New fossil mammal remains from the Chorrillo Formation (Maastrichtian, Upper Cretaceous), Santa Cruz Province, Argentina

Study of the outcrops of the Chorrillo Formation (Upper Cretaceous, Patagonia, Argentina), resulted in the discovery of a large number of new vertebrates, including mammals, such as the first Cretaceous platypus from South America, gondwanatherians, and a large therian. The aim of the present contribution is to describe new mammalian remains from the Chorrillo Formation. The new specimens include remains of the gondwanatherian Magallanodon baikashkenke, the new species Magallanodon terrwerr, the first report of meridiolestidans (Orretherium, a small indeterminate meridiolestidan and a large indeterminate mesungulatoid), new remains of the therian Patagomaia chainko, and new poscranial remains of indeterminate mammals. The analysis of the Magallanodon terrwerr sp. nov. allowed to modify previous identifications of the isolated molariforms of gondwanatherians. In addition, the tooth assigned to Orretherium expands the knowledge of the upper molariform anatomy of this species. The remains assigned to an indeterminate mesungulatoid are larger than any known Mesozoic meridiolestidan. In comparisons with other fossiliferous localities, Chorrillo/Dorotea Formation mammal assemblage is one of the more diverse known faunas, represented by 6 different taxa, which is only surpassed by the Los Alamitos Formation, with 11 described species. Further, the presence of a monotreme, a very large therian, and Magallanodon indicate that these southern assemblages were far different from those coming from northern Patagonia, a hypothesis supported by other faunal evidence.