The Origin and Evolution of Mammals - Moodle

64 THE ORIGIN AND EVOLUTION OF MAMMALS
on the grounds of an incomplete secondary palate.
Their most distinctive feature is the structure of the
postcanine teeth. Compared to those of Procynosuchus,
these have lost the cingular cusps. There is also no
anterior accessory cusp as occurs in several other
Triassic cynodont families, but a well-developed
posterior accessory cusp lies immediately behind
the somewhat recurved main cusp. Perhaps galesaurids
were adapted for dealing with a more
robust form of insect prey.
Thrinaxodontidae
Thrinaxodon (Fig. 3.20(a) and (b)) is far the commonest
and most studied cynodont of the Lystrosaurus
Assemblage Zone fauna of the Karoo. It also occurs
in the Fremouw Formation of Antarctica (Colbert
1982). Structurally Thrinaxodon is the same grade as
Galesaurus, which is to say intermediate in a number
of its features between Procynosuchus on the one
hand and the eucynodonts to follow. It was somewhat
smaller than Procynosuchus, and the postcanine
teeth were reduced in number. The more
posterior, molariform postcanines have a sharp
main cusp plus an anterior and a posterior accessory
cusp in line with it, which developed from the
enlarged end members of the row of cingulum cuspules
along the internal base of the tooth. The dentary
bone is relatively larger, and its coronoid
process rises right up into the temporal fenestra; the
adductor fossa has expanded to occupy most of the
external surface of the dentary behind the tooth row.
The temporal fenestra has enlarged considerably
compared to Procynosuchus, and the zygomatic arch
was much deeper and more robustly built, and
bowed dorsally, indicating the presence of a
substantial masseter muscle. There are several significant
differences in the postcranial skeleton
compared to Procynosuchus, most conspicuously
the appearance of large costal expansions on
the hind, proximal part of the ribs, each of which
overlaps the one behind and bears a strong ridge
for muscle attachment. There are also accessory
articulations between adjacent vertebrae, with a
peg below each postzygapophysis fitting into a
groove below the prezygapophysis of the next vertebra
behind. The functional significance of this
arrangement of the axial skeleton is obscure.
Jenkins (1971b) believed that it was a method for
more effectively applying muscle forces causing
lateral bending of the vertebral column; Kemp
(1980a), in contrast, proposed that the effect would
be quite the opposite by making the column more
rigid. At any event, a full set of costal plates is
evidently the primitive condition for Triassic cynodonts
generally, and they are variously reduced or
completely lost by the Middle and Upper Triassic
groups. The limbs of Thrinaxodon were probably
approaching mammalian in pose, particularly the
hindlimb where the knee was evidently turned
well forwards. The tail is reduced, indicating the
increased reliance on muscles from the ilium and
body fascia for the power stroke of the stride.
Platycraniellidae
There is a third well-established genus of Lystrosaurus
Assemblage Zone cynodont, which is Platycraniellus
(Abdala unpublished manuscript). It is remarkable
for having an extremely short snout and broad temporal
fenestrae (Fig. 3.20(d)). In fact, comparatively it
has the widest skull of any cynodont. The secondary
palate is complete and extends posteriorly as far as
the end of the tooth rows, a feature characteristic of
later, more advanced forms. Unfortunately, few
details of the dentition or the structure of the lower
jaw are clear in the one reliably identified specimen.
It may be that Platycraniellus is an aberrant basal
member of the more advanced cynodont taxon
Eucynodontia.
Eucynodontia
All the remaining cynodonts form a monophyletic
group Eucynodontia, in which there has been further
evolution of the temporal fenestra, lower jaw,
and by inference the jaw musculature. The dentary
has increased in relative size to such an extent that
the postdentary bones are reduced to a small, vertically
oriented compound sheet or rod of bones set
into a recess occupying the medial face of the dentary.
The coronoid process rises right up to the level
of the top of the sagittal crest, there is a large angular
process ventrally, and an articular process posteriorly
that reaches towards the jaw articulation.
The hinge bones, articular bone at the hind end
of the postdentary complex, and quadrate held in
a recess in the squamosal are relatively minute.