The Origin and Evolution of Mammals - Moodle

198 THE ORIGIN AND EVOLUTION OF MAMMALS
are characterised by the lingual shift of the
hypoconulid to lie close to the entoconid, the
‘twinned’ condition.
Pediomyidae
Forms such as Pediomys and Aquiladelphis have more
specialised molar teeth (Fig. 6.4(c)), in which the
upper stylar shelf is reduced and the trigon basin of
the upper and talonid basin of the lower are relatively
enlarged at the expense of other parts of the
teeth, enhancing somewhat the crushing function
at the expense of the shearing.
Stagodontidae
In contrast to the pediomyids, the stagodontids
tended to accentuate the shearing action of the
molars (Fig. 6.4(d)), indicating a more carnivorous
diet. This was achieved by a definite carnassial
notch in the protocristid crest between protoconid
and metaconid cusps of the lower molars.
The Stagodontidae are best known from Didelphodon,
one of the largest of the Mesozoic tribosphenidans,
with individual molars about 6 mm in length.
Glasbiidae
This family is based solely on the genus Glasbius,
which Clemens (1966, 1979) originally thought to be
a specialised didelphid. It has low, bunodont molars
(Fig. 6.4(b)) suitable for a more simple crushing action,
suggesting a partially herbivorous diet. They most
closely resemble the teeth of the caroloameghiniids,
a South American Palaeocene family (page 204).
Asian Cretaceous marsupials
For many years, Cretaceous marsupial fossils were
believed to occur only in North and South America.
However, the picture has changed radically on two
counts. With one possible, but doubtful exception,
the earliest South American rocks yielding marsupial
fossils are now believed to be Early Palaeocene
rather than Late Cretaceous in age. On the other
hand, definite Late Cretaceous marsupials do occur
in Asia. Trofimov and Szalay (1994; Szalay and
Trofimov 1996) described a virtually complete
specimen from the Campanian of Mongolia as
Asiatherium (Fig. 5.18(a)). They regard it as the most
plesiomorphic marsupial known, primitive in practically
all its anatomy, and placed in its own order
Asiadelphia. It does, however, possess the twinned
entoconid and hypoconulid and other dental characters
indicating that it is a more derived marsupial
than Kokopellia (Cifelli and Muizon 1997). Even
earlier, though far more incomplete material from
Uzbekistan has also been attributed to the Asiadelphia
by Averianov and Kielan-Jaworowska (1999).
Marsasia is dated as Coniacian which makes it
about 88 Ma, but unfortunately it is represented so
far only by several toothless fragments of lower jaw
and one with the fourth molar in place.
Although not as old as the Northern American
Kokopellia, these specimens dispel any simple view
that marsupials originated in North America and
were restricted to that continent during the whole
of the remainder of the Cretaceous Period. Krause’s
(2001) identification of a Late Cretaceous tooth
from Madagascar as a marsupial is very doubtful
(Averianov et al. 2003), but is a timely reminder that
Late Cretaceous mammalian faunas are extremely
poorly represented outside North America and
Mongolia.
Ameridelphia: the South American radiation
In stark contrast to the situation in North America,
South America was home to a major radiation of
marsupials from the Palaeocene onwards, which
included all the insectivorous, small omnivorous,
and carnivorous mammals until the arrival of
certain placental mammals in the later part of the
Cenozoic. It is not yet certain exactly when the
South American marsupial fossil record begins.
Gayet et al. (2001) have recently reported on a Late
Cretaceous site, dated as Middle Maastrichtian,
from Pajcha Pata in southern Bolivia. Amongst the
vertebrate microfossils collected, there are two very
poorly preserved tribosphenic teeth, so incomplete
that it is impossible to say to which taxa they belong,
not even whether or not they are marsupials. They
may prove to be australosphenidans, even monotremes,
or perhaps further specimens will reveal that
they are indeed marsupials, in which case they will
be critical for understanding the earliest phase of
marsupial history in South America.
Apart from the possibility of Pajcha Pata, Laguna
Umayo in Peru is the oldest marsupial bearing
locality in South America. For a long time it was