The Origin and Evolution of Mammals - Moodle

collected from the Lystrosaurus Assemblage Zone
are of Lystrosaurus, and it has been identified in
contemporaneous early Triassic beds worldwide
including, Russia, India, China, and Antarctica.
Thulborn (1983) identified a few fragments from
Australia as the quadrate and tusk of a dicynodont,
almost certainly Lystrosaurus (King 1983).
For many years, Lystrosaurus was believed to
have been an amphibious animal, living and feeding
in freshwater swamps and lakes. However,
King (1991) and Cox (1991) showed that this was
probably a misinterpretation of the significance
of such anatomical features as the dorsally placed
nostrils and eyes, and subsequently King and
Cluver (1991) offered an alternative interpretation
of its mode of life. They argued that the change
from a Dicynodon-like ancestral state that had
occurred consisted of simultaneously shortening
the skull, deepening the temporal region, and elongating
the downturned snout. These modifications
in geometrical shape resulted in an increased bite
force being applied by the adductor muscles to the
jaws. Tougher terrestrial vegetation could thus be
dealt with, by a combination of the slicing action of
the maxillary rim acting against the lateral edge of
a dentary beak, and the crushing and shredding
that occurred between dentary and palate. Features
of the postcranial skeleton such as the short, broad
hand, and a rather wide knee joint indicate a degree
of digging ability rather than being specialisations
for aquatic life as was once supposed.
The Triassic radiation of kannemeyeriids. Lystrosaurus
was restricted to the narrow time zone either side of
the Permo–Triassic boundary, but the clade to which
it belongs continued to diversify and produce a
range of large forms throughout the Triassic Period,
as reviewed by Keyser and Cruickshank (1979), Cox
and Li (1983), and King (1988, 1990). These constitute
the family Kannemeyeriidae, which never
became as diverse or abundant as the Permian
dicynodonts, no doubt partly because they shared
the large terrestrial herbivore habitat with a number
of other taxa, gomphodont cynodont therapsids,
rhynchosaurs, and early dinosaurs. Nevertheless,
kannemeyeriids had a worldwide distribution,
occurring in several parts of Africa, North and
South America, Europe, India, and in China where
EVOLUTION OF MAMMAL-LIKE REPTILES 51
they are especially abundant (Lucas 2001). They all
retained the short snout and basicranial axis characteristic
of Lystrosaurus, but had developed a high,
narrow intertemporal crest. Body size was increased,
with skull lengths anything from 25–60 cm. The
postcranial skeleton was similar to that of the
Permian dicynodonts but, as befits large-bodied
herbivores, kannemeyeriids tended to have a relatively
short, barrel-shaped trunk, and very stout
limbs. The pelvic girdle is remarkable for a huge
anterior expansion of the ilium and reduced pubis,
which, according to Cruickshank (1978) was an
adaptation for rearing up on the hind legs to feed
from higher branches of the shrubs and trees.
Kannemeyeria (Fig. 3.14(d)) is characterised by a
narrow and pointed snout, very high intertemporal
crest, and well-developed tusks. Specimens of this,
or very closely related genera rivalled Lystrosaurus
in their abundance and cosmopolitan distribution.
They are common in the Lower-Middle Triassic
Cynognathus Assemblage Zone of South Africa, and
in the equivalent Lower Ermaying Formation of
China (Lucas 2001). Very similar forms have been
found in, Indian and Russian beds of presumably
the same age (Bandyopadhyay 1988; King 1990) and
possibly, though less certainly, South American
(Renoux and Hancox 2001).
A second kannemeyeriid group is represented
by the Chinese Lower Triassic Shansiodon, and the
closely similar east African Tetragonius (Fig. 3.14(e))
and South American Vinceria. These are smaller
forms with a broad skull, and short, blunt, ventrally
directed snout. Throughout the Middle Triassic, kannemeyeriines
and shansiodontines, continued to
radiate worldwide. New groups also evolved,
including stahleckeriines such as the Brazilian
Stahleckeria (Fig. 3.14(f)) and the Zambian
Zambiasaurus, which are characterised by the
absence of tusks and a very deep posterior part of
the skull. One tantalising cranial fragment of kannemeyeriid
from Russia that may be a stahleckeriine
was named Elephantosaurus by V’iuschkov (1969),
who estimated that it came from a skull that could
have been as much as a metre in length. If so, it was
by a considerable margin the largest dicynodont
ever, and equal to the largest of the dinocephalians.
The last of the kannemeyeriids occur in the Upper
Triassic, specifically the early part of the Norian