We report a diverse assemblage of tribosphenidan mammals from several localities in the Cloverly Formation (Lower Cretaceous, Albian) of Montana and Wyoming. This unit is of historical significance for yielding well-known dinosaurs (e.g., Deinonychus antirrhopus, Tenontosaurus tilletti) and early mammals (e.g., Gobiconodon ostromi, Montanalestes keeblerorum). We provisionally identify 13 taxa (five of which are formally recognized), including Pappotherium pattersoni, a new species of the deltatheroidan Oklatheridium (O. wiblei, sp. nov.), a eutherian (Montanalestes, previously named), and two new basal tribosphenidans (Argaliatherium robustum, gen. et sp. nov., and Carinalestes murensis, gen. et sp. nov.). An unnamed taxon, represented by associated but almost edentulous dentaries, is interpreted to have had four incisors, a single-rooted canine, three premolars, and four molars, indicating that the metatherian tooth formula was established by the Albian. In addition, an indeterminate lower molar fragment preserving twinned talonid cusps and a buccal postcingulid provides the earliest evidence for Marsupialiformes. We also provide a more detailed description of the associated dentaries (holotype) of Montanalestes keeblerorum. The mammalian fauna from the Cloverly Formation shares several taxa with the roughly contemporaneous Trinity Group of Oklahoma and Texas, an observation that also applies to the dinosaur fauna, suggesting some degree of latitudinal homogeneity among described terrestrial vertebrates in this part of the North American Early Cretaceous.
We report a new deltatheroidan mammal from the Upper Cretaceous of Henna, China. The new taxon, Lotheridium mengi, is based on a nearly complete skull and associated lower jaws with full adult dentition. Deltatheroidans are known mostly from fragmentary specimens from Asia and North America. Previous views on deltatheroidan relationships were diverse, but recent studies favored their metatherian affinity. The new specimen represents the most complete skull known for deltatheroidans and provides additional evidence that deltatheroidans already had the distinctive metatherian dental formula and replacement pattern and several other derived metatherian features, supporting the metatherian status for this clade. The new species also indicates that deltatheroidan mammals were more diverse and had broader geographical distributions than previously thought.
Deltatheroidans are a specialized group of basal metatherians with a long fossil history and a broad geographic distribution across Asia and North America. The group was among the first Mesozoic mammals known by relatively complete cranial material, recovered from the Gobi Desert by the American Museum expeditions in the 1920s. Deltatheroidans exhibit trends in their dentition towards apparent specializations for carnivory, including strongly developed postmetacrista, pronounced postvallum-prevallid shear, and a reduction of the ultimate molar. While the oldest undisputed deltatheroidans hail from the Early Cretaceous of North America, Upper Cretaceous rocks in central Asia have yielded the best preserved specimens and the bulk of the diversity. Here we describe a new deltatheroidan, Tsagandelta dashzevegi, gen. et sp. nov., from the Upper Cretaceous Baynshiree Formation of eastern Mongolia. This taxon adds to the diversity of the group in central Asia and provides further evidence of uniformity among the Late Cretaceous vertebrate faunas across the Gobi Desert.
