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''Repenomamus'' (Latin: "reptile" (reptilis), "mammal" (mammalis)) is a genus of opossum- to badger-sized gobiconodontid
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
containing two species, ''Repenomamus robustus'' and ''Repenomamus giganticus''. Both species are known from fossils found in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
that date to the early
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
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
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s, though it is not possible to determine if it actively hunted live dinosaurs or scavenged dead ones. ''R. giganticus'' is among the largest mammals known from the Mesozoic era.


Classification and discovery

The fossils were recovered from the
lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...
of the
Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...
in the Liaoning province of China, which is renowned for its extraordinarily well-preserved fossils of
feathered dinosaurs A feathered dinosaur is any species of dinosaur possessing feathers. While this includes all species of birds, there is a hypothesis that many, if not all non-avian dinosaur species also possessed feathers in some shape or form. It has been su ...
. They have been specifically dated to 125–123.2
million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago ...
, during the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...
period. ''Repenomamus'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
triconodont Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the p ...
s, a group of early
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s 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 In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
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 Gobiconodontidae is a family of extinct mammals that ranged from the mid-Jurassic to the early Late Cretaceous, though most common during the Early Cretaceous. The Gobiconodontids form a diverse lineage of carnivorous non-therian mammals, and inc ...
; 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 The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
mammals represented by reasonably complete fossils (though '' Kollikodon'' may be larger, and ''
Schowalteria ''Schowalteria'' is a genus of extinct mammal from the Cretaceous of Canada. It is the earliest known representative of Taeniodonta, a specialised lineage of non-placental eutherian mammals otherwise found in Paleocene and Eocene deposits. It is ...
'', '' Oxlestes'', ''
Khuduklestes ''Khuduklestes'' is a genus of extinct mammal of uncertain affinities from the Late Cretaceous of China. It is rather similar to the also carnivorous and taxonomically uncertain ''Oxlestes'', being slightly smaller. Description ''Khuduklestes'' ...
'' and '' Bubodens'' reached similar if not larger sizes), adults of ''R. robustus'' were the size of a
Virginia opossum The Virginia opossum (''Didelphis virginiana''), also known as the North American opossum, is the only opossum living north of Mexico, its range extending south into Central America. It is the northernmost marsupial in the world. In the United S ...
with an estimated mass of while the known adult of ''R. giganticus'' was about 50% larger with a total length of around 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 ''Graciliraptor'' (meaning "graceful thief") is a genus of theropod dinosaur from the early Cretaceous Period. It is a microraptorine dromaeosaurid. The type species ''Graciliraptor lujiatunensis'' was first named and described in 2004 by Xu X ...
''. Features of its shoulder and legs bones indicate a sprawling posture as in most of small to medium sized living
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n mammals, with
plantigrade 151px, Portion of a human skeleton, showing plantigrade habit In terrestrial animals, plantigrade locomotion means walking with the toes and metatarsals flat on the ground. It is one of three forms of locomotion adopted by terrestrial mammals. ...
feet. Unlike therian mammals, ''Repenomamus'' had a proportionally longer body with shorter limbs. The
dental formula Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolog ...
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 A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...
and a specimen of ''R. robustus'' discovered with the fragmentary skeleton of a juvenile '' Psittacosaurus'' preserved in its
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
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 ''Gobiconodon'' is an extinct genus of carnivorous mammal from the Middle Jurassic to Early Cretaceous. It weighed , had a skull of in length, and had in presacral body length. It was one of the largest mammals known from the Mesozoic. Like o ...
'', ''
Jugulator ''Jugulator'' is the thirteenth studio album by English heavy metal band Judas Priest. It was released in Japan on 16 October 1997 and the rest of the world on 28 October 1997. It was their first studio album since ''Painkiller'' in 1990 an ...
'' and even ''
Triconodon ''Triconodon'' ("three coned tooth") is a genus of extinct mammal from the Early Cretaceous of England and France with two known species: ''T. mordax'' and ''T. averianovi''. First described in 1859 by Richard Owen,R. Owen. 1859. Palaeontology. ...
'' 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 Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
mammals, ''Repenomamus'' had
epipubic bone Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them). They first ...
s, meaning that it gave birth to or laid eggs that hatched into undeveloped young like modern
marsupials Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
and monotremes.


See also

*
Mammaliaformes Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent c ...
*
Cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide varie ...
s * '' Cynognathus'' *
Evolution of mammals The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked l ...


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 Eutriconodonts 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