The Origin and Evolution of Mammals - Moodle

elieved to be Late Cretaceous in age, but this is
disputed and it is more probably Early Palaeocene
(Marshall et al. 1997). In any event, the fossils themselves
are sparse and fragmentary, there being a
possible tooth of the didelphid Peradectes and a possible
member of the Pediomyidae (Sigé 1972). Taken
at face value, these identifications are very significant
because both taxa otherwise only occur in the
latest Cretaceous, Maastrichtian, of North America.
This suggests that a faunal interchange occurred,
presumably from north to south, at a time very
close to the end-Cretaceous, although the temporal
resolution is not good enough to say whether just
before, at, or just after that event. A richer and
younger site in the same formation as Laguna
Umayo has been described at Chulpas, and here
there are reported to be as many as eight marsupial
taxa, including a polydolopoid (Sillustania), a primitive
caroloameghiniid (Chulpasia), caenolestids,
and didelphids. All but the last of these groups are
only known from South America (Crochet and Sigé
1996; Muizon and Cifelli 2001). The Chulpas fauna
is tentatively dated close to the K/T boundary
and presumably represents the beginning of the
indigenous radiation that followed the initial immigration
of marsupials from North America.
Far the richest Palaeocene localities on the continent
occur in the Santa Lucia Formation at Tiu Pampa
in Bolivia, which is either Early (Muizon 1998) or
possibly early Late (Marshall et al. 1997) Palaeocene
in age, and at Itaboraí in Brazil, where the fissure-fill
fauna is slightly younger. Both sites have yielded
abundant and diverse marsupial faunas, including
six or seven indigenous South American families
between them. There are also representatives of
the modern marsupial family Didelphidae, and the
genus Khasia which is a member of the biogeographically
challenging Microbiotheria to which
the modern Dromiciops belongs. Other, less-rich
Palaeocene mammal localities occur in the San Jorge
Basin of Patagonia, some of which are Early
Palaeocene (Marshall et al. 1997).
There is inevitably a great deal of disagreement
about the detailed phylogenetic interrelationships
and thus taxonomy of ameridelphians,
particularly as many of them are still represented
only by teeth and jaws. However, the majority of
authors accept three lineages (Aplin and Archer
LIVING AND FOSSIL MARSUPIALS 199
1987; Marshall et al. 1990; McKenna and Bell 1997).
Two of these have living members and have already
been noted, although they both have a much
wider range of fossil forms. Very generally speaking,
the didelphimorphs retain primitive, sharp-cusped
tribosphenic teeth; the sparassodonts have carnivorous,
carnassial teeth; and the paucituberculates
have broader, more bunodont teeth implying an
omnivorous, somewhat rodent-like habit.
Didelphimorphia
Didelphimorphs consist of small- to medium-sized
opossums. They are generally primitive in nature,
mostly retaining the full marsupial dental formula,
and possessing simple incisors and basic tribosphenic
molars. The most controversial character
is the dilambdodont condition, which is to say the
V-shaped centrocrista, the crest on the upper molars
that connects the metacone to the paracone
(Fig. 6.1(b)). This has been taken by some authors to
indicate that the didelphimorphs are a monophyletic
group (Aplin and Archer 1987; Muizon and
Cifelli 2001). However, a very similar dilambdodont
condition is also found in primitive Australian
dasyuromorphs and certain other primitive marsupials
(Godthelp et al. 1999). As discussed later, this
suggests the alternative view that dilambdodonty is
primitive for both the ameridelphian and the australidelphian
marsupials, and indeed that the didelphimorphs
may be ancestral to both and therefore
paraphyletic. There are actually certain possibly
unique characters of the didelphimorph ankle joint
(Szalay 1994), although this evidence is very limited
because of the few adequately preserved postcranial
skeletons.
The Tiupampan form Szalinia (Fig. 6.5(a)) is the
most primitive South American didelphimorph
because its molar teeth are intermediate in structure
between the basal North American marsupial
Kokopellia, and more typical didelphimorphs from
Tiu Pampa such as Andinodelphys and Pucadelphys
(Muizon and Cifelli 2001). For instance, the
centrocrista of Szalinia is only incipiently V-shaped.
Pucadelphys is in fact much the best-known
Tiupampan marsupial; several well-preserved
skulls and skeletons have been found (Marshall
et al. 1995). It is a typical didelphid in size and general
appearance, with a skull length about 3 cm