The family Cricetidae is represented in the present
South American fauna by 44 genera and 179 species, amounting
to about 20% of the living species of mammals of this continent.
The overwhelming majority of these genera and species are South
American endemics. The most widely accepted hypothesis maintains
that this impressive diversification took place in South
America from a North American emigrant stock that entered South
America by the Upper Pliocene as a part of the massive migration
of Nearctic mammals following the establishment of the Panamanian
land bridge. Recently , another hypothesis was proposed, which
advocates an earlier arrival of the cricetids into South America.
To test these alternative views, this study surveyed
the pattern of diversification of the South American cricetids;
described a new and extensive fossil collection, mostly from
the Plio-Pleistocene sequence of the Buenos Aires Province,
Argentina, and explored the phylogenetic relationships and the
major classification of the group.
It was found that the South American cricetids belong
mostly to a distinct subfamily, the Sigmodontinae. The 41
genera and 176 species of this subfamily can be grouped into
seven clearly distinguishable tribes. One of them, the Oryzomyini,
comprises very primative cricetids, comparable to the
Ologoceneand Early Miocene cricetodontines of the Northern
Hemisphere. Nineteen fossil taxa are described, among which
there are eleven new species and two new genera. The earliest
known fossil record of the Sigmodontinae is lower Late Pliocene,
Montehermosian age. By those times, the living genera Bolomys
and Phyllotis were already full-fledged, the latter represented by an advanced species of the subgenus Auliscomys. In the
immediately overlying, late Pliocene Chapadmalalan stage,
other living genera and subgenera were found. The Early
Pleistocene fossil forms belong mostly to extinct species related
to living ones. From the Middle Pleistocene onwards, the
living species predominate. One species is found in the whole
sequence from the latest Pliocene to the Recent.
The modern character of the Upppr Pliocene and Lower
Pleistocene South American cricetid faunas, suggests an older
in situ, evolution of the Sigmodontinae than the fossil record
indicates. The hypothesis holding that the cricetids are rather
modern invaders in South America, is rejected as inconsistent
with modern character of the Late Pliocene
and Early Pleistocene fossils, and with the pattern of tribal,
generic and specific diversification of the group in the living
fauna. The origin of the Sigmodontinae is considered as dubious.
The tentative hypothesis is advanced that they probably differentiated
in South America from a cricetodontine ancestor, which
entered the continent as a waif immigrant probably by Early
Miocene times. This type of immigration is widely accepted
for South American monkeys, caviomorph rodents and some procyonids.
The absence of Sigmodontinae in the known deposits of the Miocene
and Early and Middle Pliocene of South America can be explained
by sampling bias. It is suggested that by those times the
representatives of this subfamily evolved in areas of South
America which hitherto have not yielded an adequate fossil record of
the history of mammals. As regards the geographical provenance
of the early sigmodontinae stock, this is considered to be an
open question. It is suggested that they might have originated
either in North American or in African cricetodontinesý so far
not discovered