The Origin and Evolution of Mammals - Moodle

postcranial skeleton, and Graybeal et al (1989) on the
inner ear structure, the osteology of Morganucodon is
extremely well studied (Fig. 5.3(a)–(c)). It was a small
animal with a skull 2–3 cm in length and a presacral
body length of about 10 cm. In general appearance
it would have looked like a shrew or mouse.
The skull has all the superficial appearance of a
small, modern mammal characterised by the absence
of a postorbital bar, slender zygomatic arch, and
expanded braincase consisting largely of the parietal
bone above. The sidewall of the brain case is formed
by a prominent anterior lamina of the periotic behind
that is pierced for the V 2 and V 3 branches of the
trigeminal nerve, and an epipterygoid, or alisphenoid
in front. As in the non-mammalian cynodonts,
the latter bone is a broadened, vertical pillar from the
base of the skull right up to the roofing bones.
The lower jaw is also characteristically mammalian
in form, with fully developed coronoid,
angular, and articular processes of the dentary. However,
unlike most subsequent fossil and modern
mammals, there is a double jaw articulation
(Fig. 5.3(b)). The fully formed mammalian jaw joint
between a dentary condyle and a squamosal glenoid
fossa is visible laterally. Medially to, and supported
by the dentary and squamosal bones respectively,
lie the reduced but still easily recognisable articular
and quadrate hinge bones of the reptiles. Indeed, the
postdentary bones are altogether cynodont-like,
though reduced in size, and they lie in a medial
trough of the dentary. Crompton and Luo (1993)
have demonstrated the presence of a slender
reflected lamina closely attaching to the hind
edge of the angular process in a specimen of the
Chinese species Morganucodon oehleri (Fig. 4.9(h)).
Unfortunately it is not possible to be sure whether
or not or a retroarticular process of the articular was
also present, as predicted by Allin’s theory of the
evolution of the middle ear. Certainly there was a
large promontorium on the ventral side of the ear
region of the skull, housing the enlarged cochlea and
implying that Morganucodon had a significantly
enhanced hearing system (Luo et al. 1995).
The dentition (Fig. 5.3(b)) is what would be
expected in an animal living primarily on insects
and other small, easily dealt with invertebrates. The
cusps of all the teeth tend to be sharp, and the molariform
teeth are primarily adapted for a cutting or
THE MESOZOIC MAMMALS 143
shearing action rather than crushing and grinding.
There are usually four incisors in both the upper
and the lower jaw, followed by modest canines. All
of them have wear facets indicating that the upper
and lower front teeth actively worked against each
other in the course of food collection. There are five
upper and four lower premolars, each with a large,
laterally flattened and sharp main cusp, a smaller
posterior accessory cusp behind it, and practically
no trace of a cingulum.
The three molariform teeth in both upper and
lower jaw together formed a shearing system
designed to shred up food, as first analysed by
Crompton and Jenkins (1968). Upper and lower
molars have the same basic structure of a large, laterally
compressed, and sharp main cusp. Two posterior
accessory cusps and a single anterior accessory
cusp are also present, and lie along a line parallel to
the jaw. The upper molars possess a cingulum of
fine cuspules that completely surrounds the crown;
the lower molars differ in that their cingulum is
restricted to the inner, lingual edge of the crown, and
the individual cuspules are larger but less numerous.
The wear facets on the teeth show that accurate,
extensive occlusion occurred between the opposing
molar teeth. (Fig. 5.3(c)). The main cusp of a lower
molar worked against the gap between the main
cusp and the anterior accessory cusp of the corresponding
upper molar. At the same time, the main
cusp of the upper tooth worked against the equivalent
space between the main cusp and the posterior
accessory cusp of the lower. As a consequence of
the wearing effect of tooth-to-tooth contact, flat facets
with sharp free edges developed on the opposing
teeth that come to behave like the opposing blades
of a pair of scissors. Any resilient food caught
between them is liable to be sliced, or sheared into
two. As was described earlier, the essence of the
mammalian design of postcanine teeth relates to
the direction of movement of the lower molars during
occlusion. As one of them shears against its
opposing upper tooth it is forced to move very
slightly inwards as well as upwards, while retaining
contact. To achieve this, the lower jaw as a
whole must follow a triangular pathway as viewed
from behind: upwards and inwards during occlusion,
downwards as the jaws opened, and laterally
ready for the next bite. Only the molar teeth on one