The Origin and Evolution of Mammals - Moodle

72 THE ORIGIN AND EVOLUTION OF MAMMALS
The tritylodontid skull has a strong look of a
mammal about it because of the loss of the postorbital
bar so that the orbit and temporal fenestra are confluent,
and the extremely large temporal fenestrae
separated from one another by a very deep sagittal
crest. The dentary bone occupies almost the whole
of the lower jaw and has enormous coronoid and
angular processes, while the postdentary bones are
reduced to a delicate compound rod set into a
groove on the medial side. However, there is still no
contact between the dentary and the squamosal, the
jaw hinge consisting of the tiny articular and the
weakly supported quadrate bone. The superficially
rodent-like dentition is highly specialised for a
herbivorous diet. Canines are absent but have been
functionally replaced by an enlarged pair of second
incisors. In primitive members such as Oligokyphus
(Fig. 3.23(a)) this is the largest of the three pairs
present but in other forms such as Kayentatherium
(Fig. 3.23(d)) it is the sole remaining pair. After a
long diastema in both the upper and the lower
jaws, there is a row of large, complex postcanine
teeth (Fig. 3.23(b)). The crowns of the upper postcanines
have three longitudinal rows of sharp, crescentic
cusps, while the lowers have two such rows.
The tooth rows are parallel and when the jaws
closed, occlusion occurred on both sides simultaneously.
The two rows of cusps of the lower teeth fitted
into the troughs formed by the three rows of
cusps of the uppers, and the action consisted of
powerfully dragging the lower jaws backwards
while the teeth were in contact. The individual
crescentic cusps of the upper teeth had the concave
edge facing forwards, while those of the lower teeth
faced backwards, and so the net effect was a filing
action between the two opposing occlusal surfaces,
allowing very fine shredding of even quite tough
vegetation.
The complete postcranial skeleton of Oligokyphus
is known (Kühne 1956; Kemp 1983) and is virtually
mammalian in form (Fig. 3.23(c)). The shoulder girdle
is modified by reduction of the coracoid bone and
a large, laterally reflected acromion. The humerus is
extremely slender. In the pelvis (Fig. 4.8(b)), the
ilium has almost lost the posterior process, and the
extensive anterior process has a lateral ridge of
mammalian form separating dorsal from ventral
regions. The femur has an inturned spherical head
and discrete trochanter major and trochanter minor,
and is thus fully mammalian in structure.
Tritheledonta
Tritheledontans (Fig. 3.24) constitute another highly
derived group of non-mammalian cynodonts. They
are very small, presumably insectivorous forms
occurring from the early Upper Triassic Carnian
stage through to the Lower Jurassic (Lucas and
Hunt 1994). The skull and, to the limited degree it
has been described, the postcranial skeleton are
remarkably mammalian in general appearance. As
in the tritylodontids, the prefrontal and postorbital
bones have been lost, and therefore there is no postorbital
bar separating the orbit from the temporal
fenestra. Unlike the tritylodontid condition, the
temporal fenestra is long, the sagittal crest low
and broad, and the zygomatic arch slender, characters
probably correlated with the small size of the
animals. Other mammalian features include the
virtual disappearance of the lateral pterygoid
processes of the palate. One of the phylogenetically
most significant derived features of all is a contact
between the articular process of the dentary and the
squamosal, and therefore the presence of the new,
secondary jaw articulation immediately lateral to
the quadrate-articular hinge. This critical feature
(Fig. 3.24(d)) was first claimed for Diarthrognathus
(Crompton 1963) but not universally accepted.
However, it has been amply confirmed subsequently
in Pachygenelus (Allin and Hopson 1992;
Luo and Crompton 1994). The tritheledontid dentition
also possesses certain otherwise uniquely
mammalian characters. One is that the histological
structure of the enamel is prismatic. A second is
that wear facets are present on the inner faces of the
upper and the outer faces of the lower postcanines,
indicating not only that they had a direct, shearing
occlusion, but also that, as in primitive mammals,
there was a unilateral action of the jaws, only one
side of the dentition being active during any one
instant. However, unlike mammals generally, the
upper and lower wear facets do not match exactly,
so the occlusion cannot have involved the precise
action between specific parts of opposing teeth that
distinguishes the more advanced group. Little can
yet be said of the postcranial skeleton for want of a
description of material whose existence has been