RANGE EXPANSION OF 2 SOUTH-AMERICAN MOUSE OPOSSUMS (THYLAMYS, DIDELPHIDAE) AND THEIR BIOGEOGRAPHIC IMPLICATIONS

The range expansion of two South American mouse opossums (Thylamys, Di delphidae) is reported. On the basis of morphological characters it is concluded that forms identified as Marmosa karimii from the Cerrado o f Brazil (State of Mate Grosso), correspond to Thylamys velutinus, a t axon currently recognized for the Atlantic Rainforests of Brazil. Addi tionally, thylamyines collected in northern Chile (Province of Tarapac a) and previously reported as Marmosa elegans, represent Thylamys pall idior, a form previously thought to be restricted to the Andean Prepun a of Argentina and Bolivia. The occurrence of this Andean mouse opossu m in areas of northern Chile represents a new didelphimorph marsupial for that country. The range expansions of both thylamyines are based o n series of specimens collected in northern Chile and central Brazil d eposited in the National Museum of Natural History, Smithsonian Instit ution USA. The range expansion is discussed in light of the historical biogeographic events that affected the southern part of South America during the Plio-Pleistocene, as well as the floristic relatedness of the semi-desertic biomes of the continent.