The Origin and Evolution of Mammals - Moodle

near parasagittal gait (Fig. 5.11(e)), with the elbow
turned back, the forefeet lying under the body, and
the whole limb moving in a parasagittal plane. The
principal reasons they gave for their interpretation
were the reduction of the coracoids, the posteroventral
position of the glenoid and its small size relative
to the head of the humerus, and the mobile
clavicle–interclavicle joint, all of which are matched
in a modern non-cursorial therian such as Didelphis.
They were not explicit about whether their proposed
mode of action of the forelimb is compatible
with Jenkins and Krause’s (1983) arboreal hypothesis,
but presumably it is not.
A third possible mode of locomotion was proposed
by Kielan-Jaworowska and Gambaryan (1994;
Gambaryan and Kielan-Jaworowska 1997). At least
as far as the Asian genera they looked at, such as
Nemegtbaatar, are concerned (Fig. 5.11(a)), they
argued against a parasagittal mode and in favour of
a sprawling mode in which both the humerus and
femur are held horizontally, at an angle of around
60 o to the animal’s sagittal plane, so that both the
elbow and knee are out to the side. They envisage a
form of locomotion similar to, though not exactly
matched by modern ricochetal mammals. It consisted
of a leaping mode in which the hind limbs
acting synchronously create an upward and forward
thrust, while the forelimbs, similarly acting
together, absorbed the energy of the animal’s landing
prior to the next leap. According to their account,
most of the unique anatomical features of the multituberculate
skeleton can be explained by this form
of progression. The relatively vertical pelvic girdle,
the lateral orientation of the femur, and the form
of the musculature as reconstructed by them indicate
that thrust was produced only to a limited extent by
normal mammalian retraction of the femur. Much
of it was generated by adduction, a drawing inwards,
of the femora, coupled with extension at the knee
and ankle joints, in somewhat frog-like fashion. On
landing, the forelimb began in an outstretched position,
the humerus partially adducted and the forearm
extending antero-laterally. Absorbtion of the
kinetic energy of the leap occurred as the distal ends
of the humeri were forced upwards. Towards the
end of this phase, the elbow joint extended restoring
the height of the front of the animal between its
forelimbs and preparing it for the next propulsive
THE MESOZOIC MAMMALS 159
phase of the hindlimb. Kielan-Jaworowska and
Gambaryan’s interpretation of locomotion in multituberculates
would presumably only apply to rapid,
or perhaps predator avoidance locomotion. No
doubt these animals were also capable of slow
leisurely ambulation, and it seems likely that a more
conventional walking mode with the legs acting
successively occurred as well.
Yet another mode of locomotion is found in the
Late Eocene Asian genus Lambdopsalis, which has a
number of indications of a fossorial mode of life
(Kielan-Jaworowska and Qi 1990; Meng and Miao
1992). The head is broad and flattened, some of the
cervical vertebrae fused, and the humerus massive
with a huge condyle for articulation with the as yet
unknown ulna (Fig. 5.11(d)).
It is thus evident that the multituberculates
evolved a range of locomotory adaptations. That
there should be arboreal, parasagittal, ricochetal,
and fossorial types adds yet further to the view that
they were the Mesozoic ecological equivalent of the
latter day rodents.
One of the initially unexpected discoveries about
multituberculates in recent years has been their
possession of true ear ossicles detached from the
dentary bone (Fig. 5.12(b)). Meng and Wyss (1995)
described the bones of the Chinese form Lambdopsalis,
showing a close similarity to the corresponding
elements of modern mammals. The angular bone
forms an ectotympanic for support of the tympanic
membrane, and the articular and prearticular are
fused to form the malleus. Even the position of the
groove for the chorda tympani nerve is the same.
Of the modern mammals, the similarity is greatest
to the monotremes, because the ectotympanic and
therefore the ear drum is oriented horizontally
rather than vertically. Hurum et al. (1996) confirmed
this detailed similarity to modern mammals in the
Mongolian Chulsanbaatar, although suggesting that
the ectotympanic and tympanic membrane were
less horizontally inclined than in Lambdopsalis.
The quality of hearing has been assessed by Meng
and Wyss (1995), who suggested that the uncoiled
cochlea and the inflated vestibule of multituberculates
indicate rather poor reception of highfrequency
air-borne sound, and that conduction of
low-frequency sound through the bones of the
skull was important.