The Origin and Evolution of Mammals - Moodle

morganucodontan teeth. However, despite the
highly modified dentition and its Upper Jurassic
age, the skull of Haldanodon proved to be rather
surprising in the primitive features it possesses.
These are relatively large articular and quadrate
bones and also stapes, a large orbital fissure, which
is the space anterior to the epipterygoid and posterior
to the interorbital septum, and a relatively large
septomaxilla bone in the snout. This led Lillegraven
and Krusat (1991) to argue that docodontans must
have diverged at a very early stage of mammalian
evolution, prior to the Morganucodonta and possibly
even the Sinoconodon lineages. However, the
more recent cladistic analyses, for example, Rougier,
Wible and Hopson (1996), and Luo et al. (2002;
Kielan-Jaworowska et al. 2004), find that Docodonta
are more derived than Morganucodonta, although
still occupying a basal position relative to all other
mammal taxa (Fig. 5.23).
Hadrocodium
Hadrocodium (Fig. 5.6) is represented by the almost
complete sub-adult skull of a tiny mammal from
the Lower Jurassic of the Lower Lufeng Formation
of Yunnan in China, the locality that has also
yielded Sinoconodon and the Chinese Morganucodon.
As described by Luo et al. (2001b), Hadrocodium
(a)
(b)
PM M 1
M 1
B
A
C
M 2
THE MESOZOIC MAMMALS 149
combines yet another suite of primitive and more
derived characters. The skull is a mere 1.2 cm in
length from which the body weight is estimated to
have been about 2 g, which compares closely with
the smallest of the living mammals. The dentition is
generally morganucodontan-like with the main
cusps of the molars in a linear row. However, unlike
morganucodontans there is no postdentary trough
in the dentary, which presumably indicates that the
postdentary bones and quadrate had lost their contact
with the dentary and become independently
attached ear ossicles. The promontorium marking
the housing of the cochlea organ is more prominent.
In two other important characters, Hadrocodium is
more advanced than morganucodontans. The jaw
articulation is relatively more anteriorly placed,
and brain is distinctly enlarged, even after making
allowance for the allometric relationship of brain to
body size in mammals. These are all features of
mammals that otherwise do not appear in the fossils
record for another 45 million years. Thus Hadrocodium
is an important form in reconstructing the pattern
of evolution of modern mammalian characters, combining
as it does a relatively primitive dentition with
a derived structure of the ear, brain, and braincase. It
suggests that convergent evolution of advanced
auditory characters occurred during the early radiation
of the group.
Figure 5.6 (a) Skull of Hadrocodium in lateral and dorsal views. Skull length approx. 1.2 cm. (b) Enlarged view of last upper premolar and first
molar in occlusion with first two lower molars (Luo et al. 2001a).