The Origin and Evolution of Mammals - Moodle

separate families. Lycosuchidae (Fig. 3.17(a)) is a basal
group of short, broad-snouted forms, and the longer
snouted Scylacosauridae (Fig. 3.17(b)) are related to
the rest of the therocephalians, which he refers to as
Eutherocephalia. At any event, this paraphyletic
primitive group of ‘pristerognathids’ consists of generally
fairly large animals, with skull length of the
order of 20–30 cm. As in the gorgonopsians, though to
a lesser extent, the dentition is dominated by large
upper and lower canine teeth. The number of upper
incisor teeth varies from five to seven in different taxa,
but there are always only three lower incisors. The
postcanine dentition is better developed than in the
gorgonopsians, although it does tend to be reduced to
as few as five modest teeth in several genera such as
Lycosuchus. These primitive therocephalian groups
disappeared after the Tapinocephalus Assemblage
Zone, coincidentally with the rise of larger sized,
superficially very similar gorgonopsians.
They were replaced in the later part of the
Permian by a mixture of both small, generalised
therocephalians, and some highly specialised new
kinds. Among the specialists, the whaitsiids, such
as the South African Theriognathus (Fig. 3.18(a)) and
the Russian Moschowhaitsia, were relatively large
animals. They retained well-developed incisors and
canines, but the postcanine dentition was greatly
reduced or completely absent. The snout is constricted
behind the canines, where the palatal exposure
of the maxilla forms a broad, heavily ridged and
pitted surface that opposes a similarly broad, concave
dorsal surface of the dentary bone; the postcanine
dentition had evidently been replaced by
directly opposing keratinised tooth plates (Watson
and Romer 1956; Kemp 1972b). Other unique features
of whaitsiids include a partial secondary palate
formed by maxillary processes meeting the vomer,
partial or complete loss of the paired suborbital fenestrae,
closure of the interpterygoid vacuity, and a
great broadening of the primitively rod-like epipterygoid
connecting the palato-quadrate to the skull roof.
All these new characters can be interpreted as devices
for strengthening the skull against the action of its
own jaw musculature (Kemp 1972b). Whaitsiids were
probably highly specialised as hyaena-like scavengers,
capable of crushing the bones of abandoned
carcasses between nutcracker like jaws.
Moscorhinus (Fig. 3.18(c)) was another fairly large
form, about 25 cm in skull length. Here, the skull is
EVOLUTION OF MAMMAL-LIKE REPTILES 57
very broad and low, with a short, massive snout
and extraordinarily expanded vomer in the palate.
Like whaitsiids, it too has tended to reduce the
number and size of the postcanine teeth (Durand
1991). Euchambersia (Fig. 3.18(e)) may be a close relative,
having a similarly broad, short-snouted skull.
Uniquely in this genus there is an extraordinary,
deep recess on either side of the snout, between
the canine and the front of the orbit, the walls of
which are covered in fine foramina. The canine
tooth has a groove down its outer face indicating
that Euchambersia possessed a pair of maxillary
poison glands coupled to fangs for delivering
venom. The postorbital bars and zygomatic arches
are very weakly developed implying that the jaw
musculature was poorly developed and the jaws
therefore unable to deal with struggling live prey.
Altogether, Euchambersia appears to have been a
therapsid analogy of snakes.
For the rest, therocephalians are small-sized,
generalised carnivorous animals. The smallest
ones were at one time referred to as ‘scaloposaurs’
(Fig. 3.17(d) and 3.18(b)), but Hopson and
Barghusen (1986) abandoned this taxon because
many and perhaps all of them were believed to be
the juvenile individuals of a variety of genera.
They erected in its place the taxon Baurioidea, of
which the early Triassic, Lystrosaurus Assemblage
Zone Oliveria (Fig.3.17(c)) is a typical, welldescribed
example (Mendrez 1972; Kemp 1986).
The skull is about 10 cm in length. There are six
upper incisors, a modest canine, and a row of ten
small postcanine teeth. A partial secondary palate
has formed by contact between the maxillae and
the vomer towards the front of the palate, which is
continued posteriorly by a pair of ridges that presumably
indicates the attachment of a soft secondary
palate. The intertemporal roof is narrow and
incipiently crested, while the zygomatic arches are
slender and not flared laterally as occurs in gorgonopsians
and cynodonts. The postcranial skeleton
of an immature regisaurid (Fig. 3.17(d))
described by Kemp (1986) is that of a very slenderlimbed,
agile creature with skeletal proportions
comparable to a modern mammal. Several other
very similar latest Permian and earliest Triassic
baurioids are known that differ from Oliveria only
in such details as the degree of development of the
secondary palate (Hopson and Barghusen 1986),