The Origin and Evolution of Mammals - Moodle

56 THE ORIGIN AND EVOLUTION OF MAMMALS
(e)
Figure 3.17 (continued ).
Therocephalia
The therocephalians were another group of carnivorous
therapsids, and many of the earlier ones bear a
superficial resemblance to the gorgonopsians. Others,
however, were significantly smaller, sufficiently so to
be regarded as small-prey carnivores or even committed
insectivores, while one specialised late group
evolved adaptations for herbivory. Like most other
major therapsid groups, the earliest record of the
Therocephalia is from the lowest of the South African
fossil-bearing horizons, the Eodicynodon Assemblage
Zone, from which the incomplete skull of a small,
primitive form has been described (Rubidge et al.
1983). For the rest, the group occurs reasonably commonly
throughout the remainder of the Late Permian,
and more rarely in the Lower Triassic. Geographically,
the majority of specimens are South African, but there
are several Russian genera, and also records of the
taxon from other parts of southern Africa, China
(Sun 1991), and even the Lower Triassic of Antarctica
(Van den Heever 1994).
The characters of the Therocephalia include
enlargement of the temporal fenestra in a medial
direction, greatly reducing the intertemporal skull
roof to a narrow bar although, as discussed later,
this character is shared with cynodonts and is part
of the case for a relationship between these two
groups of therapsids. Unequivocally therocephalian
characters (Hopson and Barghusen 1986; Van den
Heever 1994) include the following:
● paired suborbital vacuities in the palate
● broad vomer
● stapes lacking a foramen and dorsal process
● in the lower jaw, the reflected lamina of the angular
combining a free dorsal margin with a series of
strong ridges radiating from the central region
● reduced, horizontal lumbar ribs
● anterior process on the ilium and an additional
trochanter on the femur
● Digital formula precociously and independently
reduced to the mammalian formula of 2.3.3.3.3.
(Kemp 1986; Fourie 2001).
The earlier, Tapinocephalus Assemblage Zone therocephalians
were formerly classified as the Pristerognathidae
(Hopson and Barghusen 1986), but in his
review, Van den Heever (1994) regards the group as
paraphyletic and has abandoned it in favour of two