The Origin and Evolution of Mammals - Moodle

crown-group therians) is accepted by most as the
central phylogenetic framework (Wible and Hopson
1993; Ji et al. 1999; Kielan-Jaworowska et al. 2004).
There are nevertheless several outstanding problems
concerning certain groups. The most intractable
of these is the phylogenetic position of the multituberculates,
a group whose high diversity and
great longevity define its fundamental importance
in mammal evolution. The similarity and inferred
mode of action of the teeth to those of haramiyidans
led to the view that these two groups are indeed
related and should be combined in a high-level
taxon Allotheria (Hahn 1973; Butler 2000). If true,
then the primitive nature and early occurrence of
haramiyidans indicate that Allotheria diverged
right at the base of the mammal radiation, and that
multituberculates evolved a range of derived therian
mammalian characters of the skull and postcranial
skeleton convergently. A second hypothesis
is that multituberculates are the sister-group of the
monotremes. This is based on the selected evidence
of the braincase (Wible and Hopson 1993) and ear
structure (Meng and Wyss 1995), and no cladistic
analyses that use all characters supports it. Third, a
number of particularly postcranial characters shared
by multituberculates and therian mammals led to
the hypothesis that multituberculates shared a common
ancestor with some or all of the trechnotherians
(Hu et al. 1997), despite the complete absence of
any points of unique similarity between the multituberculate
and therian dentitions. The most
detailed cladistic analysis to date is that of Kielan-
Jaworowska et al. (2004), who find slightly more
support for the trechnotherian relationship than for
the haramiyid relationship, but the difference is
barely significant. The problem arises from the multituberculates’
combination of highly specialised herbivorous
adaptations of the dentition and jaw
musculature, superimposed upon a mosaic of primitive
and derived mammalian characters. Only new,
more plesiomorphic specimens with other combinations
of characters such as a less derived dentition
are likely to result in resolution of the problem.
There is no serious doubt about the monophyly
of the Trechnotheria, characterised by their triangulated
molars and features of the lower jaw, braincase
and ear region. The one question mark concerns
the earliest supposed member, Kuehneotherium. Its
THE MESOZOIC MAMMALS 181
lower jaw is very primitive in structure, suggesting
it may be a more basal mammal that independently
evolved triangulated teeth. Determination of its
true relationships will depend on finding cranial
material. Aside from the Kuehneotherium issue,
there is an uncontroversial sequence of stages illustrating
the evolution from a very primitive version
of the therian tooth in ‘symmetrodontans’ through
a basal ‘eupantothere’ such as Amphitherium, a more
progressive ‘eupantothere’ such as Peramus, to the
definitive basal tribosphenidans represented by
aegialodontids such as Kielantherium. However,
there is little doubt that these intermediate stage
taxa are paraphyletic, their definitions being based
only on ancestral stages in tooth and jaw evolution.
Continuing to utilise the names ‘Symmetrodonta’
‘Eupantotheria’ and possibly ‘Aegialodontidae’ is
a matter of convenience rather than an issue of
conceptual difficulty.
The interrelationships within the Tribosphenida
are confusing. There appears to have been a burst
of radiation starting in the Early Cretaceous and
producing several separate lineages, of which only
three are represented by more than isolated teeth or
jaw fragments. These are the placentals, marsupials,
and deltatheroidans, and there is debate about
their interrelationships. A sister-group relationship
between the latter two is supported by a number of
dental characters, leading to their frequent inclusion
in a taxon Metatheria. However, the overall
cladistic picture indicates that the placentals are
more closely related to the marsupials.
The most recent contentious issue is whether
those mammals with tribosphenic molar teeth are a
monophyletic group, or whether a separate, independent
origin of that form of tooth occurred in the
southern continents of Gondwana, within a group
named Australosphenida, and which includes
amongst others the fossil and living monotremes.
As usual, it will take further relevant fossil material
to decide one way or the other.
Evolutionary pattern
Given the generally agreed relationships outlined,
it is possible to infer a number of interesting aspects
of the evolutionary pattern of Mesozoic mammals,
the first of which occurs right at the very beginning
of the story, when considerable diversification was