New interatheres (Mammalia, Notoungulata) from the Late Oligocene Salla beds of Bolivia

摘要

Two new Deseadan interatheriine genera (Interatheriidae, Notoungulata) from the late Oligocene Salla Beds of Bolivia are described. Both are monotypic and one is known from a partial skeleton, a rarity among known pre-Santacrucian interathere taxa. Phylogenetically, both taxa nest well within Interatheriinae, showing characteristically bilobed p3-4. Both taxa also have derived characters (hypselodont cheeckteeth, persistent lingual sulcus on upper molars) relative to basal interatheriines such as Santiagorothia and Proargyrohyrax but are clearly plesiomorphic with respect to younger, more highly derived Santacrucian interatheriine taxa such as Interatherium and Protypotherium. New species Brucemacfaddenia boliviensis is on average larger than the other new Salla interatheriine, Federicoanaya sallaensis, although they do overlap in size. Distinguishing between the two new taxa based purely on molar morphology is confounded by lack of diagnostic chatters on the molar teeth and the overlap in size between the taxa. We overcome this difficulty of identifying specimens that preserve only molars by using discriminant analysis. We present a few of the simpler yet still robust discriminant functions we used so that future workers have a means of identifying problematic specimens. Analysis of Salla interathere specimens and stratigraphic provenance indicates both taxa experienced a modest increase in body size upsection, the driving mechanism for which remains unknown, but could be environmental changes or simple drift. These two new taxa help emphasize the fact that while the Salla fauna shares elements with roughly contemporaneous Deseadan faunas from more southerly latitudes, important faunal distinctions mark the two regions as well.