The holotype and only known maxillary dentition of Deltatheroides cretacicus, collected in 1925 from the Upper Cretaceous Djadokhta Formation of Bayn Dzak, Mongolia, has teeth that are badly damaged and worn, obscuring many details of their structure. Reported is a new specimen of D. cretacicus collected in 2000 from the Red Rum (Kholbot) locality near Ukhaa Tolgod, Mongolia that preserves four molars. Although the teeth are worn and have suffered some damage, they preserve details missing from the holotype. Comparisons with other deltatheroidans reveal that D. cretacicus is most similar to Deltatheridium pretrituberculare, known from Bayn Dzak, Ukhaa Tolgod, and the younger Barun Goyot Formation.
In 1998, we published the first broadscale phylogenetic analysis of basal metatherians and relevant outgroups. That analysis is updated here with amendments taken from the new specimen of Deltatheroides cretacicus as well as from relevant literature appearing since our publication. As in 1998, the consensus trees in the updated analyses identify deltatheroidans as a basal clade of Metatheria, and Deltatheroides and Deltatheridium as more closely related to each other than to Sulestes from the Coniacian of Uzbekistan. Two isolated upper molars from the Albian–Aptian of Oklahoma and the Oldman Formation of Alberta, Atokatheridium boreni and cf. Deltatheroides sp., respectively, fall at a tetrachotomy with Deltatheroides and Deltatheridium, supporting a North American occurrence for a clade generally held to be Asiatic.