The Origin and Evolution of Mammals - Moodle

74 THE ORIGIN AND EVOLUTION OF MAMMALS
reported. Gow (2001) has described some isolated
bones of Pachygenelus, showing that the anterior
edge of the scapula blade has everted and shifted
posteriorly to a sufficient degree as to regard it as a
true, mammalian scapula spine. The humerus and
femur are very similar indeed to those of the basal
mammal Morganucodon.
The main character diagnosing the family
Tritheledontidae is the dentition, which is quite
specialised in some respects. It is best known in
Pachygenelus (Gow 1980; Shubin et al 1991), where
only two incisors are present in the upper and
lower jaws (Fig. 3.24(a)). The canines are unique
amongst non-mammalian cynodonts in that the
lower one works against the antero-lateral rather
than the inner side of the upper, a feature that is
typical of mammals. There are seven postcanine
teeth in both upper and lower jaws, with incipiently
divided roots. The uppers are circular in
crown view and have a single, dominant main cusp.
An anterior and a posterior accessory cusp are present,
offset to the inner side of the main cusp and
connected by an internal cingulum. The lower teeth
are slightly larger, laterally compressed, and the
anteriorly positioned main cusp is followed by up
to three accessory cusps along the line of the jaw.
These also have an internal cingulum. One unexpected
feature concerns the relative thickness of the
enamel. In the occluding regions of the teeth it is thin
and soon wears away, leaving the thicker enamel
edges that surround the crown exposed as sharp cutting
edges. The postcanine teeth of Diarthrognathus
are essentially similar to those of Pachygenelus
though differing in detail (Gow 1980).
Chaliminia (Bonaparte 1980) is a poorly known
form from the Upper Triassic of the Rio Grande do
Sul in Brazil, but has upper postcanine teeth sufficiently
similar to those of Pachygenelus for it to be
regarded as a member of the Tritheledontidae.
Riograndia (Bonaparte et al. 2001) is represented by
several specimens of a very small form of skull
from the same locality (Fig. 3.24(b)). It has a very
peculiar dentition, but which is comparable to
Pachygenelus in several respects. There are only
three, procumbent upper incisors, of which the second
is the largest, and also three lowers, but here
the first one is much larger than the other two.
The canines are small. Both the upper and lower
postcanines are blade-like and bear up to nine
cuspules arranged in line. Each tooth is then set at
an angle to the line of the tooth row, a condition seen
to a small extent in Pachygenelus but taken to
extremes here. Presumably Riograndia is a specialised
tritheledontid, perhaps adapted for a diet of softer
invertebrates than insects.
In addition to members of Tritheledontidae as
diagnosed by the reduced incisors and the postcanine
tooth structure, there are several small,
superficially tritheledontid-like specimens from the
Rio Grande do Sul, Brazil that have been described
by Jose Bonaparte and his colleagues. Prozostrodon
(Bonaparte and Barberena 2001) is a slightly larger
form that is represented at present by the anterior
half of a 7 cm long skull, with the lower jaw and a
few fragments of the postcranial skeleton. It is
primitive compared to all other tritheledontans in
some respects, notably retention of small prefrontal
and postorbital bones, although the postorbital bar
is absent and the frontal bone does form the orbital
margin as in tritheledontids. The lower jaw has a
well-developed articular process of the dentary, but
it is unknown whether contact between this bone
and the squamosal occurred. The dentition of
Prozostrodon lacks the derived characters of the
members of the family Tritheledontidae. There are
five upper and four lower simple incisors, followed
by well-developed canines. The seven upper postcanines
have up to four linearly arranged cusps and
lack a cingulum. There are 10 lower postcanines, of
which the more complex posterior ones have a small
anterior accessory cusp, the main cusp, and two
posterior accessory cusps all in line, and also a welldeveloped
internal cingulum. They have a similar
appearance to the postcanine teeth of Thrinaxodon
and to the basal mammal Morganucodon.
Bonaparte and Barberena (1975, 2001) described a
very small, poorly preserved Rio Grande do Sul skull
as Therioherpeton (Fig. 3.24(c)), which they regard as a
member of a separate family Therioherpetontidae.
Again the postorbital bar is lacking, but as in
tritheledontids the postfrontal and postorbital
bones have been lost completely and the zygomatic
arch has the same very slender construction. The
differences from tritheledontids are that the anterior
orbital and interorbital region of the skull is
more completely ossified by meeting of the frontal,