The Origin and Evolution of Mammals - Moodle

American Morrison Formation genus Ctenacodon is of
a similar age and degree of primitiveness, although
it is not apparently closely related to Paulchoffia.
Indeed, it illustrates well the plesiomorphy of
‘Plagiaulacida’ because of its mosaic of primitive
and advanced features. On the one hand it has
retained a primitive molar structure with few cusps
and unornamented enamel, but on the other it
has more advanced, shearing lower premolars.
The ‘advanced’ multituberculates consisting of
most of the forms once classified as Ptilodontoidea
and Taeniolaboidea together constitute the Cimolodonta,
which dominated the Late Cretaceous and
early Tertiary radiation of the group. Dental characters
defining Cimolodonta include loss of the first
upper incisor, and reduction of the number of premolars
to four uppers and two lowers, coupled
with a tendency to concentrate the shearing action
on the hindmost ones only. Several of the groups,
probably independently, evolved a third row of
cusps on the upper molars. Kielan-Jaworowska and
Hurum (2001) recognise a number of subgroupings
of cimolodonts. One includes the Ptilodontoidea,
which are easily recognised by a huge, arch-shaped
lower premolar bearing heavy vertical striations
(Fig. 5.9(d)). Most, though confusingly not all
ptilodontoids have modified the dental enamel to a
microprismatic type in which the prisms are small
and densely packed, in contrast to the gigantoprismatic
enamel of most other groups. Ptilodontoids
were predominantly, though not exclusively North
American and were one of the groups that survived
well into the Tertiary. Another advanced grouping
includes a series of largely Asian forms, the
djadochtatherioids (Fig. 5.9(c)) plus the cosmopolitan
taeniolabidoids. A character shared by all these
is restriction of the enamel on the lower incisors to
the antero-ventral surface, which gives them a selfsharpening
ability analogous to rodent incisors.
Other characters are distinctly separate molar cusps
and smooth molar enamel. However, all these features
do occur convergently in other multituberculate
forms that are not clearly recognisable as members of
any of these groups.
Functional biology
This inability to resolve the phylogenetic interrelationships
of the multituberculates arises in large
THE MESOZOIC MAMMALS 155
part because they are a very conservative group,
with much minor convergent change but little in
the way of clear cut, consistent evolutionary trends
evident. Functionally, however, more and more
subtle differences have been coming to light in such
things as implied diet and mode of locomotion.
Two aspects of their biology important for understanding
the nature of multituberculates are first
that they were small mammals, mostly in the mouse
to rat range and even the Tertiary ‘giants’ were no
larger than a good-sized marmot; Krause (1982)
estimated that the head plus body length of the
ptilodontoids ranged from 6 to 22 cm. Second,
multituberculates were very abundant. Typically,
something like 75% of the specimens found at Late
Cretaceous or Palaeocene mammal localities are
multituberculates. On both these points a comparison
with modern rodents is compelling, quite apart
from any anatomical similarities,
Krause (1982) gave the first detailed analysis of
the jaw function of a multituberculate, in this case
the North American form Ptilodus, mainly by looking
at the dental wear facets and the microstriations
across them (Fig. 5.10(a)). He showed first that the
upper and lower incisors could not have made
direct contact, and that the wear on them consisted
only of a general abrasion of the tips. Therefore,
despite their relatively large size, they could not
have been used for gnawing as in modern rodents,
but only for collecting and ingesting food items.
The premolars underwent a crushing–slicing cycle,
with the large, striated, blade-like fourth lower premolar
cutting against the occlusal surface of the
large, multicusped upper fourth premolar. This was
a strictly orthal movement and resulted in horizontal
wear of the blade of the lower tooth and the
rows of cusps of the upper tooth. The action of the
molars was a grinding cycle (Fig. 5.10(b)) in which
the lower jaw was protracted, the lower molars
brought into occlusion with the upper molars, and
then the jaw powerfully retracted. This led to a
forceful trituration of food caught between the
upper and lower rows of crescentic cusps occupying
the molar crowns. Although not possible to be
certain of the diet of an animal with this kind of
dental action, Krause proposed that it would have
been more likely a mixed, omnivorous one than
strictly herbivorous. Evidence from the extent of