A review of the molar morphology and phylogenetic affinities of Sillustania quechuense (Metatheria, Polydolopimorphia, Sillustaniidae), from the early Paleogene of Laguna Umayo, southeastern Peru

Abstract

The early Paleogene mammalian assemblages known from the red mudstones of the Lower Muñani Formation at Laguna Umayo, southeastern Perú, add significant information on the early phases of the evolution of Metatheria in South America. Two early Cenozoic vertebrate associations have been found from the Laguna Umayo area: localities LU-3 and Chulpas, both including metatherians. The Chulpas local fauna includes at least three indeterminate didelphimorphians and three polydolopimorphians: Chulpasia mattaueri, Chulpasia jimthorselli, and Sillustania quechuense (Sillustanidae). The Laguna Umayo associations have alternatively been referred to the Late Cretaceous or the early Paleogene. The highly derived, enigmatic morphology of the only known upper molar of Sillustania quechuense granted the recognition of a new family of South American metatherians: Sillustaniidae. Crochet and Sigé (1993) preliminary assumed that Sillustania was probably a caenolestoid paucituberculatan, but in their formal description they recognized the Sillustaniidae as a new family belonging to the polydolopoid polydolopimorphians. A review of the holotype and tentatively referred specimen of Sillustania quechuense led us to reassess its upper molar morphology and homologies, as well as its phylogenetic affinities. The discussion of these aspects constitutes the main purpose of this work.