New specimen reveals deltatheroidan affinities of the North American Late Cretaceous Mammal Nanocuris

Deltatheroida is a small, mostly Asian, clade of Cretaceous mammals with a complicated systematic history and a poor North American fossil record. The recently described latest Cretaceous Nanocuris improvida is a relatively large, morphologically distinctive taxon with carnivorous adaptations; it was tentatively referred to its own family Nanocuridae within Eutheria. Here, we describe a new specimen from the uppermost Cretaceous Lance Formation of Wyoming that is referable to Nanocuris improvida, the second occurrence for the taxon. New morphological information from the specimen allows us to refer five additional Lancian specimens, some of which have been previously described as deltatheroidans, to Nanocuris improvida and ?Nanocuris sp. Results from a phylogenetic analysis remove Nanocuris from Eutheria and reveal strong support for its deltatheroidan affinities and for its nested position within Metatheria. As a result, we subsume Nanocuridae within the deltatheroidan family Deltatheridiidae. Within Deltatheridiidae, Nanocuris forms a clade with the only other North American deltatheroidan in the analysis, Atokatheridium from the Aptian-Albian of Oklahoma. With three Asian deltatheroidans as outgroups to the North American clade, we find support for the hypothesis that deltatheroidans originated in Asia by the Aptian-Albian and undertook a single dispersal event to North America by the Aptian-Albian.