The Origin and Evolution of Mammals - Moodle

54 THE ORIGIN AND EVOLUTION OF MAMMALS
(a)
the powerful canine action. Laurin (1998) doubted
whether the quadrate of gorgonopsians had this
propalinal ability, but he does not comment on how
otherwise the wear facets on the sides of the incisors
could have formed, since the lower jaw is not long
enough for the lower incisors to reach the uppers
without an anterior shift. At any event, the gorgonopsian
dentition was certainly adapted for
dealing with relatively large, active prey that presumably
consisted in the main of the abundant
smaller dicynodonts.
The enlargement of the temporal fenestra and by
inference of the associated adductor musculature
was achieved in a uniquely gorgonopsian manner
by a posterior extension of the fenestra well beyond
the hind limit of the skull roof and occiput, simultaneously
with a lateral extension creating a zygomatic
arch (Fig. 3.15(e)). Corresponding enlargement
of the areas of attachment of the musculature to the
Lycaenops
(b) (c)
Inostrancevia Clelandina
Figure 3.16 Gorgonopsians. (a) Skeleton of Lycaenops ornatus. Presacral length approx. 125 cm (Kemp 1982, after Colbert) (b) Dorsal and
lateral views of Inostrancevia alexandri. Skull length approx. 50 cm (Sigogneau-Russell 1989 after Tatarinov). (c) Ventral view of skull of Clelandina
scheepersi. Length approx. 18 cm (Sigogneau-Russell 1989 after Sigogneau).
lower jaw was achieved in part by evolution of
a well-developed coronoid process extending above
the dorsal margin of the jaw. More radically, adductor
musculature had also invaded the external surface
of the jaw, as indicated by the powerful
dorso-ventral ridge and concave area behind it on
the reflected lamina of the angular. Musculature
from the outwardly bowed zygomatic arch must
have attached to this region of the lower jaw, thus
creating an analogue of the mammalian masseter
muscle, an elaboration of the jaw-closing musculature
not otherwise seen until the cynodonts independently
evolved a comparable arrangement.
The postcranial skeleton of gorgonopsians
(Fig. 3.16(a)) (Colbert 1948; Kemp 1982; Sigogneau-
Russell 1989) is that of a relatively long, slender
limbed, and agile version of what otherwise illustrates
the basic condition of therapsids, discussed
in a later chapter.