New data on the Groeberiidae: unique late Eocene–early Oligocene South American marsupials

ABSTRACT

We report here on newly collected materials comprising the best known remains of the extinct South American mammal Groeberia minoprioi. Comparison with the holotype of this species and that of Groeberia pattersoni, all of which are from the same Divisaderan (late Eocene or early Oligocene) locality and level, provides the following new conclusions: (1) A pair of elongated palatal vacuities, the cheektooth formula, and the macropodoid-like mandibular inflection are the most conclusive features of marsupial affinities for Groeberia. (2) The unique combination of autapomorphies present in this taxon justifies placement of the monotypic Groeberiidae in a new order, Groeberida. The most unusual of these autapomorphies is the presence of two pairs of somewhat gliriform incisors inserted in unusually developed premaxillae, together with the posterior projection of the mandibular symphysis, which forms a basal osseous floor for the co-ossified rami. This combination of features results in a skull architecture unmatched among other mammals. (3) The dental structure and inferred masticatory mechanics are suggestive of dietary preference for hard items. (4) Analysis of the depositional history of the Divisadero Largo Formation indicates marked seasonality.