The Origin and Evolution of Mammals - Moodle

a self-sharpening property. As the dentine wears
away at a higher rate than the enamel, the latter
remains as a sharp, cutting edge at the front of the
tooth. The glenoid fossa of the jaw articulation in
these forms is extended forwards increasing the
maximum anterior shift of the lower jaw. Their
upper and lower incisors could therefore come into
direct contact for active biting, unlike those of
Ptilodus. The lower premolar teeth are smaller than
those of ptilodontoids, confirming that the premolars
had to some extent given way to the incisors
in the initial preparation of the food, possibly
indicating a diet of seeds that required de-husking
prior to mastication.
Several practically complete and many partial
postcranial skeletons and isolated bones of multituberculates
are known (Fig. 5.11). The skeleton
shows a number of similarities to that of modern
therian mammals, but at the same time there are
several quite different features. This has led to a
divergence of views both about the functional
anatomy, and about how far the postcranial skeleton
does or does not support a relationship
between multituberculates and therian mammals.
In the forelimb, the scapula is narrow with an outturned
anterior spine which, as interpreted by
Sereno and McKenna (1995), means there was no
development of a supraspinous fossa. However,
Kielan-Jaworowska and Gambaryan (1994) regard
the antero-medial face of the spine as an incipient
supraspinous fossa, onto which the equivalent of
the supraspinatus muscle already attached. The
coracoid is very reduced as in therians, but remains
as a distinctive, pointed process. The glenoid cavity
faces more ventrally than laterally, and is only
about half the diameter of the bulbous, upturned
head of the humerus. Clavicles and interclavicles
are present. According to Sereno and McKenna
(1995) the elbow joint has a trochlea form similar to
that seen in modern therians, and which restricts
movements to a hinge action about a single axis. In
Nemegtbaatar, by contrast, Kielan-Jaworowska and
Gambaryan (1994) described the condyles on the
end of the humerus for both the radius and the ulna
as convexities, and the radial condyle as spherical.
From this they infer that the elbow was capable of
a wider variety of movements than just strictly
hinging about one axis.
THE MESOZOIC MAMMALS 157
The pelvis and hindlimb are also distinct from
other mammals in several respects (Krause and
Jenkins 1983; Kielan-Jaworowska and Gambaryan
1994). The narrow iliac blade attaches at a relatively
very high angle to the line of the sacral vertebrae,
while the pubo-ischiadic plate extends deeply
below the acetabulum. The acetabulum itself is
open dorsally. A pair of epipubic bones is present,
although this may be a primitive feature of mammals
generally. The articulating head of the femur
is on the end of a distinct neck that is set at an angle
to the shaft of the bone. A very prominent
trochanter major extends well proximal to the head,
and a smaller trochanter minor lies on the underside,
where the neck and trochanter major meet.
There is no sign of a third trochanter. The tibia is
unusual in being wider from lateral to medial surfaces
than from anterior to posterior, and both tibia
and fibula possess a hook-like lateral process on the
proximal end of the bone.
There are considerable differences in the interpretation
of locomotor function in multituberculates.
Jenkins and Krause (1983) argued for an
arboreal mode of life in Ptilodus, on the basis of the
structure of the pes, and the tail (Fig. 5.11(c)).
Within the ankle, the joint between the calcaneum
and the astragalus permitted extensive flexionextension.
The joint between the distal end of the
tibia and the calcaneum consisted of a pair of facets,
which permitted the calcaneum to abduct, causing
rotation of the distal end of the pes laterally and
then posteriorly to the extent that the foot could
point almost backwards. They also recognised the
possibility of extensive, independent movement of
the first digit. Together, these movements would
have allowed the foot to be turned backwards and
to gain purchase while the animal descended head
first down a tree. The tail is very long, possesses
well-developed haemal arches on the caudal vertebrae,
and there are long neural spines on the sacral
vertebrae from which powerful tail muscles could
have originated, all of which indicate a prehensile
tail.
A second proposed mode of locomotion in multituberculates
arises from Sereno and McKenna’s
(1995) description of a complete, articulated forelimb
of the djadochtatherioid Bulganbaatar. They
argued that its structure indicates a fully therian