The Origin and Evolution of Mammals - Moodle

(a) (d) (c)
(b)
Leontocephalus
V
PAL
Kemp (1969b) analysed the functioning of the jaws
in detail and showed from wear facets preserved on
the opposing incisor and canine teeth of a spectacularly
well-preserved specimen of Leontocephalus that
the lower jaw was capable of two modes of action.
The simplest one consisted of opening the jaws
extremely widely, by as much as 90�, to be sufficient
for the huge upper and lower canines to clear one
another enough. The lower jaw was then powerfully
adducted so that the sharp, serrated-edged canines
would sink into the prey to disable it. The second
mode of bite was more precise and consisted of
a propalinal shift forwards of the lower jaw, so that
now when it closed the four serrated lower incisors
on one side passed between the five similarly
PT
a.pt.
2
1
3
EVOLUTION OF MAMMAL-LIKE REPTILES 53
(e)
Arctognathus
refl.lam.
ext.add.mand
ext.add.zyg.
Figure 3.15 Gorgonopsian skull and jaw mechanism. (a), (b) Skull of Leontocephalus intactus in dorsal and ventral view. (c) Lateral view of skull
and lower jaw of Arctognathus. (d) Interdigitation of incisor and canine teeth from positions 1–3, and the action of opposing single upper and
lower incisors. (e) Ventral and lateral views of the disposition of the main adductor mandibuli musculature a.pt, anterior pterygoideus muscle.
ext.add.mand, external adductor mandibularis muscle. V, vomer. PAL, palatine. refl.lam, reflected lamina of the angular. PT, ptepygoid. (Kemp 1969).
serrated upper incisors (Fig. 3.15(d)). The effect of
the interdigitated incisor bite would have been to cut
a jagged edge in the flesh and therefore enable
chunks to be more readily torn off and swallowed.
To achieve the necessary antero-posterior movement
of the jaw, a mobile articulation between the
quadrate and the squamosal had evolved, whereby
the ball-shaped body of the quadrate bone rotated in
a socket-shaped squamosal recess about a transverse
axis. The effect was to permit the lower part of the
quadrate to shift forwards or backwards, which in
turn caused the whole lower jaw to move forwards
or backwards. By this means, the jaw could shift forwards
for the incisors to intermesh, or backwards
so that the incisors did not get in the way of