Coloniatherium

Abstract: In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium, and the early Paleocene Peligrotherium. The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents.

Abstract: The great diversity of mammals from the Los Alamitos Formation (Campanian–Maastrichtian) in Rio Negro Province, Argentina has provided significant information about the evolution of South American dryolestoids. Among these mammals the alleged dryolestid Leonardus cuspidatus Bonaparte was described based on a fragment of maxilla with four molariforms. A right mandibular fragment with two molariforms from the same site as the maxilla is here assigned to L. cuspidatus. The lower molariforms are compatible in the expected morphology with those from the holotype. Even though referred to Dryolestidae, Leonardus shows unique features: (1) the presence of a huge and dome-like stylocone, disconnected and more centrally placed from the other cusps than in other dryolestoids such as Groebertherium, but contacting the preparacrista in the first preserved molar; (2) the absence of cingulae in both upper and lower molars, the latter being similar in shape to the former; (3) the presence of three roots in at least one of the lower molars, which has only been documented in the mesungulatid Coloniatherium for the m1. These characters confirm the diversity and uniqueness of the South American Mesozoic mammals.