A new basal “didelphoid” (Marsupialia, Mammalia) from the Early Paleocene of Tiupampa (Bolivia)

ABSTRACT

Szalinia gracilis, gen. et sp. nov. is a new “didelphoid” marsupial from the early Paleocene beds of the Santa Lucía Formation at Tiupampa (Bolivia). It is characterized by a short rostrum and a strong anteroposterior compression of the molars. Stylar cusp C is small and stylar cusp A is very large, strongly projecting anteriorly. Like most of the other marsupial genera from Tiupampa, S. gracilis presents greater similarities with other South American “didelphoids” (e.g., lncadelphys, Tiulordia, Marmosopsis, Derorhynchus, Carolopaulacoutoia) than with any of the North American basal marsupials (Kokopellia, Aenigmadelphys, Anchistodelphys, Iqualadelphys, and the Alphadon group). Szalinia is, in many respects, more plesiomorphic than the “didelphoids” from Itaboraí (late Paleocene) and is part of a basal stock for the South American “didelphoids.” Szalinia appears to be morphologically intermediate between basal North American taxa (e.g., Kokopellia, Aenigmadelphys, Anchistodelphys, Iqualadelphys) and the later “didelphoids” of South America. This is consistent with a North American origin of the South American “didelphoids.” Limited available data on Late Cretaceous and early Paleocene South American tribosphenic mammals suggests that their dispersal from North America was complex and may have involved several distinct events. The fact that Szalinia and structurally similar taxa from Tiupampa form a diverse assemblage that already possesses some apomorphies of South American “didelphoids” suggests that they arrived before the “condylarths” and pantodonts also present at Tiupampa.