The Origin and Evolution of Mammals - Moodle

a mixture of progressively more specialised herbivores.
The enigmatic Russian Estemmenosuchus, usually
included may well not be a dinocephalian at all.
Brithopia
Brithopians (Fig. 3.9(a) and (c)) are the least modified
and therefore most pelycosaur-like dinocephalians.
They constitute the most abundant group of the earliest
Late Permian therapsid fauna of Russia, and
there are some superbly preserved, complete specimens.
The Isheevo form Titanophoneus is a large
animal, up to 2 m in length. The canine teeth,
both upper and lower are much larger than the rest
of the dentition, although the intermeshing incisor
teeth are also well developed. The postcanine dentition
on the other hand is reduced in both number
and individual size of the teeth. The temporal
fenestra illustrates the primitive dinocephalian
condition. It is enlarged, mainly in a dorso-ventral
direction so that the lower temporal bar is depressed,
while the area available for muscle attachment
extends onto the dorsal surface of the parietal bone
and also the posterior-facing surface of the postorbital
bone immediately behind the orbit. Seen
from above, the intertemporal bar has become quite
narrow. Evidently the adaptation for carnivory in
Titanophoneus consisted of a dorso-ventrally elongated
adductor musculature capable of producing
a rapid bite, the energy of which would have been
dissipated by the anterior dentition. The heels on the
incisor teeth are small and would have assisted in
the use of the incisors crudely to tear off pieces of the
prey’s flesh. The postcranial skeleton of Titanophoneus
is also a primitive version of the therapsid condition.
The limbs are relatively long and slender, the shoulder
joint simplified, the ilium enlarged, and the
femur S-shaped as in other therapsids. However,
such pelycosaurian features as undifferentiated dorsal
vertebrae and a very long tail are retained. Several
other Russian brithopian genera have been known
for several decades, but only recently have members
of the group been discovered elsewhere. A Chinese
specimen was described by Jinling et al. (1996),
and Australosyodon (Rubidge 1994) is known from a
single, badly crushed skull from the lowest fossiliferous
part of the South African Beaufort sequence,
the Eodicynodon Assemblage Zone. The overlying
Tapinocephalus Assemblage Zone of South Africa
EVOLUTION OF MAMMAL-LIKE REPTILES 37
contains the more specialised brithopian Anteosaurus
(Fig. 3.9(e)). This is a massive animal, with a skull
length up to 80 cm, composed of heavy bones and a
marked boss on the postorbital. The whole postorbital
region of the skull is deepened, creating space
for an enlarged adductor musculature. The dentition
is similar to that of other brithopians except for a
further reduction in the postcanine dentition.
Titanosuchia
The titanosuchians are large, heavily built forms
that constituted the dinocephalian radiation and
dominance of the terrestrial fauna during the
Tapinocephalus Assemblage Zone of South Africa.
The earliest form, Tapinocaninus, actually occurs in
the Eodicynodon Assemblage Zone (Rubidge 1991).
A small number of closely related Russian genera
occur in contemporaneous deposits. They evolved
varying degrees of adaptation towards herbivory,
involving reduction of the canines, increase in the
size of the heel at the base of the incisor teeth, and an
increase in the number of postcanine teeth to as many
as twenty. The jaw articulation shifted forwards to
reduce the length of the jaws to an even more marked
extent than occurs in brithopians. Like Anteosaurus
among the latter, the thickness of the cranial bones
increased to a dramatic extent. As befits gigantic
animals, the postcranial skeleton is modified for
weight bearing in several respects (Fig. 3.9(b)). The
vertebrae are short and wide. The shoulder girdle is
massively constructed, the humerus has large, flattened
ends, and the radius and ulna are similarly
broad, flat bones. The pelvic girdle is relatively short
but tall, and again the hindlimb bones very broad.
Unexpectedly, the dinocephalians have independently
acquired the mammalian digital formula of
2.3.3.3.3. (Hopson 1995). No detailed analysis of the
mechanics of locomotion of dinocephalians has been
made. Kemp (1982) claimed that the forelimb was
probably still used in a sprawling fashion, but that
the hindlimb was at least capable of a more erect gait
as interpreted in other therapsids. In fact, given the
size of these animals it is probable that a more or less
fully erect gait was obligatory.
The members of the family Titanosuchidae,
namely Titanosuchus and Jonkeria (Fig. 3.9(b)), are the
least modified of the titanosuchians. The canines,
though reduced, are still distinct, and the temporal