The Origin and Evolution of Mammals - Moodle

38 THE ORIGIN AND EVOLUTION OF MAMMALS
fenestra has not been secondarily reduced in size by
expansion of the surrounding bones, as occurs in
the more advanced, tapinocephalid dinocephalians.
The incisor teeth bear very well-developed heels,
and when the lower incisors interdigitate between
the uppers, opposing edges of the heels contact.
According to Kemp (1982), the effect of this would
be to create a continuous slit in the food, rather than
simply punch a line of holes, so that tearing up the
food would be more effective. The titanosuchid
postcanine dentition is very distinctive. There are
numerous, small, leaf-shaped teeth that bear serrated
edges and although the uppers do not make
direct contact with the lowers when the jaws closed,
they would presumably have a good grasping function,
perhaps of plant material. The diet is not clear
but it may be assumed to have been quite catholic.
The Tapinocephalidae are the second family of
titanosuchians and are the specialist herbivores
(Fig. 3.9(f)). The canine is no larger than the incisors,
and the interdigitation of upper and lower
teeth extends to the whole tooth row. The way the
teeth worked was quite elaborate, as inferred by
Efremov (1940) in Ulemosaurus. The size of the
crown was reduced but the heel is elongated in a
lingual direction to form a short, wide shelf. The
side edges of the heels continued to have a cutting
action as in titanosuchids and the lingual elongation
of the heel increased the length of cutting edge
available, permitting the animal to cut up vegetation
into finer pieces. In addition, the tapinocephalid
incisors had a degree of crushing or grinding ability
both where the blunt points of the crowns met
the opposing heels and perhaps also between the
outer faces of the lower incisor against the lingual
face of the uppers. The incorporation of the postcanines
into the cutting tooth row would have
considerably increased the rate of cropping of the
vegetation consumed, although it must have been
quite soft, leafy material.
The extremes of pachyostosis are found in the
tapinocephalids, where the thickness of the skull
bones increases to a remarkable extent. Barghusen
(1975) found that the frontal and parietal bones in a
32 cm long skull of Moschops are no less than 11.5 cm
thick. The bones surrounding the temporal fenestra
also increased in massiveness, secondarily reducing
the size of the fenestra and increasing the intertemporal
width. Barghusen (1975) showed how the
pachyostosis was designed for the habit of head
butting. The occipital condyle lies relatively far forwards
on the ventral side, so the skull would habitually
have been carried with the nose pointed
downwards and the thick intertemporal region facing
forwards. Thus head-to-head ritual, competitive
contact between individuals would occur over
this highly strengthened area of the skull.
There are several genera of tapinocephalids,
including Tapinocaninus, the earliest Titanosuchian.
They differ in the shape of the skull, the extent of
the widening of the intertemporal region and the
degree of pachyostosis. Avenantia is relatively primitive
for it has retained a narrow intertemporal roof
with an area for external muscle attachment still
exposed, and the degree of pachyostosis is slight.
Moschops, and the very similar Russian Ulemosaurus
have a greatly reduced temporal fenestra and correspondingly
wide intertemporal width, coupled
with very heavy pachyostosis. Riebeekosaurus combines
a long, low snout with a narrow intertemporal
region but only moderate pachyostosis. The
most highly evolved genus of all is Tapinocephalus
itself, with its low, broad snout, enormously wide
intertemporal width, and massive pachyostosis.
Styracocephalus (Fig. 3.10) has been a problematic
genus for it combines the tapinocephalid features of
pachyostosis and reduced temporal fenestra with
such primitive features as a small but distinct
canine and a cluster of palatal teeth. It is also
unique in possessing a pair of bony bosses above
the orbits and a pair of possibly horn-bearing protuberances
at the back of the skull. Earlier workers
regarded Styracocephalus as a relative of the brithopian
Burnetia, while King (1988) concluded tentatively
that it is was some kind of dinocephalian. With the
help of some new material, Rubidge and Van den
Heever (1997) confirmed the latter interpretation
and their cladistic analysis placed it as the sistergroup
of the rest of the Titanosuchia.
Estemmenosuchia
The dinocephalians considered thus far form a
coherent group with no serious controversy about
the interrelationships between the various members.
The one major problem concerns the nature of
Estemmenosuchus (Fig. 3.7(g)). This exclusively
Russian form appears in the Ocher/Isheevo fauna as
part of the early radiation of therapsids. It is a large