Mammals from the Late Cretaceous of Argentina and the radiation of the South American dryolestoids

GUILLERMO ROUGIER; AGUSTIN MARTINELLI

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Mesungulatum houssayi, Bonaparte 1985, was the first Mesozoic mammal described from South America and mesungulatids have been found in all Late Cretaceous the localities subsequently discovered where more than a single mammalian specimen has been found (Rougier et al., 2011) and are at the moment the most distinctive and ubiquitous mammals of the SA Mesozoic. Mesungulatids represent a morphologically diverse group of taxa which in most cases have originally been identified as units of familiar level, including Mesungulatidae, Reigitheriidae and Peligrotheriidae. Both, Peligrotheriidae and Reigitheriidae are monotypic, while Mesungulatidae includes two species of Mesungulatum, Coloniatherium, plus a couple of yet unnamed specimens species. Other taxa have been referred to the group but the status is uncertain. Mesungulatids are characterized by a great development of the pre- and post- cingula, bunodont cusps, robust and almost bifid metaconid, mesiodistally compressed lower roots, thick prismatic enamel with a small AP zone, complex last premolar and complete lack of talonid. Most of these forms have supernumerary roots in the penultimate and last premolars as well among other features. Mesungulatids range in age from the Campanian to the Paleocene and include small forms such as Reigitherium and relative giants as Peligrotherium. The petrosals and basicranium known indicate the retention of a primitive braincase made mostly by the anterior lamina and derive inner ear structures with a minimum of one cochlear turn in Peligrotherium and Coloniatherium. Mesungulatids like other SA dryolestoids lack evidence of Meckels cartilage, coronoid bone, and postdentary elements. Mesungulatum and allies are members of a derived clade of SA dryolestoid with forms like Leonardus, Groebertherium and yet unamed taxa from Candeleros Formation as basal members of this radiation. The low number of molars and premolars (3-3) in Coloniatherium, Peligrotherium and other features suggest that the mesungulatids may have arisen from a paurodont-like morphotype with small number of postcanines, and sub-equal roots. A broad phylogeny of non-tribosphenic mammals still recovered the more gracile forms are as part of a monophyletic SA assemblage sharing as most distinctive trait the lack of metacone (a feature recognized by Bonaparte, 1994). However, because the incomplete nature of most of the SA material, alternative resolutions, are just a few steps longer. The more complete materials of mesungulatids now available allow to evaluate positional variations and propose taxonomic synonyms which significantly reduce the mammalian Late Cretaceous diversity.