The Origin and Evolution of Mammals - Moodle

70 THE ORIGIN AND EVOLUTION OF MAMMALS
one being merely an elongated secondary palate.
The frontal and palatine bones meet in the front
wall of the orbit, there is no pineal foramen, and
none have any sign of costal plates on the ribs.
However, the situation is confused by the presence
of these latter three characters in the more advanced
traversodontids. Most authors recognise two
probainognathian families, the Chiniquodontidae
and the Probainognathidae, but there are two forms
that are apparently basal members of the group.
One of these is Lumkuia, from the Lower Triassic of
South Africa and the other Ecteninion, from the
Upper Triassic of South America.
The skull, jaws, and partial skeleton of Lumkuia
(Fig. 3.21(b)) were discovered in the Cynognathus
Assemblage Zone of South Africa (Hopson and
Kitching 2001). It is modest-sized, the skull being
about 6 cm in length. Lumkuia is more derived than
the contemporary carnivore Cynognathus for it lacks a
pineal foramen and has no costal plates on the ribs.
Its postcanine teeth resemble quite closely those of
chiniquodontids, with high, slightly recurved crowns
that are laterally compressed, and bear small anterior
and larger posterior accessory cusps. However, it
lacks the chiniquodontid features of a greatly elongated
secondary palate, and the angulation of the
ventral cranial margin.
Ecteninion (Fig. 3.21(e)), from the Upper Triassic
Chañares Formation of Argentina (Martinez et al.
1996), also lacks these characteristics of the chiniquodontids.
Its most distinctive feature is the postcanine
dentition, consisting of seven teeth, of which
the last three are the best developed. Each of the latter
is transversely flattened and overlaps the next
tooth behind. There are three sharp cusps in line, the
first the largest and the third barely extending above
the jaw margin. The more anterior postcanines are
similar in form but much smaller. The lower postcanine
teeth are not visible in the one known specimen
of Ecteninion, but it would be a reasonable guess that
they are similar to the uppers, and that the whole
postcanine dentition was specialised for masticating
relatively soft animal matter.
Chiniquodontids (Fig. 3.21(d)) are a carnivorous
group, recognisable by the degree of angulation in
the ventral margin of the skull where the maxilla and
zygomatic arch meet. In a recent review, Abdala and
Giannini (2002) concluded that the chiniquodontid
specimens attributed to the familiar genera
Probelesodon and Belesodon are actually different
growth stages of the single genus Chiniquodon. An
adult specimen has a skull length up to about 25 cm,
and bears sharp, well-developed incisors and
canines. The 7–10 postcanine teeth are bluntly sectorial
with a strongly recurved principal cusp and
a posterior accessory cusp behind.
Probainognathus (Fig. 3.21(c)) has sometimes been
included in the chiniquodontids, but more usually it
is placed in its own family. It is a small carnivore with
a skull only about 7 cm in length. The postcanine
teeth have a linear series of three cusps, the central
one being the largest, and there are internal cingular
cusps around the base. They are therefore very similar
to those of the Lower Triassic Thrinaxodon and also
the early mammal Morganucodon. In the first publication
on Probainognathus, Romer (1970) described a
direct contact between the hind end of the dentary
and the squamosal bone, which is technically the
mammalian jaw hinge. Taken with the dental structure,
this led to the once widely held view that
Probainognathus was the mammal ancestor. Subsequently,
however, Crompton (1972b; Luo and
Crompton 1994) showed that the contact is actually
between the surangular bone and the squamosal, a
contact that occurs in other eucynodonts. Nevertheless,
the dentary of Probainognathus extends closer
to this pre-existing contact than in any other nonmammalian
cynodonts except for tritheledontans
described below.
Tritylodontidae
The Tritylodontidae (Fig. 3.23) are a highly specialised
and derived family of herbivores that
occurred worldwide during the Upper Triassic and
Lower Jurassic (Anderson and Cruickshank 1978).
Specimens are known from South Africa, Europe,
North and more dubiously South America, China,
and Antarctica (Lewis 1986). Isolated tritylodontid
teeth have also been described from the Early
Cretaceous of Russia, which extends the temporal
range of the group by a very considerable degree
(Tatarinov and Matchenko 1999). They varied in
size from the less than 5 cm skull (Fig. 3.23(e))
of Bocatherium (Clark and Hopson 1985) to the
South African Tritylodon and North American
Kayentatherium with skulls up to 25 cm.