Repenomamus

''Repenomamus'' (Latin: "reptile" (reptilis), "mammal" (mammalis)) is a genus of opossum- to badger-sized gobiconodontid mammal containing two species, ''Repenomamus robustus'' and ''Repenomamus giganticus''. Both species are known from fossils found in China that date to the early Cretaceous period, about 125-123.2 million years ago. ''R. robustus'' is one of several Mesozoic mammals for which there is good evidence that it fed on vertebrates, including dinosaurs. Though it is not entirely clear whether these animals primarily hunted live dinosaurs or scavenged dead ones, evidence for the former is present in fossilized remains showcasing the results of what was most likely a predation attempt by ''R. robustus'' directed at a specimen of the dinosaur '' Psittacosaurus lujiatunensis''. ''R. giganticus'' is among the largest mammals known from the Mesozoic era, only surpassed by ''Patagomaia''.

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 eutriconodonts, a group of early mammals with no modern relatives. ''R. robustus'' was described by Li, Wang, Wang and Li in 2001, 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 2001. 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 some of the largest known Mesozoic mammals represented by reasonably complete fossils (though '' Kollikodon'' and ''Patagomaia'' may be larger, and ''Schowalteria'', '' Oxlestes'', '' Khuduklestes'' and '' Bubodens'' reached similar if not larger sizes). Adults of ''R. robustus'' were the size of a Virginia opossum. It had body length without tail of for complete specimen with estimated skull length of , although there is more partial specimen that had skull. Estimated mass of ''R. robustus'' is .

The known adult of ''R. giganticus'' was about 50% larger than ''R. robustus'', with a body length of and total length over (skull reaching , trunk of and preserved tail in length) and an estimated mass of . These finds extend considerably the known body size range of Mesozoic mammals. In fact, ''Repenomamus'' was larger than several small sympatric dromaeosaurid dinosaurs like '' Graciliraptor''. Supplementary Information/ref> 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 , though a more recent study indicates instead that it was .

Paleobiology

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; a recorded attack on an '' Archaeornithoides'' by a '' Deltatheridium'' predates its description.

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''. The intertwined nature of the fossil, similar to the Fighting Dinosaurs fossil of Mongolia, was likely a byproduct of an altercation between the two animals in which the mammal was most likely the instigator of an ongoing predation attempt. 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. 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-placental mammals, ''Repenomamus'' had epipubic bones, implying that it gave birth to undeveloped young like modern marsupials, or laid eggs like modern monotremes.

See also

* Mammaliaformes
* Cynodonts
* ''Cynognathus''
* Evolution of mammals

References

External links

*
Prehistoric badger had dinosaurs for breakfast
. Michael Hopkin. ''Nature.com''. January 12, 2005.
*
Fierce mammal ate dinos for lunch
. ''BBC News''. January 12, 2005.

{{Taxonbar, from=Q131960

Cretaceous mammals
Gobiconodontidae
Early Cretaceous mammals of Asia
Fossil taxa described in 2000
Yixian fauna
Taxa named by Jinling Li
Taxa named by Wang Yuan
Taxa named by Yuanqing Wang
Taxa named by Chuankui Li
Prehistoric mammal genera