Animals of this genus were plantigrade, which is a foot posture not associated with running quickly after prey. Also, the legs were relatively short compared to the body. In many ways, their body shape may have resembled modern day Tasmanian devils.
They were probably carnivorous and, unlike other known Mesozoic mammals, probably ate young dinosaurs. A specimen of R. robustus has been discovered with a young Psittacosaurus preserved in its stomach. The shape of Repenomamus teeth are also consistent with the hypothesis of carnivory.
Size of a small dog
The larger species, R. giganticus, holds the distinction of being the largest mammal known from the Cretaceous. A few other described mammals, such as an early giant platypus, Kollikodon, may have been larger, but they are only known from small fragments so their actual size is in question. R. giganticus was more than 1 m (3 ft) long and weighed about 12–14 kg (26–31 lb). The other species, R. robustus, was less than 0.5 m (20 in) long and weighed 4–6 kg (9–13 lb).
These finds are considered important, because they expand the ecological niches known to be inhabited by mammals during the 150 million year reign of the dinosaurs. Previously, the only known mammals of this time period were small nocturnal insectivores, not unlike modern-day shrews. It had been assumed that the niches of animals larger than 1 m (3 ft) in length were filled entirely by dinosaurs, and were off limits to mammals until after the Cretaceous-Tertiary extinction event wiped out the dinosaurs and allowed the diversification of mammals during the Cenozoic. Repenomamus was an exception to this rule.
The smallest dinosaurs are either early bipeds, or members of the lineage that evolved into the birds during the Jurassic period. R. giganticus is larger than a small handful of feathered dinosaurs also from the same rock formation in China, including Microraptor and Parvicursor, and is roughly the same size as the earlier Compsognathus. Repenomamus is also larger than numerous species of Jurassic and Cretaceous birds, which are also considered dinosaurs by cladists.
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 128–139 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, Li 2005. The two known species are the sole members of the family Repenomamidae, which was also described in the same paper in 2000.
- J. Li, Y. Wang, Y. Wang, & C. Li. 2000. A new family of primitive mammal from the Mesozoic of western Liaoning, China. Chinese Science Bulletin 46(9): 782–785. (2001).(abstract, in English)
- Yaoming Hu, Jin Meng, Yuanqing Wang, Chuankui Li. 2005. Large Mesozoic mammals fed on young dinosaurs. Nature, 433: 149–152. (abstract)