The Origin and Evolution of Mammals - Moodle

76 THE ORIGIN AND EVOLUTION OF MAMMALS
cynognathids were the plesiomorphic sister group
of all the rest of the eucynodonts on the basis of certain
primitive characters, such as the lack of a deep
embayment between occiput and zygomatic arch.
The analyses of a number of authors have agreed
with this, notably Rowe (1986) and Martinez et al.
(1996). However, others, primarily Hopson and
Barghusen (1986; Hopson 1991; Hopson and
Kitching 2001) argued for a monophyletic clade
Cynognathia, consisting of cynognathids plus diademodontoids
but excluding the more advanced carnivorous
eucynodonts. The latter, formally including
Mammalia, constitute their Probainognathia which
is thus the sister group of Cynognathia.
A third dispute, which also has yet to be resolved,
concerns the position of the tritylodontids. This issue
is related to the broader issue of which eucynodontian
taxon is the closest relative of Mammalia,
but for the moment will it be treated separately.
When Kühne (1956) gave the first detailed, comprehensive
description of a tritylodontid, based on the
fragmentary but virtually complete remains of
Oligokyphus from England, he had no doubts about
the prevailing view that they were closely related to
the mammals, on the basis of such characters as the
loss of the prefrontal and postorbital bones and
postorbital bar, the multirooted, complex postcanine
teeth, and the huge dentary bone almost, but
not quite contacting the squamosal. At about the
same time, however, Crompton and Ellenberger
(1957) proposed that tritylodontids were highly
derived ‘gomphodont’ cynodonts, and opinion
shifted to this view when Crompton (1972a) published
his detailed study of the form and function
of the teeth of cynodonts, showing that both traversodontids
and tritylodontids had an occlusal mechanism
based on transverse crests on the postcanine
teeth and a posteriorly directed bilateral power
stroke of the lower jaw. From this standpoint it
must be inferred that those mammalian characters
of tritylodontids not found in traversodontids were
acquired independently of the mammals. In 1983,
Kemp listed the numerous derived characters
shared by tritylodontids and the early mammal
Morganucodon, including not only several cranial
features but also a virtually identical postcranial
skeleton, and accordingly proposed a return to the
older view that the two groups are related, and
therefore that traversodontids and tritylodontids
had evolved a specialised herbivorous dentition
convergently. This view gained qualified supported
shortly thereafter from a detailed cladistic analysis
produced by Rowe (1986, 1988). Since then, the
majority of analyses have agreed that tritylodontids
are related to mammals rather than to diademodontoids,
including Wible’s (1991) re-evaluation of
Rowe’s data, Luo’s (1994) cladistic analysis of dental
and cranial characters, Luo and Crompton’s
(1994) extremely detailed study of quadrate characters,
and Luo’s (2001) analysis of the structure of the
inner ear. The recent analyses of eucynodontian
interrelationships by Martinez, May, and Forster
(1996), and of gomphodont cynodonts by Abdala
and Ribeiro (2003) also conclude that the tritylodontids
are not ‘gomphodonts’. On the other hand Sues
(1985) and Hopson and his colleagues (Hopson and
Barghusen 1986; Hopson 1991; Hopson and Kitching
2001) and Battail (1991) continued to support the
diademodontoid relationship of tritylodontids.
Surprisingly and despite considerable efforts, no
proposed resolution of this question has been
universally accepted, as discussed shortly.
The final question concerning cynodont interrelationships
is which group is most closely related to the
monophyletic taxon Mammalia, as usefully represented
by Morganucodon, by far the best known of the
early mammalian taxa. For those authors who support
the diademodontoid–tritylodontid relationship,
such as Hopson and Kitching (2001) and Rubidge
and Sidor (2001), the only contender is a tritheledontan,
taking this group to include now the
various Rio Grande do Sul genera such as
Therioherpeton, Brasilodon, and Brasilitherium as well
as Tritheledontidae. However, among those who
accept that tritylodontids are related to mammals,
there are differing views about whether the tritylodontids
or the tritheledontids are actually the
closer. The problem is that these two advanced nonmammalian
cynodont taxa differ in the combination
of mammalian and non-mammalian characters
that they exhibit, so whichever view is correct, convergence
of certain mammal characters must have
occurred.
Characters that support a sister group relationship
between tritylodontids and the mammals
include several braincase characters (Wible 1991;