The Origin and Evolution of Mammals - Moodle

(Fig. 6.5(c)). The dentition exhibits the characteristics
of the family Didelphidae, including a fully
expressed V-shaped centrocrista. Pucadelphys was
part of an extensive radiation of small didelphid
opossums well under way during the Palaeocene;
there are four or five Tiupampan genera, and Itaboraí
has yielded nine (Marshall et al. 1997). During the
Eocene and Oligocene there was a marked decline
in didelphid diversity although this is due at least
in part to the relatively few fossil-bearing localities
yet unearthed for this period of time (Flynn and
Wyss 1998). From the mid-Miocene onwards, the
known diversity increased, reaching its maximum
at the present day.
Didelphids have also been reported from
Antarctica, in the Eocene La Meseta Formation of
Seymour Island (Reguero et al. 2002). Furthermore,
they are the only marsupials to have appeared in
the northern continents and Africa during the
Cenozoic. The very generalised Peradectes, and various
closely related genera, occur sporadically until
the Miocene in deposits in North America, Europe,
Asia, and Africa, although never diversifying, or
contributing a particularly significant part of the
mammalian faunas of those areas. One noteworthy
European occurrence is in the Eocene Lagerstätten
of Messel in Germany of completely preserved
specimens (Koenigswald and Storsch 1992). Shortly
afterwards they disappeared from the Old World
(Zeigler 1999) and didelphids remained exclusive
to South America until the secondary invasion
of Central and North America during the Plio-
Pleistocene, when they participated in the northern
dispersion during the Great American Biotic
Interchange (page 285).
The Sparassocynidae is the only other family of
didelphimorphs usually recognised, and it consists
solely of the genus Sparassocynus, a Miocene and
Pliocene taxon (Reig et al. 1987). While the molar
teeth are basically similar to those of typical
didelphids, including the V-shaped centrocrista,
they have been modified for more specialised
carnivory (Fig. 6.5(d) and (e)). Both the protocone
and the talonid are reduced and carnassial crests
have evolved. Sparassocynids possess a curiously
specialised ear region with a highly expanded
tympanic bulla and a very large ectotympanic sinus
above the middle ear, of unknown functional
LIVING AND FOSSIL MARSUPIALS 201
significance. The palatal vacuities are lost. The
relationships of Sparassocynus have long been
debated, including the suggestion at one time that
they are related to borhyaenoids. However, they are
now generally regarded as aberrant didelphimorphs
with certain features converging on those
of the borhyaenids (Reig et al. 1987), and worthy of
their own family status.
Sparassodontia (Borhyaenoidea)
This is an entirely extinct order containing the specialised
carnivorous marsupials of South America,
some of which achieved the size and extremes of
adaptation found in the larger placental felids. The
molar teeth are specialised for shearing. In the upper
molars, the centrocrista is straight, in contrast to
the dilambdodont V-shape of the didelphimorphs.
The protocone and the stylar region are reduced,
while the metacone and postmetacrista expanded,
which tends to create a longitudinal, carnassial
cutting edge. In the lower molars, the posterior part
of the tooth, metaconid and talonid, are reduced.
There are also well-defined sparassodont cranial
characters, such as the absence of a palatal vacuity,
and postcranial skeletal characters such as reduced
epipubic bones, and details of the ankle structure
(Marshall 1978).
Discovering the origin and relationships of this
group has been made difficult by the convergent
development of carnivorous adaptations of the
teeth in other predominantly carnivorous marsupial
taxa. At one time it was believed that sparassodonts
were related to the thylacines of Australia, despite
the fairly obvious superficiality of the similarities in
tooth and limb structure. Another, more carefully
considered view was that sparassodonts were the
sister group of the Cretaceous North American
stagodontids (page 198), but this was necessarily
based solely on details of molar morphology, since
no skull or postcranial skeleton of a stagodontid has
yet been found. One of the most interesting possibilities
is the view of Marshall and Kielan-Jaworowska
(1992) that sparassodonts are the most basal group
of marsupials, with plesiomorphic similarities to the
deltatheroidans. As described earlier (page 171),
Deltatheroida is a Late Cretaceous group of Asian
and North American tribosphenidans that may be
the sister group of all marsupials. Others disagree.