Nueva evidencia sobre la relación filogenética entre los osos pleistocenos de América del Sur y del Norte

The subfamily Tremarctinae (Ursidae: Camivora), distributed exclusively in America, comprises five genera: Plionarctos from the upper Pliocene of North America, Arctodus, that groups the two North American species; Arctotherium and Pararctotherium with an uncertain number of South American species; and Tremarctos with one fossil species in North America and a living one in South America. One of the characters usually taken into account in the definition of the subfamily Tremarctinae is the premasseteric fossa in the lateral outer side of the mandible body, which occupies almost all the thickness of the mandibular body. This description agrees with the observations in Tremarctos and Arctodus, but in the Argentine and Bolivian species of Arctotherium and Pararctotherium this fossa is extremely reduced or absent. In order to compare the premasseteric fossa of Tremarctos and Arctodus with that of the South American fossil genera, it was measured the volume of this fossa and related with the thickness of the mandibular ramus, between the second and third lower molar. The results show the great difference of the structure between Tremarctos and Arctodus on one side, and Arctotherium and Pararctotherium, on the other. lt is proposed that in the South American genera the premasseteric fossa is drastically reduced, disappearing in some cases. This may be considered a derived character for the group Arctotherium-Pararctotherium. Consequently, the opinion of some authors who include all the extinct genera of South American 'bears as subgenera of the genus Arctodus would be invalidated. Museo de La Plata