The Origin and Evolution of Mammals - Moodle

palatal, and orbitosphenoid bones, and the more
derived palatal structure involving reduction of the
pterygoid bones and closure of the interpterygoid
vacuity. The upper postcanine teeth of Therioherpeton
are laterally compressed and have incipiently
divided roots. Unfortunately, no lower jaw or dentition
is yet known for the genus, although enough
of the postcranial skeleton of Therioherpeton is preserved
to indicate features that are closely comparable
to the tritylodontid Oligokyphus and the early
mammal Morganucodon (Kemp 1983). The vertebrae
have a relatively large neural canal, and the cervical
vertebrae are broad. The ilium has a long anterior
process but no posterior process, and the femur is
very similar in its slender build, bulbous inturned
head, and the appearance of the trochanters.
The most recent additions to the Rio Grande do Sul
tritheledontan fauna are Brasilodon and Brasilitherium
(Bonaparte et al. 2003). These are of particular interest
because their dentitions are close in structure to
the presumed ancestral mammalian condition.
They also differ slightly from one another. In the
case of Brasilodon (Fig. 3.24(e)), the crowns of the
postcanines are symmetrical about the main cusp,
with equally well-developed anterior and posterior
accessory cusps in line, and an anterior and a posterior
cingulum cusp on the inner face. The upper
postcanines of Brasilitherium (Fig. 3.24(f)) are like
those of Brasilodon, but the lowers differ. They are
asymmetrical, with the main cusp set further forwards,
one small anterior accessory cusp, and up to
three posterior accessory cusps. There is also an
anterior cingulum cusp. The authors consider these
two genera to be more closely related to Mammalia
than are tritheledontids such as Pachygenelus, a
question taken up below.
In addition to the reasonably well-defined
small, insectivorous-type advanced families
Tritheledontidae and Therioherpetontidae of the
Upper Triassic, there are several records of jaws and
isolated postcanine teeth of superficially comparable
forms. Several of these hail from North America
and historically have been referred to as dromatherians;
various European forms have been considered
members of this same group (Battail 1991;
Hahn et al. 1994). Sues (2001) reviewed the material
of Microconodon, which is the most complete dromatherian,
but which even so is represented only
EVOLUTION OF MAMMAL-LIKE REPTILES 75
by three incomplete dentaries and a few isolated
teeth. Beyond the conclusion that it is, indeed, an
Upper Triassic eucynodont with basically triconodont-like
postcanine teeth, he was unable to interpret
its relationships. None of the other taxa referred
to this group fare any better. It may be that they
are all part of the Upper Triassic radiation of
Tritheledontans, or conceivably that there were
other, as yet ill-understood lineages of advanced,
near-mammalian eucynodonts at that time.
Interrelationships of Cynodontia and the
phylogenetic position of Mammalia
For all the importance of establishing exactly where
within the Cynodontia the Mammalia cladistically
nest, with the implications it holds about its mode
of origin, there remains considerable disagreement
about both this issue, and about other fundamental
aspects of the interrelationships of the cynodont
groups described. A brief history of the situation
may help clarify the debates. For many years it had
been accepted that there were two lineages of cynodonts
(e.g. Hopson and Kitching 1972; Crompton
and Jenkins 1973; Battail 1982). One consisted of a carnivorous
group including Procynosuchus, Thrinaxodon,
cynognathids, chiniquodontids, and tritheledontids
and from which the mammals had supposedly
evolved. The other consisted of the herbivorous, or
‘gomphodont’ cynodonts, namely the diademodontids
and traversodontids, and also the tritylodontids
which were therefore inferred to have evolved
their otherwise mammalian characters convergently.
Consequent upon a detailed description of
Procynosuchus, Kemp (1979; 1982) questioned what
amounted to a diphyletic origin of advanced
cynodonts. He proposed that cynognathids, diademodontids,
and all the other cynodonts with
greatly enlarged dentaries, carnivores and herbivores
alike, constituted a monophyletic group and
that Procynosuchus and Thrinaxodon were its successive
plesiomorphic sister groups. The hypothesis of
a monophyletic Eucynodontia (Kemp 1982) for these
advanced cynodonts has been corroborated by all
subsequent authors apart from Battail (1991).
The next area of dispute concerned the position
of Cynognathidae within Eucynodontia, and this
has not yet been resolved. Kemp (1982) argued that