Repenomamus

Julius T. Csotonyi Artwork

***Repenomamus*** is a genus of mammals from Cretaceous China. *Repenomamus* contains two species; one is cat-sized *Repenomamus robustus* and the other almost badger-sized *Repenomamus giganticus*. Both species fossils found in China that date to the early Cretaceous period, about 125-123.2 million years ago. Indicating quite significant advancement for mammalians in the age of dinosaurs. Despite how it is showed in media, they where still quite small compared to a human. It was the first evidence of a Mesozoic mammal that was bigger than a mouse. In fact, *Repenomamus* was 3 feet long and weighed 26 to 31 pounds. This made it bigger than most of the dinosaurs of Liaoning, where it lived. In fact, there is even evidence that some dinos should have hidden from *Repenomamus*. In the stomach of *Repenomamus* were the bones of a baby *Psittacosaurus*. This is evidence that mammals were starting to get their revenge on dinsaurs by eating them.

## Classification and discovery

The fossils were recovered from the lagerstätte of the Yixian Formation in the Liaoning province of China, which is renowned for its extraordinarily well-preserved fossils of feathered dinosaurs. They have been specifically dated to 125–123.2 million years ago, during the Early Cretaceous period.

*Repenomamus* is a genus of triconodonts, a group of early mammals with no modern relatives. *R. robustus* was described by Li, Wang, Wang and Li in 2000, and *R. giganticus* was described by Hu, Meng, Wang and Li in 2005. The two known species are the sole members of the family Repenomamidae, which was also described in the same paper in 2000. It is sometimes alternatively listed as a member of the family Gobiconodontidae; although this assignment is controversial, a close relationship to this family is well-founded.

## Description

Individuals of the known species in *Repenomamus* are the largest known Mesozoic mammals represented by reasonably complete fossils. However, other mammals such as Oxelestes and Kolikodon the monotreme both reached around the same or even the larger than Repenomamus. Adults of *R. robustus* were the size of a Virginia opossum with an estimated mass of 4–6 kg (8.8–13 lb) while the known adult of *R. giganticus* was about 50% larger with a total length of around 1 m (3 ft 3 in) and an estimated mass of 12–14 kg (26–31 lb). These finds extend considerably the known body size range of Mesozoic mammals. In fact, *Repenomamus* was larger than several small sympatric dromaeosaurid dinosaurs like the genus *Graciliraptor*. 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. Unlike therian mammals, *Repenomamus* had a proportionally longer body with shorter limbs.

The dental formula was originally interpreted as 3.1.2.42.1.2.5, though a more recent study indicates instead that it was 3.1.1.52.1.2.5.

## Paleobiology

Despite its looks, features of the teeth and jaw suggest that *Repenomamus* species 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; a recorded attack on an *Archaeornithoides* by a *Deltatheridium* predates its description. Speciations towards carnivory are known in eutriconodonts as a whole, and similarly large sized species like *Gobiconodon*, *Jugulator* and even *Triconodon* itself are thought to have tackled proportionally large prey as well; evidence of scavenging is even assigned to the former.

Like most other non-therian mammals, *Repenomamus* species had epipubic bones, meaning that it laid eggs that hatched into undeveloped young like all-non-therians.

## Appearance in other media

### *Jurassic Park*

*Repenomamus* is mentioned in the *Jurassic World Dinosaur Field Guide* as an enemy to the *Psittacosaurus*. This is likely a reference to the fossil in which a baby *Psittacosaurus* was found in a *Repenomamus*.