The Origin and Evolution of Mammals - Moodle

has figured large in discussions of early therapsid
evolution, being regarded as more advanced than
Biarmosuchus. Kemp (1982) and Hopson and
Barghusen (1986) both expressed as cladograms the
view that Biarmosuchus was more basal, and that
Eotitanosuchus was more closely related to advanced
carnivorous therapsids. In her review of primitive
carnivorous therapsids, Sigogneau-Russell (1989)
went so far as to place the two genera in different
Infraorders, which is to say the least astonishing.
At the other extreme, Ivakhnenko et al. (1997;
Ivakhnenko 1999) reviewed all the material of
Biarmosuchus and Eotitanosuchus and came to the
conclusion that the two are the same species, differing
only insofar as they are different growth
stages. The skull length of the type specimen of
Biarmosuchus tener is about 16.5 cm while that of
the type specimen of Eotitanosuchus olsoni is over
twice the length, at about 34.5 cm. Other specimens
from the same locality lie between these extremes
and illustrate a morphological transformation.
Whether they really are conspecific may be debated,
but there can be little doubt that they are extremely
similar and certainly should be treated as congeneric,
with Eotitanosuchus a junior synonym of
Biarmosuchus.
Li and Cheng (1995) have described the Chinese
genus Biseridens as a member of the group, but no
biarmosuchians have yet been found in the
South African Eodicynodon Assemblage Zone.
However, there are specimens from later dates in
South Africa that are included in the group
(Sigogneau-Russell 1989; Hopson 1991). Hipposaurus
is from the Tapinocephalus Assemblage Zone, which
lies immediately above the Eodicynodon Assemblage
Zone, and for which the skull and most of the
skeleton has been described (Boonstra 1965;
Sigogneau 1970, 1989). It has retained the primitively
convex dorsal outline of the skull, relatively small
temporal fenestra, broad intertemporal region and
dentition of simple sharp teeth dominated by a large
canine found in Biarmosuchus. A small number of
minor derived characters shared between the two
have been noted by Hopson (1991), including details
of the zygomatic arch and fusion of the fourth and
fifth tarsals of the hind foot. There are several other
South African biarmosuchians, including Ictidorhinus
(Fig. 3.8(b)), which occurs in the Dicynodon
EVOLUTION OF MAMMAL-LIKE REPTILES 33
Assemblage Zone at the very end of the Permian.
A well-preserved skull of Lemurosaurus (Fig. 3.8(c))
has been described recently by Sidor and Welman
(2003) and confirms the presence of interdigitating
incisor teeth and a sclerotic ring in the group.
The family Burnetiidae was erected originally
for two genera, one South African and the other
Russian. Burnetia from the Dicynodon Assemblage
Zone of South Africa, and Proburnetia (Fig. 3.8(d))
from the Late Tatarian of Russia are extremely similar
primitive carnivores. They are relatively small
therapsids, with a skull length around 20 cm and
are characterised by heavily pachyostosed skulls
bearing numerous bony protuberances and bosses.
Taxonomically burnetiids were at one time linked
to dinocephalians on the basis of the pachyostosis,
and at another to gorgonopsians on the basis of
their carnivorous dentition and generally primitive
skull. However, appreciation of the absence of any
discernable derived characters between burnetiids
and gorgonopsians led Sigogneau-Russell (1989) to
consider them as biarmosuchians. With the assistance
of new South African specimens, Rubidge
and Sidor (2002) endorsed the latter view, and a
cladogram of all the biarmosuchians and burnetiids
by Sidor and Welman (2003) points to Lemurosaurus
as the sister group of Burnetiidae. This particular
genus has incipient bosses on various parts of the
skull suggesting an early stage in the full development
of the burnetiid pachyostosis.
Dinocephalia
The dinocephalians (Fig. 3.9) are a group of
large, heavily built animals that are among the
first therapsids to appear in the fossil record in
Russia, South Africa, and China, at the beginning
of the Late Permian. The very doubtful possibility
that fragmentary remains of Guadalupian age in
North America may be dinocephalians has been
mentioned earlier and isolated teeth have been
found in Brazil (Langer 2000). After a brief period
as the commonest therapsids, dinocephalians disappeared
well before the close of the Permian, not having
been discovered later than the Tapinocephalus
Assemblage Zone of South Africa or its equivalent in
Russia. Historically, the carnivorous and herbivorous
dinocephalians were separated into two groups,