The Origin and Evolution of Mammals - Moodle

that contains the independent ancestry of both the
caenolestids and the australidelphians. Szalay and
Sargis (2001) argued this from the postcranial skeletal
structure, suggesting that the didelphid carpus
and tarsus bone patterns are ancestral to the basic
australidelphian patterns.
There have been two comprehensive cladistic
studies based on morphological characters. Springer
et al. (1997a), before going on to discuss the molecular
evidence, analysed 102 morphological characters
by PAUP and found that the most parsimonious tree
generated did not even recognise Ameridelphia or
Australidelphia as monophyletic. The dasyuromorphs
came out as the sister group of all other living
marsupials; didelphids as the sister group of the
caenolestids plus the remaining australidelphians;
Dromiciops as the sister group of the diprotodonts.
However, most of these groups are weakly supported,
and rather than elucidating relationships,
their cladogram illustrates the considerable limitations
of morphology alone to resolve marsupial
interrelationships. Horovitz and Sánchez-Villagra’s
(2003) analysis of 230 morphological characters is
far the most detailed to date (Fig. 6.3(a)), and it also
embeds Dromiciops deeply within Australidelphia
as the sister group of diprotodonts. However
several of their groups are very weakly supported,
and it must be concluded that as far as morphology
is concerned, there is still merit in Aplin and Archer’s
(1987) refusal to accept any supraordinal taxa, but
simply to list the respective australidelphian and
ameridelphian orders.
Turning to the more recently acquired molecular
evidence of relationships, this too has been marked
by inconsistency, although as more sequences
are becoming available, agreement seems to be
emerging. One of the first was a hybridisation DNA
study by Kirsch et al. (1991), which supported
Australidelphia, and again found Dromiciops with
diprotodonts. Springer et al. (1998) published a
study using a variety of rRNA, cytochrome b,
nuclear IRBP, and protamine P1 sequences, and
they were able to propose a complete resolution of
the interrelationships of the living marsupial
orders. Their cladogram supported quite strongly
the caenolestid plus australidelphian clade, with
didelphids basal to it. The most unexpected point
was that the peramelemorphs come out as the sister
(a)
(b)
LIVING AND FOSSIL MARSUPIALS 195
Mayulestes &
Pucadelphys
Didelphimorphia
Paucituberculata
Dasyuromorphia
Notoryctemorphia
Peramelemorphia
Microbiotheria
Diprotodontia
Didelphimorphia
Paucituberculata
Microbiotheria
Diprotodontia
Peramelemorphia
Notoryctemorphia
Dasyuromorphia
Figure 6.3 (a) The morphological based cladogram of Horovitz
and Sánchez-Villagra (2003). (b) The molecular cladogram of
Amrine-Madsen et al. (2003).
group of the rest of the Australidelphia. Dromiciops
emerges as the next most basal australidelphian
group. Finally, Notoryctes is the sister group of the
dasyuromorphs.