The Origin and Evolution of Mammals - Moodle

fossil mammals, which might help clarify this
question, are exceedingly scarce in North America
(Cifelli and Davis 2003).
At any event, during the Late Cretaceous the
North American marsupials radiated into 4 families,
and about 19 genera, all of them relatively
small forms, but variously adapted, some for insectivory,
some for a rather more carnivorous habit,
and some for a more cosmopolitan or omnivorous
diet. Incidentally, it should be remembered in this
context that the Tribosphenida as a whole were less
abundant and diverse than the contemporary multituberculate
mammals. It needs also to be borne in
mind that so very little is known about mammals
of the Cretaceous in the Gondwanan continents,
including Africa, that a major surprise is always
possible. The reported, though disputed presence
of a Late Cretaceous marsupial in Madagascar is
a reminder of this.
The K/T mass extinction event has been described.
As far as the marsupial mammals are concerned,
the thriving North American radiation was all but
eliminated, and the only marsupials represented on
that continent in the Palaeocene were the didelphid
Peradectes, and one or two closely related genera.
Possibly, as discussed earlier in the chapter, the
mass extinction of the marsupials was due to competition
from similarly adapted placental immigrants
from Asia. Whatever the reason, didelphids
did nevertheless persist and indeed radiated in a
small way through the Eocene and possibly into the
Miocene. Members of the family also appeared
sporadically in Europe, Africa and Asia during this
time, presumably by dispersal from North America.
Although important as small carnivorous members
of certain Old World mammalian faunas, they
never achieved significant taxonomic diversity, and
have not been recorded in these regions since the
Miocene (McKenna and Bell 1997).
South America
The next major point in known marsupial history
occurred around the time of the K/T boundary.
Having all but disappeared from North America,
they left the Laurasian and African world virtually
exclusively to the placentals. However, at about the
same time, marsupials first appear in the South
American fossil record. No marsupial, or placental,
LIVING AND FOSSIL MARSUPIALS 217
mammals have been found in undisputed Cretaceous
rocks anywhere in South America. The mammalian
fauna that was present, as most richly represented
by the Campanian or Maastrichtian Los Alamitos
Formation of Argentina, consisted solely of nontribosphenidan
Mesozoic mammal groups that are
more typical of Jurassic and Early Cretaceous
deposits elsewhere, and which are represented here
mostly by subgroups endemic to South America.
Whether the very earliest South American marsupials
are strictly latest Cretaceous or Early Palaeocene
is not certain, but what is evident is that the fauna
included possible members of two groups characteristic
of the Late Cretaceous of North America,
peradectids and pediomyids. Add to these the
very slightly later appearance in South America of
possible relatives of the specialised North
American genus Glasbius, and the conclusion seems
inescapable that the South American fauna was
derived by immigration from North America close
to 65 Ma. During the Late Cretaceous, the Caribbean
Archipelago was forming, and provided a potential
dispersal route by island hopping. Dispersal of several
other groups of terrestrial vertebrates, including
dinosaurs, had indeed been in progress during the
latest Cretaceous (Rage 1988; Gayet et al. 1992).
Within a very short space of time, less than five
million years, a truly indigenous South American
marsupial fauna had evolved from the presumed
immigrant ancestors. There was a radiation of
primarily insectivorous and omnivorous didelphids,
that retained the primitive tooth form. Simultaneously,
the medium and large carnivore mammal
niches started to be filled by the sparassodonts
(borhyaenoids), a group which was also probably
derived from the immigrant didelphid ancestor.
The paucituberculates may be related to the original
Glasbius-like immigrant, and they evolved into
a series of small- and medium-sized omnivore, and
specialist grain, fruit, and tuber-eating herbivores.
However, the South American marsupial radiation
was limited by them having to share their continent
with certain placental mammal groups, which had
also arrived at the start of the Palaeocene. These
contributed the medium- to large-sized ungulate
herbivores, and also certain specialists, such as
anteaters and sloths, that constitute the Xenarthra.
Later in the Cenozoic, marsupials had to contend