)). There is no reason to doubt that
Robertia had retained the ancestral form of dicynodont
postcranial skeleton, which remained relatively
conservative within the group (King 1988).
The first strikingly unique feature is the occipital
condyle of the skull, which is markedly trefoilshaped,
unlike the kidney-shape of other therapsids.
Kemp (1969a) showed that in dicynodonts it
was part of a complex joint involving the atlas and
the axis vertebrae that permitted rotation of the
head about a longitudinal and a transverse axis, but
by a mechanism sufficiently different in detail from
that found in other therapsids, to indicate that it
had an independent origin from the pelycosaur
state. Robertia has a full-length ribcage, with no tendency
to develop reduced, immobile lumbar ribs.
The tail is very short. The limbs are short compared
to therapsids generally, with humerus and femur
longer than the respective lower limb bones. On the
other hand, the limb girdles are relatively advanced,
in the sense of developing mammal-like characteristics
(see Chapter 4). The front edge of the slender
scapula blade is everted and there is a very welldeveloped
acromion process for attachment of the
clavicle. The coracoid plate below the acromion
process is reduced so that there is ample space for
expansion of the supracoracoideus muscle from its
primary site at the front of the coracoid to the anterior
part of the internal surface of the scapula. Thus
there appears to have been a precocious, independent
development of a mammal-like ‘supraspinatus’
muscle. The pelvic girdle has an enlarged and anteriorly
extended ilium coupled with reduction of the
pubis, indicating that the ilio-femoralis muscle had
expanded and was taking over the role of the presumably
greatly reduced caudi-femoralis muscle in
femoral retraction. Confirmation is offered by the
very distinctive form of the femur, which bears a
large trochanter major occupying about a third of
the shaft behind the head, but lacks any sign of a
fourth or internal trochanter. Finally, the feet are
EVOLUTION OF MAMMAL-LIKE REPTILES 45
also precociously evolved. They are very short and
the phalangeal formula is reduced to the mammalian
condition of 2 : 3 : 3 : 3 : 3.
Despite these superficially ‘advanced’ features,
the mode of locomotion appears to have been relatively
primitive. King (1981a, 1988) analysed in detail
the locomotory mechanics of the later form
Dicynodon trigonocephalus, and concluded that the
forelimb could not have operated in anything other
than a sprawling mode. The hind limb had a more
complex action, with elements of both a sprawling
and a more erect gait, but was not capable of
the simple parasagittal gait of other progressive
therapsids. The few adequately preserved fossil
trackways of dicynodonts generally confirm King’s
interpretation (Smith 1993; De Klerk 2003). They
show a plantigrade foot, with a wide forelimb trackway,
but a slightly narrower hindlimb trackway.
However, the dicynodont postcranial skeleton can
perhaps best be understood as adapted for digging,
basically to collect food lying just below the surface
of the ground, but in some specialised cases for
active burrowing.
The dicynodont radiation
The anatomy and functioning of the skull as seen
in Eodicynodon remained relatively conservative
throughout the subsequent evolutionary radiation
of the dicynodonts (Fig 3.13) although, as befits a
highly successful, diverse, and widespread group,
there is much detailed variation, associated with
different diets and habitats, in the size and shape of
the skull, the extent to which tusks and postcanine
teeth were retained or even elaborated, and the
form of the horny beak as inferred from the nature
of the bony surfaces of the jaws supporting them.
Increasing information about the diversity of the
postcranial skeleton is accumulating and can be
correlated with habitat, particularly in terms of
different degrees of digging and burrowing ability
(King 1988; Ray and Chinsamy 2003).
Biogeographically, the dicynodonts are far and
away best known as a consequence of the hundreds
of specimens recovered from the Late Permian and
Lower Triassic of the Karoo of South Africa and
other parts of southern Africa. However, by the
Triassic the group had a worldwide distribution,
occurring in all continents, including Myosaurus