The Origin and Evolution of Mammals - Moodle

Wible and Hopson 1993). The interorbital region is
largely filled in by expanded orbitosphenoid,
frontal, and palatal bones. The paroccipital process
is bifurcated and the quadrate is attached directly
to the distal end of its anterior process without the
squamosal intervening. Also, the postcanine teeth
are multi-rooted. Impressive similarities are also
found in the postcranial skeleton (Kemp 1983),
which is almost indistinguishable from that of
Morganucodon. For example, in the shoulder girdle
the virtual loss of the coracoid plate and the extensive
reflection of the acromion process of the
scapula; and in the pelvis, loss of the posterior
process of the ilium and very long anterior process
with the external ridge separating upper from
lower regions so characteristic of mammals, and
also an epipubic bone. However, as more information
emerges about the postcranial skeleton of
tritheledontids (Gow 2001), the more it looks as
if these characters are true of that group as well,
and not exclusively of mammals and tritylodontids.
Hopson and Kitching (2001) actually include seven
postcranial characters in their list defining a
tritheledontid—mammal clade, of which six are
also found in tritylodontids.
The characters of tritheledontids that support
their sister group relationship with mammals
include the secondary jaw articulation between the
dentary and squamosal, although the fully formed
dentary condyle of the mammal had not evolved,
and the detailed structure of the quadrate (Luo and
Crompton 1994). The mode of action of the postdentary
teeth of tritheledontids (Shubin et al. 1991),
involving unilateral contact between teeth of one
side of the jaw at a time approaches the mammalian
condition, even though there is not the precise
occlusal relation of specific lower and upper teeth.
The teeth themselves have prismatic enamel, and
the upper postcanines have a buccal cingulum. The
zygomatic arch is much more slender than in
tritylodontids even in specimens of the same skull
size, and there is a longer secondary palate. The
basicranial region of the skull is shortened, and the
fenestra rotunda and jugular foramen are completely
separated.
Agreement on the interrelationships of the major
eucynodont groups might be expected to come from
cladistic analyses based on large morphological
EVOLUTION OF MAMMAL-LIKE REPTILES 77
data sets. In recent year there have indeed been
several of these, and yet there is still no consensus
on any of the contentious issues. Luo (1994) used
82 dental and cranial characters and found tritheledontids
to be the sister group of mammals but
only by a very narrow margin over tritylodontids
(Fig. 3.25(a)). Martinez et al. (1996), on the basis of
68 characters found tritylodontids to be the immediate
sister group of Mammalia, and tritheledontids
the sister group of these two (Fig. 3.25(b)). Like
Luo, they also found cynognathids to be the most
basal eucynodont group. Hopson and Kitching
(2001) coded for 101 characters including dental,
cranial, and postcranial. Their cladogram continued
to support Hopson and Barghusen (1986;
Hopson 1991) by relating cynognathids to the
diademodontoids, placing tritylodontids with the
latter, and recognising tritheledontids as the sister
group of mammals (Fig. 3.25(c)). In part these
inconsistent results may be due to the different taxa
used. But the main cause lies firmly in the choice of
unit characters made. In their total of 101 characters,
Hopson and Kitching (2001) included 28 dental
characters of which no less than 17 were
concerned with the morphology of the individual
postcanine teeth. Twenty of their characters were
postcranial. In contrast, out of 68 characters,
Martinez et al. (1996) included only 13 dental characters
of which a mere five concerned individual
Cynognathidae
Diademodontoidea
Probainognathus
Tritylodontidae
Tritheledontidae
Mammalia
(a) (b)
(c)
Cynognathidae
diademodontoids plus tritylodontids
Chiniquodontidae
Probainognathus
Tritheledontidae
Mammalia
Cynognathidae
Diademodontoidea
Chiniquodontidae
Probainognathus
Tritheledontidae
Tritylodontidae
Mammalia
Figure 3.25 Three cladograms of the main cynodont groups. (a) Luo
1994). (b) Martinez et al. 1996. (c) Hopson and Kitching 2001).