The Origin and Evolution of Mammals - Moodle

196 THE ORIGIN AND EVOLUTION OF MAMMALS
Some of these hypothesised relationships have
been supported by subsequent workers. Springer
et al.’s placement of the peramelemorphs and of
Notoryctes are corroborated by the results of Krajewski
et al. (2000), although this study was actually
focussed on the interrelationships of the dasyuromorphs,
and did not include Dromiciops. Phillips
et al. (2001) also concluded that the peramelemorphs
are the sister group of the rest of the
Australidelphia, although again Dromiciops was not
included in the study. Other studies, however, have
contradicted this consensus. Palma and Spotorno
(1999) analysed 12S rRNA of several marsupials and
came to the unexpected conclusion that peramelemorphs
are related to the caenolestids, although
they were unable to resolve the relationships of the
two together with the rest of the marsupials. Amrine-
Madsen et al. (2003) have applied the largest data set
so far, consisting of five genes and a total of 6.4 kilobases,
and included Bayesian as well as Maximum
Likelihood methods of analysis (Fig. 6.3(b)). They
strongly support the monophyly of Australidelphia,
and, though less strongly, the paucituberculates
(caenolestids) as its sister group. They found they
were unable to resolve the position of Dromiciops confidently,
although there is some preference for placing
it as the sister group of all the Australian groups.
Thus, it has to be concluded that molecules have
not yet resolved the interrelationships of marsupials
much more than morphology, beyond confirming
the monophyly of Australidelphia including
Dromiciops.
The fossil record of marsupials
The taxonomy and biogeography of living marsupials
generates a number of fascinating issues that can
only hope to be answered by reference to the fossil
record. The most obvious one is how the group as a
whole arrived at such a disjunct distribution, being
found today only in Australasia and America, two
continental land masses now separated by a large,
inhospitable distance. Second is how the marsupial
mammal radiation paralleled the placental mammal
radiation in many, but not in all ways, and what this
may imply about the constraints and potentials of
mammalian adaptive evolution. Third is the effect of
secondary contact of hitherto isolated marsupial and
placental faunas, a phenomenon well illustrated by
the history of the group and often, but arguably
claimed to be an example of competitive interactions
between clades.
Origin and Cretaceous radiation
As briefly discussed earlier, the marsupial clade was
part of the radiation of tribosphenidan mammals
that commenced towards the end of the Early
Cretaceous. The earliest possible fossil marsupial
by a long way is the 125 Ma Chinese Sinodelphys
(Fig. 5.18(c)), although it appears in Luo et al.’s (2003)
cladogram as the sister group of the Marsupials and
Deltatheroidans combined. That is, as a stemmetatherian.
Otherwise, the earliest recorded fossil
marsupials, Kokopellia and other similar genera from
Utah in North America (Cifelli 1993a; Cifelli and
Muizon 1997), are dated about 100 Ma, close to the
Albian/Cenomanian boundary. The dentition of
Kokopellia (Fig. 5.18(b)) has many, but not all the
characters of the rest of the primitive marsupials,
indicating that it is the most plesiomorphic form. It
lacks the characteristic development of stylar cusps
other than the stylocone on the upper molars, and
the twinning of the entoconid and hypoconulid
cusps of the talonid of the lower molars.
A small number of specimens of more advanced
forms of marsupials have been found in rocks of
the succeeding Cenomanian stage, such as the
stagodontid Pariadens (Cifelli and Eaton 1987), and
these were followed during the course of the
remainder of the Late Cretaceous of North America
by a radiation of four families, known almost
exclusively by their teeth and incomplete jaws.
Didelphidae
This family is most abundantly represented by
Alphadon (Fig. 6.4(a)), which is often used as a model
for the ‘archetype’ marsupial (Fig. 6.1(a)). Its dentition
is generalised, as is still to be seen in the living
members of the family. The dental formula is the
basic marsupial one of 5/4 : 1/1 : 3/3 : 4/4, apart
from an occasional specialised genus. The upper
molars tend to develop the dilambdodont condition
of a V-shaped centrocrista between the paracone
and metacone, and the complete set of five
stylar cusps are usually present. The lower molars