The Origin and Evolution of Mammals - Moodle

pass through the birth canal. Relative to body size,
no fully developed cleidoic egg that small is known,
but it is of the same order of size as a marsupial
neonate.
There are few other indications of the details of
multituberculate biology. That they occupied an
adaptive zone generally comparable to the rodents
today seems beyond serious dispute. Their small
body size, grinding dentition, and considerable
diversity and abundance all point to this. They must
have much more disparate in their detailed biology
than has yet been shown, with great variation in the
details of diet and habitat occupied amongst the 70
known, and presumably many unknown genera.
The final demise of the group right at the end of
the Eocene has generally, if uncritically been attributed
to competition with advanced therian groups,
particularly the rodents, whose explosive radiation
commenced during the Eocene. Van Valen and
Sloan (1966) suggested that prior to rise of rodents,
the Condylarthra and Plesiadapiformes of the
Palaeocene had ‘weakened’ the multituberculates,
though there is little evidence for this. A number of
suggestions have been made about what might
have been the direct cause of the presumed competitive
inferiority of multituberculates in the face
of placentals, from the vague ‘multituberculates
remained significantly below the eutherian level of
advancement in nearly all areas of their biology’ of
Hopson (1967) to the specific belief of Kielan-
Jaworowska and Gambaryan (1994) that their
decline and extinction was a due to their less efficient
locomotor system. A hypothesis that the
replacement of one higher taxon by anotheris due
to competitive interaction is extraordinarily difficult,
and usually quite impossible to test (Kemp
1999). At the very least it demands a precise analysis
of the pattern of decline of one group and rise of
the other. It also requires convincing evidence of
adaptations in the two respective groups for utilising
an identical resource, over which competition
could have occurred. As it happens, Krause (1986)
has attempted just such an analysis of this case,
showing that there is indeed a significant inverse
correlation between both generic diversity, and also
abundance of individuals of multituberculates and
rodents respectively, through the Late Palaeocene
and Eocene of North America.
Basal Holotheria
THE MESOZOIC MAMMALS 161
There is a large group of Mesozoic mammals whose
molar teeth have evolved triangulation of the three
main cusps. This feature is associated with an exaggeration
of the postvallum-prevallid shearing function,
that is to say, the front part of a lower tooth
shears against the back part of the corresponding
upper. In those where it is known, there are also
certain minor characters of the petrosal bone. The
group embraces very early, primitive forms with no
more than the basic triangulated tooth, through
several intermediate grades, to the living marsupials
and placentals that possess fully expressed
tribosphenic molars. Until relatively recently,
palaeontologists usually referred to the group as
the Theria, confusingly because historically this
term was coined for the two groups of living members
alone, to distinguish them from the monotremes.
With the advent of cladistic analysis (Fig. 5.23) and
the associated demand for unambiguous, strictly
monophyletic groups, a sequence of essentially nodebased
taxa have been named to accommodate the
sequence of increasingly derived forms. McKenna
and Bell (1997) have adopted six such, of which
Kielan-Jaworowska et al. (2004) recognise four.
1. Holotheria. The group containing all the forms
that have triangulated molar cusps, and therefore
the clade that contains the common ancestor of
Kuehneotherium, the living groups, and all its
descendants. Kielan-Jaworowska et al. (2004)
exclude this taxon because they believe the evidence
that Kuehneotherium is related to the rest is
inadequate.
2. Trechnotheria. The clade consisting of the common
ancestor of ‘Symmetrodonta’, the living
groups, and all its descendants.
3. Cladotheria. The clade consisting of the common
ancestor of ‘Eupantotheria’, the living groups, and
all its descendants.
4. Zatheria. The clade consisting of the common
ancestor of Peramus, the living groups, and all its
ancestors.
5. Tribosphenida. The clade consisting of the common
ancestor of Aegialodontidae, the living groups,
and all its descendants. Kielan-Jaworowska et al.
(2004) refers to this clade as Boreosphenida for reasons
discussed below.