The Origin and Evolution of Mammals - Moodle

194 THE ORIGIN AND EVOLUTION OF MAMMALS
(a)
of the diprotodonts and caenolestids, respectively,
must be convergent. However, Kirsch had not
included the microbiothere Dromiciops in his study,
and not long afterwards Szalay (1982) challenged the
simple biogeographically supported dichotomy,
when he showed that the structure of the lower ankle
joint of Dromiciops was modified in the same way as
in the Australian marsupials (Fig. 6.2(c)). There is one
continuous joint surface of the calcaneum for the
astragalus, instead of the two separate joints found in
the remaining American groups. Szalay (1982) therefore
placed Dromiciops with the Australian forms,
separate from the American groups, and named
the two taxa Australidelphia and Ameridelphia,
respectively. Certain details of the dental morphology
are also shared by microbiotheres and the
Australian radiation (Aplin and Archer 1987;
Marshall et al. 1990), and most of the subsequent
molecular evidence strongly supports the relationship
(Kirsch et al. 1991; Springer et al. 1998; Palma and
Spotorno 1999), as also does the detailed morphological
analysis of Horovitz and Sánchez-Villagra (2003),
which used a very large database of 230 characters. It
is now universally accepted.
The interrelationships of the five australidelphian
orders among themselves are poorly understood
and consequently very controversial, for the morphological
evidence alone is extremely inconsistent.
Concerning the relationship of Dromiciops, Marshall
et al.’s (1990) cladogram placed it as the sister group
of all the other four together, Szalay (1994) placed it
as the sister group of the Dasyuromorphia alone, and
(b)
Su
CaA
Horovitz and Sánchez-Villagra (2003) placed it as the
sister group of Diprotodontia Fig. 6.3(a)) alone.
Szalay (1994) recognised a monophyletic group
Syndactyla for peramelemorphs and diprotodonts,
which none of the other authors do. The marsupial
mole, Notoryctes, has been associated variously with
diprotodonts (Marshall et al. 1990), or peramelemorphs
(Szalay 1994).
The phylogenetic relationships of the American
marsupials, as inferred from morphology, have been
no clearer. Even after the microbiothere Dromiciops
is excluded, there remains the matter of whether the
other two South American orders, Didelphimorphia
(didelphids) and Paucitubulidentata (caenolestids)
constitute a monophyletic group Ameridelphia.
Based on morphology, there are three alternative
views. One is that Ameridelphia is indeed monophyletic
(e.g. Marshall et al. 1990). The only morphological
character that supports this is the
pairing of the spermatozoa in the male reproductive
tract, and even here there are differences in the
details suggesting possible convergence (Temple-
Smith 1987). No characters of the dentition, cranial,
or postcranial skeleton clearly link them. The second
view is that the didelphids are a monophyletic
group, but that it is the sister group of the rest of the
marsupials, which is to say the caenolestids plus
the Australidelphia. Characters of the Didelphidae
so defined are limited to details of the ankle joint
and the absence of a third trochanter of the femur
(Marshall et al. 1990; Szalay 1994). The third possible
view is that Didelphidae is a paraphyletic grouping
(c)
CLAJP
Figure 6.2 Marsupial characters. (a) Comparison of the polyprotodont (above) and diprotodont (below) conditions. (b) Syndactyly of the
hindfoot of two marsupials. (c) Left calcaneum of an ameridelphian showing the separate lower ankle joint pattern (SLAJP) and an
australidelphian showing the continuous lower ankle joint pattern (CLAJP).