New marsupial (Mammalia) from the Eocene of Antarctica, and the origins and affinities of the Microbiotheria.

Abstract

We describe and comment on an isolated upper molar belonging to Woodburnodon casei gen. et sp. nov. (Mammalia, Marsupialia, Microbiotheria, Woodburnodontidae fam. nov.), from the Eocene of the La Meseta Fm (TELM 5 or Cucullaea I Member), Marambio (Seymour) Island, Antarctic Peninsula. With a body mass estimated between 900 to 1,300 g (depending on the type of equation and the possible molar locus of the type specimen), it represents the largest known Microbiotheria, living or extinct. Besides its size, other diagnostic features include a proportionally large metacone, reduced or absent para- and metaconules, and an unusual labial notch between stylar cusps C and D. Woodburnodon casei is an undoubted Microbiotheria; however, its reference to the Microbiotheriidae is discarded: almost all its morphological characters are plesiomorphic when compared with South American microbiotheriids, even with respect to the oldest representatives of this family. This suggests (a) a quite ancient and southern origin for Woodburnodon and its ancestors, and (b) that the origins and initial radiation of the Microbiotheria may have occurred from a generalized peradectoid. The new taxon, here referred to the new family Woodburnodontidae, constitutes the second microbiotherian known from these Antarctic levels and age; this confirms the
association of representatives of this order within a common, Andean-Patagonian-Antarctic biogeographic region, already present since the Late Cretaceous. Microbiotherians stand as the plesiomorphic sister-group of Bonapartheriiform marsupials, the latter including Glasbius and allied taxa.