The Origin and Evolution of Mammals - Moodle

52 THE ORIGIN AND EVOLUTION OF MAMMALS
Stage, and all were by this time restricted to South
and North America (King 1988, 1990). There are surviving
stahleckeriines, plus one final new group
(Fig. 3.14(g)), represented by the North American
Placerias and the South American Ischigualastia (Cox
1991). These possessed large skulls around 50 cm in
length, and lacked tusks. The snout is relatively long
and the occiput very deep.
The recent description by Thulborn and Turner
(2003) of six associated cranial fragments collected
in 1914 from Queensland and about which Heber
Longman stated ‘some slight resemblance might be
traced to the Dicynodonts of South Africa’ is intriguing
in the extreme. One of the fragments consists of
a piece of premaxilla, with the stump of a large tusk,
that is indistinguishable from the corresponding
region of a kannemeyeriid dicynodont. The other
pieces are less diagnostic, but all are consistent with
that identification. The date appears to be well
established as Early Cretaceous, and all the documentary
evidence points to a genuine discovery and
reliable curation of the specimen. The possibility
must therefore be entertained that kannemeyeriids
survived in Australia for no less than 110 Ma after
their last appearance elsewhere in the world.
Gorgonopsia
The remains of gorgonopsians recorded in the
lowermost of the South African Karoo fossil levels,
the Eodicynodon Assemblage Zone, are very poorly
preserved (Rubidge 1993; Rubidge et al. 1995) and it
is not until the overlying Tapinocephalus Assemblage
Zone that complete specimens have been found.
Here they occur as relatively rare, smallish carnivores,
adapted for feeding on prey by greatly
enlarged upper and lower canines that are oval in
section and carry a serrated hind edge. During the
rest of the Late Permian however, gorgonopsians
were the dominant top terrestrial carnivores, and
they are also found in contemporaneous Russian
deposits. So far they are completely unknown
outside southern Africa and Russia and, unlike the
other latest Permian therapsid groups, not a single
gorgonopsian is known to have survived into even
the base of the Triassic.
Gorgonopsians (Figs 3.15 and 3.16) varied in
body size from that of a small dog to somewhat
larger than any living mammalian predator. As
well as the enlarged canines, there are five upper
and four lower well-developed incisor teeth, while
the postcanines are reduced to at most four or five
very small, simple teeth. Several cranial characters
also diagnose the group, including the following:
● enlarged, flat-topped preorbital region of the skull
● enlargement of the temporal fenestra by lateral
and posterior extension while the intertemporal
roof remained broad and uninvaded by adductor
musculature
● preparietal bone
● vomers fused
● paired palatine bones meeting in the mid-ventral
line of the palate, and together with the pterygoids
forming a deeply vaulted palate.
The gorgonopsians also retain many primitive therapsid
features such as the carnivorous dentition,
broad intertemporal region of the skull roof, and
very conservative postcranial skeleton. As a consequence
there has been a tendency in the past to
classify them with other primitive carnivorous taxa,
such as the hipposaurids and burnetiids, which are
now included in Biarmosuchia. Recent authors now
follow the review of Sigogneau-Russell (1989) and
limit the Gorgonopsia to the well-defined forms
that have the diagnostic characters of the skull temporal
fenestra, and palate noted. Taxonomically
constrained in this way, there is remarkable conservatism
within the group. Genera differ among
themselves by little more than size, relative proportions
of the skull width, interorbital breadth, etc,
and the trivial character of number of postcanine
teeth, and all can be accommodated in the single
family Gorgonopsidae. Sigogneau-Russell (1989)
recognises three subfamilies. The Gorgonopsinae
(Fig. 3.15) consists of the majority of genera. The
Rubidgeinae (Fig. 3.16(c)) have a very broad, heavily
built skull, with wide postorbital bar, and massive
zygomatic arches. The Inostranceviinae
consists primarily of the Russian genus Inostrancevia
itself (Fig. 3.16(b)), which is the largest of all gorgonopsians,
having a skull length of over half
a metre. Its main characteristic apart from this size
is the relatively very long preorbital length.
Biologically, the gorgonopsians were superbly
adapted for a highly predaceous mode of life.