The Origin and Evolution of Mammals - Moodle

(e)
(f)
Figure 3.22 (continued).
Massetognathus
Exaeretodon
and the lumbar ribs, while in Massetognathus
(Fig. 3.22(e)) they are very reduced, fine projections,
and altogether absent in Exaeretodon. The appearance
of the vertebral column and ribcage is virtually mammalian.
The acromion process of the shoulder girdle is
well developed indicating increased invasion of the
inner surface of the scapular blade by forelimb musculature.
The hindlimb in a form such as Luangwa has
achieved a structure functionally comparable to
mammals. The ilium extends far forwards, although it
lacks the mammalian external ridge dividing the
blade into discrete upper and lower parts. The pubis
is turned back, the ischium is almost horizontal, and
the obturator foramen large.
The earliest traversodontids occur in the Lower
Triassic of Argentina (Abdala and Ribeiro 2003).
Pascualgnathus, and Rusconodon have teeth in which
the transverse ridge is only a little more prominent
than in Diademodon (Bonaparte 1970). Middle Triassic
genera are known from several regions, being represented
by the well-known Massetognathus from
Argentina, and Luangwa from Zambia. Other forms
have been found elsewhere in southern Africa, North
EVOLUTION OF MAMMAL-LIKE REPTILES 69
America (Sues et al. 1999), and Russia (Battail and
Surkov 2000). All the Middle Triassic forms have the
fully developed traversodontid version of the gomphodont
tooth, with the transverse crest dominating
the occlusal surface of the crown (Abdala and Ribeiro
2003). The last and most specialised traversodontids
are from the early part of the Upper Triassic. The
Argentine Exaeretodon (Fig. 3.22(f)) was a large cynodont,
with a presacral body length up to 1.4 m. The
dentition is remarkable for the extent to which the
adjacent teeth along the tooth row interlock. The posterior
edge of each one is concave and fits tightly
against the convex anterior edge of the next tooth
back. The result is a very strongly built triturating
surface. A diastema between the canine and the postcanine
tooth row, and the parallel arrangement of the
left and right tooth rows indicates that the dentition
had achieved a highly effective grinding ability.
Probainognathia
The Probainognathia includes a series of Lower to
Upper Triassic carnivorous eucynodonts. There are
few unique characters uniting the group, the main