The Origin and Evolution of Mammals - Moodle

156 THE ORIGIN AND EVOLUTION OF MAMMALS
(i)
(a)
(ii)
(c)
general abrasion of the teeth, including the more
anterior upper premolars and the fourth lower premolar
suggests that relatively large, quite tough
items were included, such as hard seeds. Furthermore,
by comparison with very small modern mammals,
a diet of leaves alone would be unlikely to have
been nutritionally adequate, and almost certainly a
mixture of seeds, tubers, insects, and worms would
have been necessary to maintain the relatively high
metabolic rates expected in such creatures.
Gambaryan and Kielan-Jaworowska (1995)
have reconstructed the jaw musculature of the
djadochtatherioids of Asia (Fig. 5.10(c)), which
(i)
(b)
mass.m.
(ii)
(iv)
temp.m
Figure 5.10 Jaw action of multituberculates. (a) Slicing action of premolars and (b) triturating cycle of molar teeth of Ptilodus (Krause 1982).
(c) Reconstruction of the adductor musculature of a djadochtatherioid (Gambaryon and Kielan-Jaworowska 1995). mass.m, branches of the
masseter musculature; temp.m, temporalis muscle.
(iii)
evidently had a similar mode of feeding action to
Ptilodus. Not surprisingly, the authors demonstrated
that the masseter muscle was complex, and
that its insertion on the lower jaw extended far
anteriorly compared to other mammals, giving it a
large vertically directed bite force. The coronoid
process of the dentary is well forward, so that the
temporalis muscle would have had an exaggerated
posterior component during chewing. One characteristic
of members of this group, and of the taeniolabidoids
is the restriction of the enamel layer to the
anteror-ventral surface of the lower incisor. As
in rodents and lagomorphs, this gives the tooth