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Repenomamus - the mammal that ate dinosaurs 

When: Cretaceous (~138-129 million years ago)

Where: China

What: Repenomamus is the largest known genus of mesozoic mammal. Two species are known, R. robustus and R. giganticus.  Repenomamus robustus was about the size of a living North American Opossum, and R. giganticus was about 50% as big as this, coming in at about 3 feet (~1 meter) long. This is not very big by today’s standards, but as most mesozoic mammals were rat sized or smaller, this was very large indeed for its time! Repenomamus falls within the group Triconodonta, an extinct clade of mammals that falls between the monotremes and the therians (placentals + marsupials). Their name comes from the three cusps, typically in a row, found on their upper and lower cheek teeth. Fossils of triconodonts are found from the late Triassic to the end of the Cretaceous.  Though the end Cretaceous extinction is commonly thought of as the extinction of non-avian dinosaurs, some mammal groups were lost here as well, or at least their numbers drastically redued. 

Speaking of dinosaurs, the skeleton of  Repenomamus robustus was found with bones belonging to a juvenile Psittacosaurus (a ceratopsian) clearly inside its ribs. Proof that this mammal snacked on some baby dinos! The diet of the larget mesozoic mammals has long been controversial, but here is undeniable evidence that at least some of these taxa were carnivorous and ate other vertebrates. On a more personal note, this specimen was described by Yaoming Hu, who tragically died from cancer at a relatively young age, just three years after publishing the specimen.