Repenomamus giganticus and modern opossum
Geologic period: Cretaceous
Repenomamus giganticus shown with a modern possum for comparison
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At three feet in length and perhaps as much as 30 pounds in weight, Repenomamus giganticus is the largest mammal known from the Cretaceous. Although small in comparison to some dinosaurs around at the time, Repenomamus shows that early mammals filled a number of environmental niches once thought to have been the exclusive domain of dinos and other early non-mammals.
Repenomamus may have been a hunter or a scavenger, but one thing is certain: Repenomamus dined on dinosaurs. A related species, Repenomamus robustus, has been found fossilized with dinosaur remains in its gut!