Both species are known from fossils found in China that date to the early Cretaceous period, about 125-123.2 million years ago.

[18][10] Features of its shoulder and legs bones indicate a sprawling posture as in most of small to medium sized living therian mammals, with plantigrade feet.

[19] Features of the teeth and jaw suggest that Repenomamus were carnivorous and a specimen of R. robustus discovered with the fragmentary skeleton of a juvenile Psittacosaurus preserved in its stomach represents the second direct evidence that at least some Mesozoic mammals were carnivorous and fed on other vertebrates, including dinosaurs;[18] a recorded attack on an Archaeornithoides by a Deltatheridium predates its description.

[20] More evidence suggesting Repenomamus was suited to a predatory lifestyle was later revealed when a specimen of R. robustus was uncovered alongside an adult Psittacosaurus.

This was posited on the basis that the Repenomamus involved was noted to have been latching on to the Psittacosaurus with its arms and legs while biting the dinosaur.