The evolutionary history of the South American cricetid rodents
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.