The Origin and Evolution of Mammals - Moodle

subsequent specimens revealed that there are three
premolar teeth and possibly four molar teeth
(Butler and Kielan-Jaworowska 1973), which corresponds
to the marsupial rather than the placental
dental formula, and which led to the group being
removed from the Placentalia and placed in the
informal category of ‘Theria of the metatherianeutherian
grade’. By 1990, Kielan-Jaworowska and
Nessov (1990) had come to the conclusion on the
basis of the tooth formula, and an incipient tympanic
bulla formed from the alisphenoid bone that
deltatheroidans are in fact the sister-group of the
marsupials, and that the two together should formally
constitute the taxon Metatheria. The structure
of the molar teeth (Fig. 5.19(b)) does not offer strong
support for this conclusion though. In the upper
molars, there is a very wide stylar region and a relatively
small protocone, and in the lower molars the
talonid is much narrower than in typical marsupials.
However, these could be specialisations superimposed
on a basic marsupial structure for the
enhanced shearing action of the teeth of a carnivore.
Opposing Kielan-Jaworowska and Nessov’s interpretation,
Cifelli (1993c) denied a relationship
between Deltatheroida and Marsupialia. He suggested
that not only were there no shared molar tooth
characters, but also that the other similarities may be
primitive for tribosphenidans. The most recent stage
in the argument is based on some beautifully preserved
material of Deltatheridium (Fig. 5.19(a)), in
which Rougier et al. (1998) have shown that the complete
dental formula is I4/3: C1/1: PM3/4: M4/4,
which confirms a premolar and molar count for marsupials.
A second characteristic marsupial feature is
the pattern of tooth-replacement, in which only the
last premolar is actually replaced after birth; one of
the their specimens is a juvenile form that appears to
show this was indeed the case. A third is an inflected
angular process on the lower jaw, which is characteristic
of marsupials. There are also one or two fine
details of the anatomy of the braincase supporting
the relationship.
Minor tribosphenidan groups
While the Placentalia, Marsupialia, and Deltatheroida
are the three best known and diverse Cretaceous
groups of tribosphenidan mammals, there are several
other groups known only by their particular
THE MESOZOIC MAMMALS 173
versions of the tribosphenic tooth whose relationships
both to one another, and to the three wellcategorised
groups are unclear (Fig. 5.20). Taken
together, these short-lived, minor branches of the
tribosphenidan tree have been informally referred
to as ‘Theria of the metatherian-eutherian grade’,
an expression introduced by Patterson (1956).
Historically the most important are the ‘Trinity
Theria’, named after the Trinity Formation in Texas,
and dating from the Albian stage, at the end of
the Early Cretaceous. It was comparison of the
‘Trinity Theria’ with ‘eupantotheres’ which enabled
Patterson (1956) to establish that the protocone of
the tribosphenic upper molar was a new structure,
and not the homologue of any of the triangle of
cusps of the molars of pre-tribosphenic molars.
Pappotherium (Fig. 5.19(d)) is the best known, being
represented by partial jaws along with isolated
upper and presumed but not certainly associated
lower teeth. Holoclemensia (Fig. 5.19(e)) is similar
but known only from upper molars, and Kermackia
(Fig. 5.19(f)) is based on a lower molar. Early
attempts were made to slot the ‘Trinity Theria’ into
existing taxa, notably that of Slaughter (1971), who
claimed that Pappotherium was a placental and
Holoclemensia a marsupial. However, all subsequent
authors have rejected such a simple phylogeny and
accept instead that there were several separate lineages
of tribosphenidans, most very short-lived,
radiating from a hypothetical metatherian-eutherian
grade ancestor during the Early Cretaceous
(Kielan-Jaworowska et al. 1979b; Clemens and
Lillegraven 1986; Cifelli 1993c; Flynn et al. 1999).
Other independently evolved molar teeth of no
clear affinities are found in the Late Cretaceous, such
as Falpetrus (Fig. 5.19(h)) from Wyoming and Bistius
(Fig. 5.19(i)) from Mexico (Clemens and Lillegraven
1986). Clearly, the second half of the Cretaceous
Period was a time of considerable evolutionary
‘experimentation’ as far as advanced, insectivorous
mammals were concerned (Fig. 5.20).
Australosphenida and the mystery of the
Monotremata
The greatest mystery of all concerning mammalian
evolution stretches back for 200 years: the question
of what exactly the monotreme mammals are, and