| Repenomamus | |
|---|---|
 | |
| Type ofR.giganticus,Paleozoological Museum of China | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Eutriconodonta (?) |
| Family: | †Gobiconodontidae |
| Genus: | †Repenomamus Liet al., 2001 |
| Type species | |
| †Repenomamus robustus Liet al., 2001[1] | |
| Species | |
†Repenomamus robustusLiet al., 2000 | |
Repenomamus (Latin: "reptile" (reptilis), "mammal" (mammalis)[2]) is a genus of opossum- to badger-sizedgobiconodontid[3]mammal containing two species,Repenomamus robustus andRepenomamus giganticus. Both species are known from fossils found inChina that date to the earlyCretaceous period, about 125-123.2 million years ago.R. robustus is one of several Mesozoic mammals[7] for which there is good evidence that it fed on vertebrates, includingdinosaurs. 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 byR. robustus directed at a specimen of the dinosaurPsittacosaurus lujiatunensis.[8]R. giganticus is among the largest mammals known from the Mesozoic era, only surpassed byPatagomaia.
The fossils were recovered from thelagerstätte of theYixian Formation in the Liaoning province of China, which is renowned for its extraordinarily well-preserved fossils offeathered dinosaurs. They have been specifically dated to 125–123.2million years ago, during theEarly Cretaceous period.
Repenomamus is agenus ofeutriconodonts, a group of earlymammals with no modern relatives.R. robustus was described by Li, Wang, Wang and Li in 2001, andR. giganticus was described by Hu, Meng, Wang and Li in 2005. The two knownspecies 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 familyGobiconodontidae; although this assignment is controversial, a close relationship to this family is well-founded.
Individuals of the known species inRepenomamus are some ofthe largest knownMesozoic mammals[9][6][10][11][12][13] represented by reasonably complete fossils[10][11] (thoughKollikodon andPatagomaia may be larger,[14][15] andSchowalteria,Oxlestes,Khuduklestes andBubodens reached similar if not larger sizes[16][17]). Adults ofR. robustus were the size of aVirginia opossum. It had body length without tail of 41.2 cm (16 in) for complete specimen with estimated skull length of 10.6 cm (4.2 in), although there is more partial specimen that had 11.2 cm (4.4 in) skull. Estimated mass ofR. robustus is 4–6 kg (8.8–13 lb).
The known adult ofR. giganticus was about 50% larger thanR. robustus, with a body length of 68.2 cm (27 in) and total length over 1 m (3 ft 3 in) (skull reaching 16 cm (6.3 in), trunk of 52.2 cm (21 in) and preserved tail 36.4 cm (14 in) in length) and an estimated mass of 12–14 kg (26–31 lb).[18] These finds extend considerably the known body size range of Mesozoic mammals. In fact,Repenomamus was larger than several small sympatric dromaeosaurid dinosaurs likeGraciliraptor.[18][10] Features of its shoulder and legs bones indicate a sprawling posture as in most of small to medium sized livingtherian mammals, withplantigrade feet. Unlike therian mammals,Repenomamus had a proportionally longer body with shorter limbs.
Thedental formula was originally interpreted as3.1.2.42.1.2.5, though a more recent study indicates instead that it was3.1.1.52.1.2.5.[19]
Features of the teeth and jaw suggest thatRepenomamus werecarnivorous and a specimen ofR. robustus discovered with the fragmentary skeleton of a juvenilePsittacosaurus preserved in itsstomach 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 anArchaeornithoides by aDeltatheridium predates its description.[20]
More evidence suggestingRepenomamus was suited to a predatory lifestyle was later revealed when a specimen ofR. robustus was uncovered alongside an adultPsittacosaurus. The intertwined nature of the fossil, similar to theFighting 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 theRepenomamus involved was noted to have been latching on to thePsittacosaurus with its arms and legs while biting the dinosaur.[8] Speciations towards carnivory are known ineutriconodonts as a whole, and similarly large sized species likeGobiconodon,Jugulator and evenTriconodon itself[21] are thought to have tackled proportionally large prey as well; evidence of scavenging is even assigned to the former.[6]
Like most other non-placental mammals,Repenomamus hadepipubic bones, implying that it gave birth to undeveloped young like modernmarsupials, or laid eggs like modernmonotremes.[18]
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