When they were first identified in the 1920s, they were believed to be placentals and possible ancestors of the "creodonts" (a polyphyletic group of extinct carnivorous mammals from the Paleogene and Miocene), but this was later disproven.

Nonetheless, deltatheroideans do converge on hyaenodontids, oxyaenids, carnivorans, dasyuromorphs, thylacoleonids and sparassodonts in many details of their dental anatomy, suggesting a carnivorous lifestyle.

[4][8] Deltatheroideans in this regard appear to have replaced eutriconodont mammals as the dominant carnivorous mammals of the Mesozoic, either directly through competition or occupying vacant ecological niches; in North America, Nanocuris came to succeed the larger gobiconodontids and Jugulator, while in Asia the Early Cretaceous gobiconodontid radiation is replaced in the Late Cretaceous by a deltatheroidean one.

[9] Given that all insectivorous and carnivorous mammals groups suffered heavy losses during the mid-Cretaceous, it seems likely these metatherians simply occupied niches left after the extinction of most eutriconodonts.

[11] Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 444-448.