The Origin and Evolution of Mammals - Moodle

180 THE ORIGIN AND EVOLUTION OF MAMMALS
Since the first proposal of a taxon Australosphenida,
additional Gondwana forms have been
discovered, and attributed to the group, most
notably Asfaltomylos (Fig. 5.21(c)), which extends
the geographical range of the group to South
America (Rauhut et al. 2002). It is from the Middle
to Late Jurassic of Argentina, and the only specimen
so far consists of a damaged but complete
lower jaw showing a possible six premolar teeth
and definitely three molars. Rich et al. (2001a)
described another Early Cretaceous Australian
jaw, Bishops, which resembles Ausktribosphenos and
which Luo et al. (2002) therefore include in the
australosphenidans.
Several authors disagree strenuously with the
Australosphenidan concept. Sigogneau-Russell et al.
(2001) have commented that, while a relationship
between Ausktribosphenos and Steropodon may be
well supported, Ambondro certainly does not fit at
all well, for its tooth is different in a number of
respects. Rich et al. (2002) strongly argued against
a relationship between on the one hand Teinolophos,
Steropodon and the other monotremes, and on the
other Ausktribosphenos and Bishops, disputing the
characters supposedly uniting them. They continue
to believe that the latter two are related to the
true tribosphenidans. Pascual et al. (2002) go so far as
to revert to an old view that monotreme molar teeth
are not even basically tribosphenic, but that they were
derived from a completely ‘non-therian’ ancestry.
Woodburne et al. (2003) undertook a cladistic
analysis, based on 51 characters, most dental but a
few mandibular. They found that the monotremes,
including Steropodon and Teinolophos as basal members,
are the sister group of all the therian mammals.
Furthermore, the disputed genera Ambondro,
Ausktribosphenos, Asfaltomylos, and Bishops constitute
a monophyletic group that nests within the
stem placentals.
So far only incomplete lower jaws and partial
lower dentitions of putative australosphenidans
have been found. It is therefore too early to be confident
that it really is a well-founded taxon, let alone
that it evolved independently such an apparently
complex dental structure as the tribosphenic tooth,
complete with neomorphic protocone and huge
expansion of the talonid. More material will doubtless
test this nevertheless exciting, and at least
biogeographically satisfying concept. Meanwhile,
the solution to the mystery of the monotremes
continues to be elusive.
An overview of the interrelationships
and evolution
Interrelationships
Several cladistic analyses of the interrelationships of
Mesozoic mammals (Fig. 5.23) have been published
in recent years and there is good agreement on a
number of issues, notably concerning the abandonment
of any vestige of the old, simple dichotomy into
‘therian’ and ‘non-therian’ mammals. In its place, an
increasingly crownward sequence consisting of
Sinoconodon, morganucodontans, docodontans, eutriconodontans,
and Trechnotheria (stem-group plus
Haramiyida
Adelobasileus
Sinoconodon
Morganucodonta
Docodonta
Hadrocodium
Kuehneotherium
Australosphenida
Shuotherium
Tinodon
Eutriconodonta
Multituberculata
Spalacotheriidae
Dryolestoidea
Vincelestes
Peramus
Aegialodontidae
Deltatherida
Marsupialia
Placentalia
Figure 5.23 Simplified version of Luo et al.’s (2002) phylogeny of
the major Mesozoic mammal groups.