Zupaytherium rex demonstrates that large predatory mammals evolved independently in Gondwana near the end of the Cretaceous. The presence of a 50+ kg mammal in the Allen Formation suggests that terrestrial food webs were more complex than previously recognized, with mammals occupying the 30–60 kg predator niche typically filled by small abelisauroid theropods in other Gondwanan faunas.
A parsimony analysis of 45 craniodental characters and 21 taxa recovers Zupaytherium within a monophyletic Gobiconodontidae as the sister taxon to Repenomamus giganticus from the Early Cretaceous of China. The South American taxon shares with Repenomamus a deep dentary, enlarged canine-like anterior premolars, and reduced postdentary trough. zupaytheriumrex
Large gobiconodontid with dentary length >12 cm; lower molars with three subequal cusps separated by deep notches; m1 trigonid length 8.2 mm; enamel with vertical ridges; femur with a distinct fourth trochanter. The South American taxon shares with Repenomamus a
MPCN-PV-9876, an associated partial skeleton including a right dentary with m1–m3, a fragmentary maxilla with M2–M4, axis vertebra, and partial right femur. enlarged canine-like anterior premolars